[98Frontier4WD]

I started engine without air filter and focused on the MAFS:

CLEANED MAFS—NO EFFECT:
I cleaned the MAFS with MAFS cleaner (it was a bit dirty after all-had to look at it with magnifying lens) -- no effect. Still stalled at about 10% of stoplights and had more hesitation and sputtering than before. ~10 episodes in a 15 mile trip.

RECREATE STALL BY FUTZING WITH MAF—NO EFFECT:
I cannot recreate the condition the mechanic showed me whereby engine was stalling by pressing on the MAFS electrical contactor. I wiggled/ bent/ pressed on the contactor and its cables, no difference, just a not so healthy idle throughout. Perhaps what the mechanic showed me was just a coincidence at the time.

TRY HOT/COLD AIR ON MAF—NO EFFECT
I alternated blowing hot and cold air on the MAF housing using a blow drier, to test my previous temperature dependence theory. Again no difference. Note: because my MAFS is right on the throttle body I had to cut a piece of cardboard to shield the MAFS air hole from the blow dryer air flow, otherwise if the blow dryer airflow gets close to the MAFS air intake it disturbs the air flow enough to confuse the MAFS and then the engine does indeed stall.

STILL NO CODES.

Rogoman, thanks for the suggestions. Here is my speculative investigation priority at this point:

Spark plugs: I’ll look at them one more time to see if they still show “rich” condition after normal drive. They are NGK. The fact that they were all uniformly showing “rich” condition on my first inspection (see photos below) makes me want to give this low priority. Spark plug carbon is likely symptom not cause.
Air filter: Low priority. Filter is still clean, and engine malfunctions the same way even without the air filter.
Fuel filter: Low priority because there is no hesitation under high engine horsepower. Also seems like a clogged filter would be associated more with “lean” combustion, not the “rich” I observe.
Fuel pressure: High priority? High fuel pressure (regulator fail) seems plausible. Would likely cause “rich” combustion.
Pump bad less likely, again that would correlate to “lean”.
Vacuum leak: Low priority? Would’t this also result in “lean” ?
Fuel injectors: Low priority. The fact that all 4 plugs seem uniformly sooted is not consistent. If I saw difference in one or two cylinders only then I would suspect injectors more.

Here is a photo of how spark plugs looked three days ago (Don’t they look “rich”?):


And here is how they looked in 2009 (I regard these as normal looking, right?):


More tests coming…

 

[98Frontier4WD]

Here is how spark plugs look today after 18 mile drive( 10mi freeway + 8mi city). About 10 short episodes of sputtering at highway speeds and two stalls at stoplights.

Do these still indicate “rich” combustion?

 

[98Frontier4WD]

Thanks.
So perhaps I only have rich condition at idle and other instances where throttle is closed. I'll investigate the IACV then...

 

[98Frontier4WD]

NOISY / UNRELIABLE MAF SIGNAL?

I think I may actually have a noisy / unreliable MAF signal.

Look at this first video.

With the engine NOT running and the ignition on, I taped the MAF air hole at the throttle body with a piece of electrical tape (the MAF in this 98 Frontier is in the throttle body). You can actually see the black strip of electrical tape at the very top of the video.

I inserted a paper clip into the MAF signal pin (the signal that tells the computer what the airflow is) to measure the signal.

In the beginning, the signal out of the MAF is a low 0.76V. This is a low value but I have zero airflow. Not sure what the signal should be with 0 airflow (but I measured the signal at idle and it is within spec).

But now notice what happens when I press both the connector and the MAF housing. The voltage starts fluctuating, sometimes going as high as 1.30 V. I also show how striking the MAF housing makes the signal fluctuate.

I did make sure I had a good connection to the wire through the paper clip and alligator clips, to make sure I was not just measuring bad connections at the measurement apparatus.

So is this normal? Is it normal for the signal to fluctuate like that with the engine not running?

http://youtu.be/-xdD0uazA9Q

 

[98Frontier4WD]

Thanks for the pointer rogoman,
But where is the shield? It must stop before it gets all the way to the connector. Should I strip some of the cable's outer plastic or tape to get to it?
I see in the wiring diagram that the shielding is grounded through a connector in the cabin, an enclosure by the passenger feet.

 

[98Frontier4WD]

MAF SIGNAL SHIELDING IS PROPERLY GROUNDED:

I tested the MAF signal cable shielding for proper ground. It seems to be well grounded.
I peeled back the cable wrapping, the shielding started about ½” from the MAF connector.
Shielding is at 0V (well at 0.006V to be exact). Also to verify that I had a good ground I hooked a 7W light bulb (~0.6A@ 12V) to the battery (+) and grounded it through the shielding and it lit properly. So the ground looks pretty solid.


BUT SCOPE SHOWS NOISY MAF SIGNAL?

In the following video, I borrowed a potable scope to look at the MAF signal. The video shows the MAF signal while I press either on the MAF housing or the MAF connector. The engine is idling most of the time, though I do rev it up in a couple of instances. You can hear the uneven idling (yes I have a noisy belt pulley too).

Other than the electrical noise the MAF signal voltage looks within specs. Specs are 0.9-1.8V at idle and 1.9-2.3V at 2500rpm. As you can see in the video my idle voltage is about 1.6V at idle and 2.2V when engine is revved up.

This MAF signal looks noisy to me. I have never looked at any automotive signals with a scope before, so I don’t know what to expect. But still that much noise does not look normal to me.

https://www.youtube.com/watch?v=0oaSzRDEAwQ

 

[98Frontier4WD]

1) Five days ago, I did a crude test for VACUUM leaks:

I made some contraption with a glass jar and two hoses, lit an olive oil drenched paper towel in the jar, closed it and blew some smoke through the brake booster vacuum line.

I did not see any vacuum leaks, though, admittedly I could not make a lot of smoke this way. But seems to me I should have discovered any major leaks even with this crude device.


2) So (four days ago) I CHANGED THE MAF SENSOR:

Behavior changed, I no longer get the occasional hiccup at cruising speeds but problem with idle persists.

I did not want to buy an OEM MAFS just on suspicion (It is about $400) so I got a cheap one online for about $35, just to try it out.

On the positive side, the new MAFS does not have a noisy signal like my original. When I wiggle and apply pressure on the MAFS electrical connector or the MAFS housing with the engine not running and ignition on, the signal I read with a voltmeter stays constant. Also the new MAFS seems to respond to the engine being revved up -- the voltage increases. I have not looked at the signal with a scope but I’m pretty sure its more stable. Is it more accurate than my old MAFS? Well that I don’t know.

But in any case, with the new MAFS I don’t seem to get the short power hiccups I was experiencing before at cruising speed. I have now driven back and forth 3 times to work (6x20 miles) and no cruising speed hiccups. I used to get 4-5 hiccups per trip before.

However, the idle is still erratic when I go to work, but not when I return! Also, I never get an erratic idle immediately. Certainly not when the engine is warming up (fast idle). But even after the engine reaches operating temperature, still the idle is normal, in the beginning. Then, going to work, after I have driven a good 20’-25’ on the freeway and then continue on city traffic THEN I notice the rough idle at stoplights. Then the idle goes between 800 and 500 rpm and the engine feels like it’s going to stall, but never quite does. No codes! In a way, the rough idle is now worse with this new MAFS, but surprisingly, it never stalls like it used to with the previous MAFS.

Returning from work late in the evening, when the driving pattern is reverse, I get all the way home without any rough idling (idle is not perfect but I would say not pathological in this case). In this reverse driving pattern, I first drive 6 miles on city streets and stoplights and I don’t get any rough idle. The engine reaches operating temperature within the first 2 miles. Then I drive about 12 miles on freeway and then finally 2 miles on city streets and stoplights close to my house. I don’t get any rough idling on these last two miles. Basically I don’t get any rough idling at all on my return trip home.

When I changed the MAFS, I read a pending code P0171 (mixture lean), after one round trip to work (2x20miles). But it was only a pending code, and I think it’s was likely because the new MAFS is different than the old one. I cleared the code and did not get another one in the next two days driving back and forth to work.

In any case, if it were a vacuum leak, it would not take 30’ to manifest itself. Would it? I imagine once the engine warmed up (5mins at most) it would have started causing symptoms. No?


3) So (today) I TOOK APART AND CLEANED THE IACV (Idle Air Control Valve):

Was challenging to remove without removing the throttle body, but with some contortions and McGuyver screwdrivers I managed.

?? Now, QUESTION: is the IACV supposed to stay A LITTLE OPEN when the solenoid is not connected ??

In other words, is the spring inside the IACV supposed to keep the valve completely closed?
Because that was the only thing that struck me perhaps as a little suspicious. The IACV was not completely closed when the solenoid was disconnected.
Otherwise, the IACV did have a small film of carbon deposits but did not seem unusual. I cleaned it with carburetor cleaner anyway.

I also cleaned the fast idle valve, since I was there, and I also took out the four fuel injectors and cleaned them, they did not seem dirty. I put things back together, but still, rough idle after about 20-30 minutes.


4) I DISCONNECTED BATTERY:

And left it disconnected for 30’, just to give the ECU a chance to forget the … traumatic past. Engine was difficult to start afterwards, took 20” cranking before it started, very erratic and slow, then within another 30” it stabilized.

But still, erratic idle after about 30’.

Got another pending code P0171 (lean), but again, I think this might be because the new MAFS likely has different characteristics than the OEM part?


5) I VACUUM PUMPED the EGR with the engine idling:

Engine stalled. So I guess that means the EGR is working?

============================================

Any ideas? Especially whether the IACV is supposed to close all the way when the solenoid is inactive?

Here is a video of the IAVC not closing completely
https://youtu.be/E9Z6HbqpHvQ


And here is a video shaowing me applying on and off voltage to the IACV solenoid (I connect/disconnect the electrical connector)
https://youtu.be/7XAklJn3vQM

 

[98Frontier4WD]

Thanks jdg,

1) Yes, carb cleaner for vacuum leaks …I have not tried that but I did not mention that I did try the similar propane method. But it does not seem to work. I started by releasing some propane directly into the throttle intake, to see what effect on engine RPM I might expect, but there was almost no effect. It’s as if the ECU immediately senses the extra fuel and immediately readjusts, so there’s not really any noticeable change in RPM – especially since the idle is already uneven. So, I figured, if it does not work with all the propane directly into the intake, what’s the chance I’ll be able to detect a small leak? Also, I’m not sure how much propane is needed. I just used a small plumbing torch on up to max flow (what would essentially make a say 4 inch flame if you lit it). Maybe this propane trick only works on older cars?

2) I know that I cannot really see signal variations with a multimeter. But… my old MAFS had enough of a noise in its signal that I could actually see it even on a voltmeter (with the ignition on, engine not running and a piece of electrical tape over the MAFS air hole the signal should be steady – but with the old MAFS I could see variations on multimeter when I applied pressure on the MAFS connector or its housing) . Since that behavior went away with the new MAFS, I feel safe to assume that the noise was originating inside the MAFS – as opposed to some grounding issue or the ECU. Bottom line: The noisy MAFS may have been one additional cause of rough idle, but apparently not the only issue.

I suspect t my old MAFS has some failing electrical connection inside (solder gone bad). But it does not really look like you can open these things, or at least not without risking destroying them. Without having another MAFS I did not want to risk destroying my only semi-working one. The car becomes barely drivable without a MAFS. Now that I have a secondary MAFS, I may attempt to fix my original OEM part -- especially since the new one does not seem to have the original OEM characteristics (though apparently ECU “learns” to interpret its signal – hence perhaps the initial P0171 “lean” code – but always as a pending code, never a check engine).

BTW, thanks everyone for all the help. Though the problem remains elusive, I feel I’m learning a lot. Hopefully this discussion is not so far off track to be useless to others.

 

[98Frontier4WD]

STARTED GIVING ME AN OCCASIONAL PENDING CODE P0171 (LEAN) :

Of course now I don’t know if this might be caused by that non OEM cheap MAF I’ve installed (since my original Hitachi MAF seemed to have a noisy signal). But in any case,

I CHANGED THE FRONT O2 SENSOR, no effect:

Thought perhaps a failing sensor would give a lean signal, causing the ECU to exceed acceptable fuel trims and flag an occasional pending P0171. Besides that sensor had 155K miles on it, not a waste changing it, I thought. But since it had no effect, I tried to look again at vacuum leaks,

TESTED FOR VACUUM LEAKS WITH CARBURETOR CLEANER:

Used carburetor cleaner this time, and tried to get the ECU to run in open loop for the test.
I thought I could cause open loop operation by disconnecting a few sensors. So I disconnected the Throttle Position Sensor, the MAF and the front O2 sensor. But the engine refused to run with all three sensors disconnected. I had to re-connect the MAF and with TPS & O2 disconnected it seemed to run, open loop I assume? I could now detect some change in engine pitch when I sprayed carb cleaner directly into the intake.

I sprayed around several places throughout the intake, but because the idle was erratic in the first place, my test was rather inconclusive. There was a rather non descript area where if I sprayed I could somewhat detect a change in engine RPM, but not always. Also, it is possible that the carb cleaner simply evaporated from where it was being sprayed and then rose to be sucked in by the throttle intake airflow. Seems like in order to do a better test I’d have to put some tube / big hose on the throttle intake to make sure carb cleaner fumes don’t find their way in. Anyway, I have to get another can of carb cleaner to repeat the test with a better setup.

BTW, I did open my original MAF, you can cut the black plastic cover (and hope to glue it back together if you manage to fix it). I figured there was likely some bad contact inside or a solder that had gotten loose. But the entire circuit is covered with a tacky sort of silicone or polyurethane gel. Even if I managed to get the gel out, the small wires that attach to the circuitry don’t seem to be soldered. They look like they are almost braised (or something) to the leads.

====================

Well, I have the fuel pressure test left top do. Then I’m probably ready to declare defeat. Have never taken my truck to the dealer, perhaps this is the time I find out whether they’re worth the $150 they want to charge me for diagnosis.

 

[98Frontier4WD]

Thanks rogoman,

Well, maybe some data is finally coming up:

I did a crude vacuum test. Below is a video of what is happening with my vacuum. Now I recognize this is probably not the right tool to test vacuum, tomorrow I’ll look for a proper automotive vacuum gauge for the job. As you can also see in the video I hooked the vacuum gauge to the brake booster hose which unfortunately has a valve -- so I had to resort to a hack where I put a paper clip to create some small vacuum release otherwise the valve would simply retain the maximum vacuum.

I see the needle fluctuating which is not a healthy sign, but it could also be an artifact of my gauge and setup. As I said, tomorrow I’ll look for a proper gauge and hook it up to a hose without a vacuum valve.

My Vacuum video:
https://youtu.be/zoohhMJsnCg


Now since fast vacuum fluctuation could be a sticky valve, I did a google search and found this video:
https://www.youtube.com/watch?v=PI5bBV0ZFek

I did this same “paper by the exhaust” test on my truck and lo and behold, this afternoon that I had a sporadic rough idle issue (the idle is not always rough – it’s intermittent) what I saw was: The paper was being pushed back (normal) then at some point it would do a random suction (paper goes toward the exhaust-abnormal) and then immediately after the engine would try to stall -- but would not fully stall, there was a small surge towards more normal RPM and then the idle would recover till the next rough incident, which yesterday evening was about once every 5 seconds. But as I said this rough idle sometimes it’s not there at all, sometimes its mild and sporadic, sometimes is almost constant.

Here is another video I took yesterday evening where perhaps you can hear the sound the exhaust makes when the rough idle malfunction is almost constant. Great Harley sound up close like that! I had not discovered the paper trick yet when I took this video so I don’t know what a paper would have done here.
https://youtu.be/RQTGjkN_xlk

More tests tomorrow…

 

[98Frontier4WD]

Well, I’m still at a loss

VACCUM TEST:

So here is a better vacuum test. I connected the vacuum to the brake booster, but before the one way valve.
In the video you can see the rough idle:
@ 0:05 Near stall
@0:38 Near stall
@0:53 & 1:30 I revved up to 4000RPM
@1:55 Briefly opened throttle ½ way
@2:42 Near stall
This is the first time I do a vacuum test. Does the little vibration in the needle qualify as unstable vacuum?
The tinging/sputtering vibration noise you hear in the video is not the exhaust but noise from inside the vacuum gauge, perhaps the needle fluctuating?

My Vacuum test: https://youtu.be/g7GcgtUDa9o


This next video shows idling only. As I noted down the times when a near stall occurs, I see they occur almost every 30 seconds regularly. Perhaps that has some significance?

My vacuum at idle only: https://youtu.be/oBqMvjeotjI


COMPRESSION TEST:

Seeing whatever needle vibration I saw above, and seeing the paper sucked into exhaust in the “paper by exhaust” (like this guy shows on youtube: https://www.youtube.com/watch?v=PI5bBV0ZFek), and reading that it may indicate sticky valves, I decided to do a compression test.

Test was done with all spark plugs removed, throttle full open, about 7 turns of the engine (cranking to 15 turns seems to yield 3 PSI more):
Cyl 1: 162 PSI
Cyl 2: 162 PSI
Cyl 3: 158 PSI
Cyl 4: 158 PSI

I tried adding about 1tsp of oil to Cyl1 and got 170 PSI.

Nissan specs are: Standard: 178 PSI. Min: 149 PSI, Max acceptable difference between cylinders: 14 PSI.

So looks ok to me. No sticky valves? No bunt valves, no major ring and seal issues?

LOOK FOR BLOWN GASKET:

Oil looks fine, not milky
I opened the radiator and started engine cold. No coolant came out.

FUEL PRESSURE TEST:

I imagined that fuel supply issues would occur at high power, not idle. But since I have now pretty much checked everything, I went ahead and did a fuel pressure test.
I Td a fuel pressure gauge after the filter and got:
36 PSI with vacuum hose attached to fuel pressure regulator (Spec: 34) OK
45 PSI with vacuum hose NOT attached to fuel pressure regulator (Spec: 43) OK

So, all systems are checking out ok

I now do get a pending P0171 (lean) code once in a while after letting it idle for a while. I also got a P0300 but I’m not sure about the conditions since I was doing all these tests with vacuum, compression and fuel pressures.

In any case, when I took out the spark plugs to do the compression test, here is how they looked (engine had been rough idling for about 10 minutes before I took them out).

I cannot see any unevenness, implying something might be wrong with one or more cylinders.


So what is going on?

As a reminder I have also:
-Changed the MAF (ok with cheap non OEM part, since my original was noisy)—disconnecting MAF does not fix the rough idle anyway.
-Cleaned the Idle Control Valve and checked operation (spring by itself closes valve only ¾ of the way, but I assume that is normal?)
-measured the TPS resistance for linearity and the TPS switch
-Checked the fast idle settings (thermos-mechanical opening of throttle, throttle completely closed once engine warm)
-Checked the manual portion of the idle air (screw on throttle body) per shop manual procedure (If I repeat the procedure per shop manual I end up in the same ½ turn of the screw open-same as it was)
-Checked ignition timing per shop manual procedure (look for 20 deg BTDC with TPS disconnected) -- in case timing chain had skipped.
-Hand pumped the EGR valve at idle to verify that it did indeed cause engine to stall—in case EGR stuck open at idle.
-Changed front O2 sensor—in case it was about to fail and giving erroneous lean
-Put Techron in gas tank—to clean injectors
-Opened injectors to look for dirt—very little found
-Checked for vacuum leaks (not very thoroughly but vacuum measurements look good, don’t they?)

So what’s left:

The only thing I have not checked is ignition. But obviously I do have spark, so any static or simple continuity tests on ignition and coils are unlikely to reveal anything. Besides what sort of ignition issue is it that shows up only at idle? I yanked on cables, banged on distributors, disassembled distributor cap, nothing, no difference. Any ideas on the ignition?

A mystery remains, that “Paper by the exhaust test” which at my exhaust does indeed show the paper being occasionally sucked towards the exhaust pipe when engine gets into its funky idle spurt (by is this cause or effect of the fact that engine is in near stall?)

Driving me nuts !!

As an added irritation, I have to pass a smog test by the end of the month…

 

[98Frontier4WD]

Thanks. I put the injector test in the queue… as far as pinpointing a particular cylinder at fault, no the only codes I have gotten occasionally are P0171(lean) and P0300 (random misfire) and neither compression tests or spark plug appearance show any difference between cylinders

... meanwhile,

SMOG TEST PASSED:
Well, it passed the smog test. So now at least I have time to fix this.
Here is a snapshot of the smog test results:


OCCASIONAL P0171 and P0300:

Engine still works well, including reasonable idle while cold and well after it has reached operating temperature.
But once it has been running for about half hour, the rough idle starts.
I’ve been monitoring for codes and when it starts rough idling, within about 5-10’ I often get a pending P0171 (lean), but not always. Typically I don’t get the code again in subsequent trips so Engine light typically never comes on and the pending P0171 quickly clears (one or two trips, I’m not sure). Sometimes, together with the P0171 I’ve gotten also an occasional P0300 (random misfire).

drive with MAF DIDSCONNECTED test:

Another interesting fact: Rough idle is there even if I turn on the AC. However, if I disconnect the MAF then the rough idle is still there, but less than with the MAF connected. More interestingly, with the MAF disconnected when the AC compressor is on the idle is now steady at about 1000RPM.

 

[98Frontier4WD]

Well, I did capitulate and have taken the truck to two more garages now. But no success. Nobody seems to be able to find anything.

I guess the dealer is next in line to prove their worth. Hopefully I will not have to just pay for overpriced attitude.

To their credit, the two garages did not want to charge me anything since they were not successful, though I insisted on paying them $40 for the one hour or so they spent making an honest effort.

If I heard correctly, one of the mechanics told me that my Long Term Fuel Trim is at about -10% while my Short Term Fuel trim is about +24% (almost maxed out, I guess?). That seems puzzling to me. At a certain RPM after a while the LTFT should start adjusting so that STFT goes back down closer to 0. Not?

I'm thinking I should buy one of those ODB2 bluetooth or wireless tools and then download one of the many available programs (I heard Torque was good?) to verify these things myself. Anyone has any experience with those?


The garages also double checked many of the items I had originally investigated, including smoking the whole system in search of vacuum leaks but did not find any. In one mechanic’s opinion, the lean condition could cause the vacuum needle to vibrate a bit, there is not necessarily something wrong with the cylinders, valves or gasket, he said, things that were more or less excluded by the other tests anyway.

 

[98Frontier4WD]

Thanks jdg. I ordered something similar waiting on it…

Sorry for the long silence. Had to get some business travel out of the way.

Took the truck to the dealer first time Friday. But their diagnosis seems vague to me and leaves a lot unexplained.
Here is how it went:

I talked to the master tech, gave him a two page summary of symptoms and what I had already done and spent 10’ explaining verbally.
As I was talking he told me I should have done an “idle learn” after changing the MAFS. But I don’t think this ’98 Frontier has an idle learn procedure. I did not find one mentioned in the Service Manual. In any case, I cannot expect techs to know by heart all the models and years that have an idle learn procedure. He did not mention doing idle learn himself afterwards.

He later called me and thought it may be a short or something of that nature. He said the MAFS signal was really noisy “I have never seen it that bad, and wonder how does this truck even run?” were his words. He was then nice and sent me the diagnostic page through email. Here is what I got:



BTW, I’m not sure I understand the two middle signal traces (especially the Air/Fuel Alpha, isn’t that value too low? ), so if someone has something to add I’d appreciate it.

Sure, looking at that first trace, seems like there’s virtually *NO* MAFS signal. That trace looks to me more like the signal from Eddie Van Halen’s guitar amplifier.

So first mystery is how can the truck even run without major drivability issues (other than a rough idle) with a MAFS signal like that? (No MAFS codes from ECM)

Second, this does not corroborate at all with my measured MAFS signal. Below are the videos showing my voltmeter measurements. Of course it could still be that the noise is so high frequency (looks to me in the order of 10HZ ~100msec period) that I cannot pick it up with my analog or digital voltmeters. However, as you can see in the video, the MAFS seems to respond to changes in engine load as expected and is also within voltage specs. I also blew into the MAFS with ignition on, and it does indeed seem to respond as expected.
I’m thinking that the noisy signal trace I got from the dealer shows a MAFS that should be totally unresponsive, not what I observe.

In any case, here are my videos. To rule out problems in the wiring harness, I took out the ECM and measured the MAFS signal directly at the ECM input, as shown here:


First is video of measurement using a small analog voltmeter:
https://youtu.be/vdCYtiSyeIc

As you can see my video shows:
@idle my MAFS signal = 1.3V (spec is 0.9-1.8V)
@2500RPM no load my MAFS signal = 1.9V (spec is 1.9-2.3V)
@3500RPM no load my MAFS signal = 2.3V
The signal seems to increase linearly with increasing RPM – as expected.

In this second video I repeated the measurement with a digital voltmeter:
https://youtu.be/-jb_MRQKADM
@idle my MAFS signal = 1.40-1.45V (spec is 0.9-1.8V)
@2500RPM no load my MAFS signal = 1.97V (spec is 1.9-2.3V)
@3500RPM no load my MAFS signal = 2.35V
Snapping the gas (higher engine load) brings my MAFS signal up to 3.5V


As an extra test I just turned on the ignition (engine not running) and just blew into the MAFS hole in the throttle intake. As you can see the airflow registers as expected.
https://youtu.be/5ylCFTawZtk

The dealer’s suggestion was to replace the MAFS with a genuine Nissan at over $500 – just to continue testing. I already paid him $150 for this diagnosis.

I’m not expecting this cheap MAFS I put to be that good. I’m sort of expecting skewed long term fuel trims as the MAFS is likely to have a different airflow/signal characteristic. But I do expect the truck to run and idle with the fuel trims taking up the slack. My hope was to confirm that the original MAFS was the culprit and THEN go ahead and try to find an OEM part. But the evidence makes me doubtful that the MAFS is indeed the cause of my problems.

The biggest discrepancy is that I see nothing like what the dealer sees in the MAFS signal. Of course he’s (presumably) measuring the MAFS as the ECM sees it, while I’m measuring the signal at the ECM input. If there is a disparity indeed, that would point to a bad ECU. An ECU whose only malfunction is to misinterpret the MAFS signal. What is the probability of that? Even taking into account the fact that I should start looking at oddball things at this point.

I guess next thing would be to borrow a scope again and see if I can record the MAFS signal the dealer sees.

P.S. Will I be able to see traces like the one the dealer sent me with torque? Or at least quickly updating real time values?

 

[98Frontier4WD]

I JUST CANNOT SEE ANY NOISE ON THE MAFS SIGNAL WITH A SCOPE:

I went ahead and measured the MAFS signal entering the ECU (pin 54) with a scope.
I cannot see anywhere the noise that the dealer sent me (see my previous post).
Here is my scope trace for MAFS signal entering the ECU:



The scale is 2seconds per square the voltage 0.5V per square. It shows me revving up the engine from idle.
I cannot tell if the voltage levels are correct (they are within spec), but the behavior is as expected. You can see how the signal surges by depressing the accelerator. Certainly I do not see the noise. The signal looks quite smooth. Definitely nothing like what the dealer sent me ? ! ?

Here is one of the videos I took showing the scope trace:

https://youtu.be/dUm6E1512tU

And here is a video of the MAFS signal I made during rough idling. You can actually see how as the idle drops below the pre-programmed 850RPM, the ECM opens the IACV (I can hear this when the air filter is off) in an attempt to bring the idle back up (presumably injects more fuel too) and the increased airflow is seen in the increased MAFS signal.

http://youtu.be/r4VBFGXGxok


INDIVIDUAL CYLINDER AIR INTAKE VISIBLE?

Jdg, your intuition on individual cylinder air intake being picked up by the MAFS might be correct. Interestingly, the only high frequency I see is exactly when I snap the gas -- when the butterfly opens and the individual cylinder air gulps become large -- then I can see oscillation in the signal. I think these might indeed be the individual “gupls” of air intake from the cylinders, since the frequency of the oscillation is 2x the RPM. So I guess this hot wire MAFS seems sensitive enough to pick up some of these high frequency airflow variations !?! If anything, being a cheap one, I thought it would be slow in responding…

Here is the MAFS signal trace when snapping the gas:



And here is a similar video but with a fast trace, where you can see the signal oscillations when snapping the gas:

http://youtu.be/8saghlBiZYg

But other than those brief periods where the gas is snapped, I see no noise on the signal. Certainly nothing like what the dealer shop sent me.

So bottom line…

If the dealer indeed sees noise in the MAFS signal when he reads it from inside the ECM while I don’t by reading it as it enters the ECM, there are three main possibilities:

a) I am measuring wrong, but serendipitously read something that looks exactly like a reasonable MAFS signal
b) The dealer is reading the MAFS wrong from the ECM
c) The signal is ok entering the ECM but there is noise that scrambles it in the internal ECM signal conditioning, or the analog to digital converter, or something like that

a) seems quite unlikely. So does b). As I understand it, there is nothing to connect that the dealer tech might have gotten wrong. You just connect to the NISSAN consult port and the Nissan diagnostic just reads the MAFS digital value from some predetermined location in memory. Does not seem like there’s much opportunity for error.

c) would imply a bad ECM. A bad ECM where the only malfunction is scrambling of the MAFS input signal. Perhaps I’m biased but I’m also very skeptical of diagnoses that blame the ECM.

In any case,
To test this last scenario c) I wondered if I could just feed an artificial constant MAFS signal to the ECM. If the ECM is scrambling the signal then it should scramble my constant voltage too, and continue to run rough, or worse. So I just used a potentiometer as a voltage divider to create a constant voltage of about 1.7V (what the MAFS typically puts out at idle). I disconnected the MAFS, and fed the 1.7V signal directly into the ECM MAFS signal input (pin 54). THE ENGINE IDLED BEUAUTIFULLY. If the signal were getting scrambled, I should still have the rough idle, shouldn’t I? Of course with this artificial signal you cannot do much other than idle. As soon as you press the accelerator the engine stalls. Poor ECM sees no change in the airflow, probably does not increase the fuel and thus with throttle open the engine stalls. I’m pretty sure the ECM gets thoroughly confused by that.

Another thing I feel this proves is perhaps the absence of anything mechanically wrong with the engine, since I got it to idle beautifully by simply feeding it an artificial constant MAFS signal.

WIRELESS OBD2 READER:
Anyway. I got the wireless OBD2 reader today. This one:

I have not connected it yet. I was hoping to get it to work with my iPhone but none of the 4 free apps I tried seem to work that well (dropping data etc). I’ll keep looking or may have to resort to using laptop computer. I don’t mind buying an app for this, but I first want to see the limited functionality demo version work, which has not been the case yet.

Yes, nobody has looked at the injectors yet.

This is becoming quite the intriguing problem, I must say…

 

[98Frontier4WD]

jdg, thanks for hanging on with this long thread. More information is coming.

Regarding the scope trace, the time shown (2seconds to 500msec) is the scale, I.e. the time per square on the display, not the sampling rate. The sampling rate is much higher. So you would see noise even on a slow trace. It would just all smudge together.

I did the ground current carrying capacity test (tail light bulb) you mention for the MAF signal shielding (see post #13). I have not done it for the MAF ground itself.

GOT SOME NEW DATA WITH OBD2 WIRELESS DONGLE:

I gathered quite a bit of sensor data on a few trips using the new OBD2 dongle in combination with the OBDCarDoctor free app on my iPhone. It is very useful.

I’ll have to process the data and then I’ll post it. I may have finally isolated the issue, but there are also a lot of things I cannot explain… could use some help… let me process the data and I’ll post again…

 

[98Frontier4WD]

Well, here is the OBD2 data:
Refresh rate is about 4 samples per second. Good enough for this type of analysis...

 

[98Frontier4WD]

Thanks jdg, those suggestions were quite helpful.

ELECTRICAL CABLES:
Now that I have the scan tool with graphing capability I did try to yank, shake and disturb any cable I could, bang on the dashboard, shake the truck, the ECM etc. … while observing the MAFS signal through the OBD interface. No noise or disturbance was ever noticed. I repeated the test by placing electrical tape over the MAFS to make sure I had a steady signal, so that I could more easily spot any perturbation in the signal. If you don’t put tape over the MAFS opening you just see variations in natural air movement (eg. a breeze).

THERMALLY SENSITIVE MAFS?
So, unable to blame the cabling I pursued the theory of a heat sensitive MAFS. So I took the MAFS out of the throttle intake, left it hanging by its wires, turned on the ignition and used a hairdryer to start heating up the MAFS, while monitoring the signal with the OBD2 app. I was hoping to see some noise develop at some point. The MAFS opening was taped, to keep the hairdryer airflow out of the signal.

So, in about 15’, I heated up the MAFS to probably 160-170F (the temperature I thought it develops during driving) and waited 15 minutes, but I saw no noise. I decided to heat it more (did not care about destroying a $30 MAFS at this point). So I put the MAFS in a glass jar (to keep the hairdryer airflow from dispersing) together with a BBQ thermometer. Temperature inside the jar climbed to 230F. I really don’t think it heats that much normally, but what that heck, I thought, enough trouble already. I must have waited a good 30 minutes warming up the MAFS, …and then finally voila the spiking signal started!!
I verified for about 10’ that what I was seeing was indeed spikes and noise. Yes it was!. Airflow as seen by ECM was spiking between 0 and 12gr/sec even though the sensor was blocked with electrical tape!! Then I let the MAFS cool. But it kept spiking even as it completely cooled!! I must have taken no more than 10’ for the MAFS to cool, but I waited and it kept spiking for a good 30’ – spikes with MAFS cold now!!

I thought, “Could it be something else that is warming up and causing the spikes, perhaps a wire (jdg, your intuition about just turning on the ignition and waiting was quite insightful!) or the ECM?”The temperature I had to reach to get MAFS to spike seemed unreasonable, and the fact that spiking remained well after MAFS had cooled made me suspicious that perhaps some other element is heating up and causing the spikes instead of the MAFS itself.

So, without turning off the ignition and while this new MAFS was still spiking, I swapped it with the old Hitachi MAFS. No heat. I waited about 5’, got no signal spikes.

So, I put back the new MAFS which by now had not only cooled down completely but had also been disconnected for 5’. No heat. Within 1’ the new MAFS started spiking again!!!. I watched it spike for another 3’ and then,

I swapped it again for the old Hitachi MAFS. I waited about 8’ but did not see any noise/spikes. So,

I put back the new MAFS, which had spent even more time cold and disconnected. Within a few minutes it started spiking again. Stone cold and spiking!! (to be exact, not completely cold, it was about 10F above ambient temperature – I think the hot wire and circuitry inside the MAFS generate enough of their own heat to keep the device a little above ambient temperature). I observed this (new) MAFS spiking for another full hour.

As a last test, I put back the old Hitachi MAFS and waited one hour and 45 minutes. I saw no noise/spikes.

I did though verify the defect of the old Hitachi MAFS, whereby pressing on the housing generates noise on the signal. But it’s a completely different type of noise than the spiking I get with the new MAFS. So my original Hitachi MAFS does seem bad, but in a different way.

So it looks like a thermally sensitive bad (new) MAFS right?

Two things though are not convincing. A) that it took so much time and so high a temperature to cause the misbehavior. But more importantly, B) The fact that even when the MAFS signal is misbehaving a voltmeter (analog or digital) shows the same voltage as when the MAFS is healthy!! It’s as if in spite of the spikes and noise, the “average” signal is still the same, hence the voltmeters show the same. That does not seem very convincing.

So to do one more test, the next morning I just turned on the ignition with the new MAFS and let it sit there for about 3 hours (thanks for the suggestion jdg!!), without heating anything. I figured that if it were the ECM (perhaps the DAC or other input signal circuitry) or some thermally sensitive wiring that was causing the problem, I should see spikes/noise after a while. But I did not see any noise/spikes.

I’d be happy to get a new MAFS and get this over with at this point -- and stop torturing you all guys here on this very helpful forum. But I’m reluctant to shell out the $550 the dealership wants just to get another Hitachi part that likely has a design or systemic manufacturing weakness. And if I order from another party, I’m afraid I’ll get a third MAFS lemon… which will then further drive me nuts for another month or two (though I think, if anything, I’ve now learned how to debug them – so thanks for the support, regardless of what happens I have learned a lot about how to debug contemporary engines).

I think I’ll get a more definitive conclusion when I get the MAFS to misbehave and can actually see the noisy signal entering the ECM with the scope. Problem is, it’s not that easy to borrow the scope. Apparently, when I looked at the signal with the scope the last time, I must have not waited long enough and the MAFS was not misbehaving yet.

So what about that funny STFT behavior when the MAFS is working? That STFT ramping up and down rhythmically over a 30 second cycle? It could be that the noisy MAFS, when it does eventually happen, throws the ECM for a loop and teaches it bad habits which then it carries on even when the MAFS IS working well. Well, one thing at a time. The MAFS signal is definitively going crazy. Let’s fix that and we’ll see if there is anything else.

Thanks and sorry this has dragged on for so long. Hopefully it’s not a total waste.

 

[98Frontier4WD]

Well, I ordered a new MAFS. That is why I kept quiet. This time I went with the original Hitachi (or so I think, I’ll have to see what arrives).

In the meantime to offer some entertainment, I’m posting the OBD2 graphs I was able to plot.

As a reminder, I ordered a cheap OBD2 dongle from Amazon (see previous post) and paired it up with the “OBDCarDoctor” app on my iPhone. I paid the $4 to get the paid version of the app, which allows you to monitor up to SIX parameters simultaneously and also allows you to SAVE the values to a comma separated text file.

Then with modest spreadsheet manipulations I imported the text file into Microsoft Excel and created a graph. I was monitoring 6 parameters (the maximum the tool lets you simultaneously monitor) so the update rate on each parameter had dropped to around 1.3 seconds, but the information is still useful, I think.

The six parameters I monitored are: Throttle Position (TP), Engine Load (EL), Short Term Fuel Trim (STFT), Long Term Fuel Trim (LTFT), engine RPM, and vehicle Velocity.
It was a long graph which displays nicely on excel, here I’m posting it broken into 9 snapshots.

The abnormal behavior I can see is:
a) How the STFT does this rhythmic ramping up and down between 0 and +25% at idle. It’s as if it’s seeking something but I don’t understand what. The mixture is very lean during idle (I verified by reading the O2 sensor) but LTFT does not seem to make an attempt to adjust to the fuel deficit. Not sure what the ECM is doing in this mode.
b) How the mixture turns lean and STFT trends towards +25% on deceleration. But perhaps that is normal?
c) How at the end of my drive, arriving home, I waited about 9 minutes addling in front of my house, at which point, a little over 42’ since originally starting the engine, the MAFS signal goes nuts, the idle gets quite rough, combustion turns rich etc. You cannot see the MAFS in the graphs (again 6 parameters is all it lets me log simultaneously and I had already looked at the MAFS so I was more interested in what was happening with the other parameters). As a reminder, on another drive it took about 32 minutes for the MAFS to go nuts. This last run was in the evening, when its cooler so seems like it took longer, consistent with the heat sensitive MAFS theory.

In any case, these graphs seem very useful. And all for just $24 total !! (~$20 for the OBD2 dongle and $4 for the app). Seems like anyone can do it now.