Merc68k

Hi guys,

I've got a 1999 Altima (100,000 kms) with a miss off idle. If I tap the gas at idle, the revs drop slightly and nothing happens for a split second before the engine responds. I've been trying to figure it out since I got the car almost 2 years ago and I've done the following so far:

I've checked for vacuum leaks and I'm relatively sure there are none.
I've checked the fuel pressure, and it's within spec. The regulator behaves
as described in the service manual when I disconnect the vacuum line.
I tested the MAF sensor and its output voltage rises smoothly with any
increase in throttle opening.
I tested the TPS and it operates smoothly and is within spec.
The coolant temp sensor and intake air temp sensor both are about right
according to the service manual.
It has clean plugs, newish rotor, cap, and wires, and is not burning oil or
coolant.

There are no trouble codes stored, and aside from this problem and having a
slightly high idle, the car runs smoothly and has good power at all other
revs... although there is some hesitation at times at very low revs which I assume is likely related to what's causing the idle hesitation.

The problem manifests itself as a drop in revs/stumble when I tap the
throttle off idle. It's most obvious with small abrupt throttle openings,
the kind you would use to take off from a stop (it's a manual trans).
It's more severe when the car has been started between 5 and 10 minutes of
having been shut of and is still warm. With a shorter or longer stop, it
doesn't tend to be any worse than normal.

I tested the O2 sensor as well as I could with the tools I have; my analog
meter's lowest range is 10V, so it's hard to accurately monitor the sensor
with it, and my digital meter I suspect is a bit slow to show the voltage
changes from the sensor. But, with the digtal meter, the voltage seemed to
settle around .2 volts at idle, and would spike up to around .7 if I revved
the engine. I measured the resistance of pins 1 and 3 (the heater element I
suspect) of the sensor, as per the service manual's instruction, and it
reads almost 8 ohms hot, and 4.6 ohms or so at around 5 degrees C.

The service manual says it should fall between 2.3 and 4.3 ohms at 25 deg.
C, so I assume the idea is the resistance increases with temperature. If
that's the case, then at 25C, I would expect the resistance to be more than
4.6 ohms as I recorded at 5 degrees, so that is somewhat suspect.

I tried the diagnostic mode built into the ECU where the CEL blinks on and off as the sensor cycles between rich and lean, and at 2000 RPM it happens about 10 times in 10 seconds, but at idle, it might happen 1-2 times in 10 seconds.

I priced a new sensor at around $100 CDN today from the dealer, so I'm half
tempted to install one. If it doesn't fix the problem at least it will be
new and might help my fuel economy a little although it isn't poor to begin
with.

Any thoughts/suggestions as to what else I might look at?

gfriedman

O2 sensor looks good if its fluctuating 1 per second at 2K. Don't wory about the idle it's supposed to test at 2k only.

Check air filter
Try some intake and fuel injector cleaner. Alot of guys suggest seafoam but I haven't tried it.

Merc68k

The air and fuel filters are both new. I put in some Lucus upper cylinder lubricant the last time I filled up and the car does feel smoother on acceleration now, but still hesitates at idle. I have probably another 2 tanks left before that stuff's all gone so maybe it will help by the time I've used it all.