Trouble diagnosing random misfire - Nissan Forum

 
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post #1 of 3 Old Mar 13th, 2017, 04:18 PM Thread Starter
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Trouble diagnosing random misfire

Hi all. I have been working on a random misfire problem in a 1999 Pathfinder 4X4 for a while now and am hoping for some advice on where to go next. Here is what I've done so far:

1. Replaced all plugs and wires.
2. replaced distributor cap and rotor.
3. Replaced right side catalytic converters (They were falling apart)
4. Replaced right side downstream O2 sensor
5. Replaced cracked intake manifold hose
6. Cleaned egr valve and replaced gasket
7. Checked egr valve solenoid
8. Replaced air filter
9. Cleaned out MAF sensor
10. Tested fuel pump pressure
11. Replaced fuel filter
12. Checked for vacuum leaks
13. Ran torque diagnostics :
Vacuum - 20
Intake pressure - 4.5 - 5
MAF - 7.5 - 8
Timing advance - 10 - 15
Downstream 02 voltages - 0.1 - 0.3
Upstream 02 voltages are all over the place and are frequently high up to .8 or .9

It seems like the 02 readings indicate running rich but I'm not sure if that's just a symptom of the random misfires or not. The misfires don't start right away but start kicking in after a few minutes when the engine warms up. Anyone have ideas on where to go next?

Thanks.

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post #2 of 3 Old Mar 15th, 2017, 08:43 PM
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Upstream 02 sensor voltages should be all over the place if testing it with a meter. They really need to be graphed to get a proper read on them. They should cycle withing a range of .01v to possible as high as 1.1v. Readings that are high in the range indicate a rich condition. There are several things you could look at for this. First, make sure the thermostat is working so that the engine coolant is getting to proper operating temperature; if your coolant gauge runs on the lower end of the range all of the time, this is a good sign that the thermostat is stuck open. The coolant gauge should normally read very close to the middle of the range. Make sure the fuel pressure regulator is working. Regulated fuel pressure should be around 34 PSI. If the fuel pressure is too high when running, check the vacuum hose and source to the regulator at the back side of the engine. If those are good, replace the regulator. Make sure the EGI grounds are clean and tight; these are the two, 10mm head bolts on the intake plenum that secure several black wires which have eyelet connectors on the end. Remove the bolts and clean the intake contact area and eyelet connectors with light sandpaper or a Scotch pad and reinstall. Make sure you go with the original type, NGK spark plugs; despite what the parts guy wants to say, these are the best plugs for Asian engines...and most others, as well. Avoid at all cost: Splitfires, Bosch Platinums and Champions. If those are all good and looking at what you checked, the next thing I would do is replace the upstream oxygen sensors. The best oxygen sensors to use are NTK or genuine Nissan (which are made by NTK). Denso's seem to work okay, in my experience, but avoid Bosch and other aftermarket sensors. FYI, the downstream oxygens sensors are only used for catalyst monitoring and have no affect on engine management.
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post #3 of 3 Old Mar 18th, 2017, 07:22 PM
NCV
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Vacuum - 20
Intake pressure - 4.5 - 5
MAF - 7.5 - 8
Timing advance - 10 - 15
Downstream 02 voltages - 0.1 - 0.3

WITH THESE READINGS ITS SEEMS LIKE YOU ARE AT IDLE AND OPERATING TEMPERATURE.CORRECT ME IF IM WRONG . CLEANING MAF SENSORS IS NOT A FAILPROOF METHOD. YOUR MAF g⋅m/sec SEEM A BIT HIGH. 7.5 TO 8 IS NOT IDEAL AT IDLE. 7.5 T0 8 IS JUST DUMPING FUEL AT IDLE. I WOULD REPLACE MAF AND ADD ANOTHER GROUND TO THE MAF. THESE ARE NOTORIOUS ON BAD GROUNDS
Mass Air Flow
At Idle 3.3 - 4.8 g⋅m/sec
2500 rpm 12 -14.9 g⋅m/sec
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