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Jake, Nissan Technician
Category: Nissan
Satisfied Customers: 3748
Experience:  Experienced collision repair technician, Pulaski Tech graduate and DSM hobbyist.
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99 altima 4cyl. my short term fuel trim is running lean 25%

Customer Question

Customer: 99 altima 4cyl. my short term fuel trim is running lean 25% at idle @ 22.4% calc load with long term fuel trim at 0.0 closed loop. Driving at 94.1% calc load, short term fuel trim is -3.9% running rich with long term fuel trim at 5.5%.
JA: Thanks. Can you give me any more details about your issue?
Customer: have a check engine light code p0171.
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Submitted: 1 year ago.
Category: Nissan
Customer: replied 1 year ago.
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Expert:  Jake replied 1 year ago.

Hey there, thanks for the question.

Looks like you have an obvious lean condition.

What have you done so far other than look at fuel corrections? anything at all? have you checked for air leaks? especially by smoking the intake?

Customer: replied 1 year ago.
I could not find a vacuum leak. The car does not run lean while driving at full engine load at high rpm. Only Lean at idle. So far I checked electrical connections for loose or torn wires, sprayed fluid around intake manifold and injectors to see if it changed the idle. Checked plugs, distributor, plug wires, rotor button...all looks good. Getting good data from O2 sensors.
Expert:  Jake replied 1 year ago.

Then your MAF must be out of spec. If o2 readings are good and you have NO AIR LEAKS, but your fuel trims are off that bad then the MAF is likely the culprit.

Customer: replied 1 year ago.
I will clean the MAF and put a meter on it. If that fixes it I will let you know.
Expert:  Jake replied 1 year ago.

Do let me know, but trying to test a MAF with a meter is very difficult.

You can test the sweep OK with a sensitive meter, but that's not even entirely accurate.

If you disconnect the MAF at idle the STFT should always go to 25% unless there is an air leak. Since yours is already at 25% that test is kinda hard and indicates the MAF is likely faulty.

Customer: replied 1 year ago.
Now i got a p0304 with the p0171
Expert:  Jake replied 1 year ago.

That's just another lean condition code and a misfire code being he only thing new here.

if the car is missing (which you didn't mention) then unplug the injector at cylinder 4 and see if there is any change.