Hey guys whenever on a warm day my fuel trims will be just fine, and i have absolutley no knock. Trims will usually stay around .078 and -3.13 at cruise, and I've gotten them locked in around -1.78 at WOT.

Now thats on a warm day. Well i've come to notice that whenever on cold morning im coming home from work, or late nights when im going to work, that my trims are all f**ked up. They are usually stuck around -17 and sometimes even -21 at cruise. So WTF is going on???lol

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1995 R33 RB25 240sx
1998 GTP-Sold
1999 Monte Z34-Sold

Is the car fully warmed up when running the scan? Your car is normally going to dump fuel to help warm the motor.

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1997 TGP

Yes, car is at full operating temp

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1995 R33 RB25 240sx
1998 GTP-Sold
1999 Monte Z34-Sold

My guess is yes, the air temp will affect the fuel trims. As I understand how fuel trims work, that is the only way for the computer to compensate for temperature differences. When air temp changes, so does air density. The computer will have to alter how much fuel is delivered in order to maintain the appropriate air/fuel ratio as the air density changes. Unless it uses the IAT sensor to make changes that don't show up in the LTFT.

On my last car with a carburetor I had to compensate for air temp differences myself. 35 degrees would require a change of one jet size to compensate. So the computer has to do something similar. It will probably read the O2s and notice that they are getting leaner and then richen to compensate until the O2s are normal and that richening will be reflected in the LTFT.

I don't know if the LTFT fluctuations you are seeing are a normal amount or not. It seems like a pretty big difference that you are experiencing though. How much of a temperature difference are we talking about here? At -21 you are almost to the limit of what the PCM can do to alter the fuel delivery. That doesn't sound right.

Sounds like your MAFF sensor or PCM is taking a shit.

The fuel trims will move up and down with the outside temps, but not as drastic as your saying.

The only reason I say this is, once your trims are learned and are locked in by the PCM say at -1.78 the PCM will use those numbers until it relearns or averages a new set.


I'm guessing the changes you're seeing are over a couple of days and not weeks? It takes quite a while a varying speeds for the PCM to learn the trims.

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