What does fuel trim tell me??

[WhyMe]

i have watched the vid on youtube about fuel trim. How do i decipher what its telling me?

 

[Roadkingnc]

Positive fuel trim it is adding fuel to correct a lean condition
negative it is removing fuel to correct a rich contition, short term fuel trim will vary at idle between positive and negative, long term fuel trim is learned by pcm from the short term fuel trim values. If you have a vacuum leak you will have a substancial positive value in the long trim, the opposite would be true for a rich condition.

 

[Samilcar]

I had a fuel pump problem that was finally sniffed out by a consistently high long term fuel trim value. This car would occasionally throw a "system too lean" CEL code. Using a Davis Carchip in this car revealed that the long term fuel trim would max out at 25% whenever this check engine light would come on. Checking the fuel pressure manually (this particular car couldn't report a fuel pressure value through the OBDII system), revealed low fuel pressure (43psi vs a normal 55psi)caused by a slightly leaky fuel pump connection inside the tank.

 

[punisher]

Also, seeing when the trims go lean or rich can be a big help in diagnosing.

If you get rich/negative trims at idle, and lean/positive trims during cruise or accel, you look at the MAF sensor first.

Lean trims at idle that trend towards normal under accel or cruise.....vac leak.

Rich idle trim, trending towards normal under load.....stuck fuel regulator.

Normal idle trim, trending lean under load.....fuel pressure problem (low pressure).

Just generalities, but it sure is a good starting point figuring out drivability concerns.

 

[WhyMe]

Thanks for the info

 

[asand1]

In college My Fiero had short term fuel trim. I wish my shop instructor would have been able to explain it that simply.

 

[Sonataman]

My long trim will vary with what fuel is in it.
At idle
E10 gives 0 to -3
E0 gives -3 to -10

Under load
E10 gives 0
E0 gives -5.5
So it appears E0 has more energy content and so it takes less fuel.