Fuel trim

[mackgoo]

I'm trying to trouble shoot mt CEL codes 1123, 1125. I picked up a Davis Car Chip the other day. In the trouble log at the time of the code the chip lists long term and short term fuel trim. Short term is -16% or so and long term is + 16% or so. Does any one have any idea what this is telling me?

Edited November 18, 2004 by mackgoo

[Loren]

Have a look at this thread

[mackgoo]

Thanks, I'm aware of that post and appreciate the reference. I'm really trying to determine if the info the car chip is giving me can help me trouble shoot or not. What exactly is the "short term" and "long term" fuel trim?

[Loren]

The codes mean that you are getting too much gas and the DME can not compensate for it. My info comes from the Porsche ODB II manual.

[mackgoo]

I'm really trying to determine if the info the Davis car chip is giving me can help me trouble shoot or not. What exactly is the "short term" and "long term" fuel trim?

[Loren]

When you see "trim", it means "compensation". As far as I know GM invented this term. Fuel trim in the positive side indicates the computer (DME) is adding fuel and, of course, the negative side indicates computer is subtracting fuel. For example - As the pump pressure starts to decay or the fuel filter begins to clog, the computer will respond by adjusting long-term fuel trim numbers in an attempt to get short-term fuel trim back to zero correction. The computer keeps tables of both short term and long term compensation (or trim).

If you do a Google search on "fuel trim" you will likely come across more technical articles on it - but not likely aimed at Porsche cars.

[mackgoo]

Thanks for the info. I'm going to do some playing around this week end.

[PTEC]

I'm really trying to determine if the info the Davis car chip is giving me can help me trouble shoot or not. What exactly is the "short term" and "long term" fuel trim?

<{POST_SNAPBACK}>

For what its worth, Porsche does not use either of these terms. Fuel trim is a generic term used by mostly domestic car companys and ends up in most generic OBD2 code readers. Were you to look in your DME with a PST2/PIWIS tester you would not find the words long term or short term anywhere.

I dont know this for sure but I believe the generic OBD2 scanners are renaming FRA's and TRA's (Porsches oxygen sensing adaption terms.)

[mackgoo]

Can you explain these some?

[PTEC]

Can you explain these some?

<{POST_SNAPBACK}>

Without getting too in depth.

TRA is your idle range. In this range your your injection time (in ms/rev) is changed (Additively) upon the results of oxygen sensing. Using the PIWIS tester, your TRA numbers can vary from 0 plus or minus .40 before the check engine light will illuminate.

FRA is your load range (to a certain extent, no adaption is performed at high engine load and/or engine speed.) In this range the pre-control injection time is changed (multiplicative) by a percentage based upon the results of oxygen sensing. FRA numbers can deviate plus or minus .25 from 1 before illuminating the check engine light.