Quote:
Originally Posted by bownz4
This maybe completely wrong but I read this on a forum and wanted to know if it was right. Someone was saying that over time your truck will "adjust" itself to compensate for mods. For instance, when you put a CAI system on your truck supposedly it runs fairly lean. What I read was that over time your truck will actually adjust itself to where it will run less lean and try to correct itself.
I have had CAI on my truck for a couple months now and have been waiting on a custom tune, if this is true then my truck is running at a different mixture than a stock truck, and if so when I do get my custom tune should I reset my pcm to what it originally was.
I know that's a long question and I may just be splitting hairs, but I was just curious. - Thanks!
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What you heard is correct, up to a point. The PCM is constantly altering the Short Term Fuel Trim (STFT) above and below the ideal point to try to
average it at the ideal A/F mixture. But, it may not be able to achieve this, so there is a Long Term Fuel Trim (LTFT) "bias" that is applied to the STFT to "move" it where it belongs. The LTFT is stored in the PCM, the STFT is simply dependent on voltages from the O2 sensors and is not stored.
Where we get into trouble, I believe, is when the bias requirements are moved above or below some pre-set limits. And, some of the aftermarket CAIs seem to cause this condition. The MAF sensor no longer "sees" the actual amount of air being delivered into the engine. The engine runs lean, and the O2 sensors report it. The LTFT tries to adjust, but hits a limit. The engine stays lean.
Bill can adjust the fuel flow in a custom tune to compensate for the bad input from the MAF sensor. The engine no longer is in an "extreme" lean condition, and the LTFT takes care of the rest.
So, the PCM partially corrects the problem, but it needs help.
- Jack