Wondering if anyone has any insight on this. Had my mechanic hook my car up to the durametric and he noticed the short term fuel trim values were off, but the long term fuel trim values were perfect. Short term numbers were -21 and -37, so he replaced the MAF. the numbers moved but only slightly to -18 and -32. Put another new MAF in there just to be sure the other "new" one was ok and values remained the same. Those numbers apparently haven't reached the threshold to throw a CEL or a code of any kind, but i wouldn't mind working towards clearing them up.

the car doesn't exhibit much for symtoms, not quite as smooth as it should be btw 1000 and 1800 rpms when it's cold, no symptoms once fully warmed up. The car was warmed up when it put out those fuel trim numbers - oil could be checked and was driven for a little while btw readings.

any suggestions much appreciated

thanks

chris

 

You might need another mechanic to diagnose drivability or ECU control. He seems not understanding how this works.

Say: you disconnect battery or erase code and start the car. Ecu will use written value program to inject fuel. Short and Long Term Fuel Trim both start with 0 value.
Suppose your filter is little dirty or you drive in high attitude. Your car use new air value to fuel properly but off little to be rich.
O2 sensors will notice. They run little rich and send value to ECU. ECU use Short Term Fuel Trim (STFT) to compensate it. In this case -, cut little fuel.
After it - for a while, ECU will see this sustain and change it to be Long Term Fuel Trim LTFT (say -1% instead) and set STFT back to 0. If
-1%LTFT is not enough STFT will keep being - again and later ECU says
-2%LTFT. Thing keep going until desired mixture reach (within program limit). If value of LTFT is out range (normally) +,- 10%, it sends Check Engine Light. It adjusts a lot, so something is wrong and it lets you know.

In your case, you need to run for while and see LTFT value. If it is much neg, mix is rich & ecu tries to lean it out. If much positive, mix is lean & ecu tries to add fuel. And troubleshoot from there.

 

+1 to what aroonkl said......i use to suffer from the oxygen sensors fault codes when i had the two snorkel air filter box (same as the 997 turbo), after i put back my stock air filter box with a new air filter and new MAF the problem was still there for the first day then after more driving the car started idling better and no more check engine light and oxygen sensor fault, it was a matter of the ECU adapting to the new AFR's.

 

thanks for the repsponses guys. I'm still trying to understand what's going on. The LTFT numbers if i remember correctly were right about 1.0, mayb 1.02. my understanding is that is definitely acceptable. if i am reading right shouldn't my LTFT numbers be off more is the STFT numbers are off like they are?

i guess i am a little confused

 

-LTFT is the one you should concern, not STFT. (make sure your car runs for a while and it adapts to all change.)
-your LTFT would be off if STFT keep going off (continue + or -). LTFT will adjust step by step to compensate STFT. If finally LTFT adjustment (within limit) would not make STFT value happy, ECU send Check Engine light.

 

With some versions of the motronic software they store "adaptations" that include fuel trims. Those adaptations can be reset back to zero on some Motronic versions. Does the 996tt have these same "adaptations" and if so can they be reset?

 

-Most , may be all, modern ECU (electrical control unit, Motronic is one) have adaptation. It designs to help compensate minor environment and some errors. (Little dirty O2 sensor, Barometer change, dirty filter etc.)
-Most, when you disconnect battery and clear the code, they will set back all fuel trim to start with zero.

 

That explains why after i clear out the fault codes with my Durametric and start driving the car sometimes it switches off when i come to a full stop in a traffic light or intersection it happens like three or four times until the ECU adapts again to the fuel and air ratios.