P0172 problems



Hello. I have a 2000 Malibu with a 3100 V6 and I have a problem with the P0172 code. I just replaced the ICM and coils with Delphi and the code went away but long term fuel trim is at -29% still. Could it have to relearn itself again or is there a bad sensor or bad fuel pressure regulator?


Were you experiencing engine performance problems, misfires etc?


Is this by chance a flexible fuel vehicle?


It was misfiring at highway speeds but since I replaced the ICM & Coils, P0172 went away but the LTFT is staying at a flat -29.7% no fluctuating anymore like the MAF sensor or pressure regulator is faulty. Before new Delphi injectors were installed, it had done this surging and has just now gotten worse. I unplugged the MAF sensor and the LTFT went from -29.7% to -13% with vacuum line off pressure regulator also. Those 2 parts could be the problem aye?


The fuel pressure regulator has probably deteriorated inside but not enough to make it spew gas on top. MAF sensor is 18 years old so it’s probabprobably shot.


The fact that the LTFT is steady is a good sign. If anything is bad, it would be the O2 sensor on that side. The PCM was programed at the factory with expected or predicted values of needed fuel for various driving conditions. The O2 sensors actually set the amount of fuel needed to maintain optimum F/A ratio.

When long term fuel trim varies by a certain percentage of the predicted stored values for a given length of time, the appropriate code (P0171-174) is set. This variation can come from aging and wear of the vehicle to replaced parts that may be slightly different from the originals. But as long as things hold steady, I would not be concerned. Be concerned when there is a lot of variation in LTFT over a short time.


I get this mixed up sometimes, but I think negative fuel trims mean the computer - based on the o2 sensor – is having to reduce the amount of fuel injected by 30% compared with what thinks the engine should need given the maf , throttle postion, coolant temp, and ambient temperature sensor readings. In other words the o2 sensor is disagreeing with the other sensors, and the computer can’t figure out why.

I’m assuming you didn’t replace the ignition components to address negative fuel trim problem, b/c ignition problems would usually result in positive fuel trims; i.e. if the gas isn’t burned properly the o2 sensor will see extra o2 in the exhaust stream and increase the amount of fuel injected so that excess o2 gets burned, resulting in positive fuel trim.

Negative fuel trims can result from the fuel pressure being too high, faulty injectors, or the wrong spec for the injector. The engine computer assumes a known amount of fuel will be injected for a 1 msec injection pulse. If the fuel pressure is too high, then too much gas will be injected. Or if the fuel injector is rated with the wrong pounds of fuel per hour spec for the engine, same thing. Likewise, if the maf reading is higher than actual air flow, the engine computer would think it needs to inject a corresponding amount of extra gas, which the o2 sensor would complain about, and that would result in a negative fuel trim.

So a fuel pressure measurement is probably in order. If that’s ok, a maf cleaning would make sense. Still got the same problem? At that point you’re probably better off to take it to a shop that has the Chevy scan tool. they can check the calibration of the maf and fuel pressure regulators and double check you have the correct part number for the injectors.