2001 Mercedes Benz ML320 Long term fuel trim high %

  • 2001 MERCEDES BENZ ML320
  • 3.2L
  • V6
  • 4WD
  • 165,000 MILES

I bought this used vehicle. The brake booster was leaking vacuum & LT fuel trim was at 34%. ST fuel trim was around 1-2%. Ignition timing was showing about 35% at idle speed. I changed the booster & engine is now running good. However, the mil is on and LT fuel trim is still at 34%. Ign timing at idle is around 18%. I had assumed fuel trim will readjust to the normal 0% baseline. What do I need to do to get everything running normal?

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have the same problem?
Friday, October 9th, 2015 AT 9:59 PM

1 Reply


Fuel trim is a two part adaptive strategy that adjusts fuel injector pulse width (open time) to adapt to wear, defects, or changes in the system. The two parts of a computers fuel trim adaptive strategy are short term fuel trim STFT (additive) and long term fuel trim LTFT (multiplicative).

The vehicle's operator controls air intake into the engine's cylinders through the accelerator pedal and the PCM controls the amount of fuel according to these and other demands. It tries to maintain a long term air/fuel ratio of 14.7:1. If it sees that the long term ratio is not maintained at an acceptable range for STFT, it adds or subtracts fuel to the mixture to compensate. Injector on time is the result of sensor input and learned adaptive strategies implemented by the PCM. When LTFT exceeds 10%, the PCM is likely compensating for a faulty component, something beyond normal engine wear.

The illustration shows the different sensors used to calculate fuel injector pulse width. The question indicates the importance of oxygen sensor input in calculating injector pulse width. The PCM's fuel trim is based on feedback from the oxygen sensor. The PCM makes minor adjustments to the fuel ratio and checks the results with the O2 sensors input. If the oxygen sensor were stuck frozen with an erroneous and constant output of 800Mv, the injector pulse width and air/fuel ratio would be affected.

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Wednesday, October 14th, 2015 AT 5:39 AM

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