Fuel Trims



General Notes


  • You can’t exactly add the Lambda Control to Additive or Multiplicative because mathematically it won’t add up exactly.
  • Global OBDII fuel trim values won’t react the same way as other non-European cars.
  • Adaptation Values Explained
    • Additive is LTFT at idle
    • Multiplicative is a LTFT at part load
    • Lambda Control is STFT
  • Multiplicative mixture Adaptation (higher rpm)
    • greater than 8% = lean
    • less than 8% = rich
      • The negative multiplicative fuel trim reading of -4.48% means it is rich and subtracting fuel at part throttle.
  • Additive Mixture Adaptation (idle, lower rpm)
    • Measurements: ms or mg/stroke
    • greater than 0.2ms = LEAN, adding fuel
    • less than -0.2ms = RICH, subtracting fuel
    • The .68 mg/stroke means it is adding fuel at idle.


  • 📝 Some GM platforms will make a fuel trim active based on a “O2 Monitor : Ready” PID.
  • STFT is located in group 033
    • 25% of STFT can be equal to 4% of LTFT when equalized
  • LTFT (Adaptation) is located in group 032
    • Field 1 = Long Term Fuel Trim @ Idle
    • Field 2 = Long Term Fuel Trim @ Partial (under load)


Lambda PID
  • Example. When lambda is 0.85, it means the system is subtracting 15% fuel (to keep the range of correction in stoichiometric combustion)


Lean Condition (too much air)

I. Vacuum leak (on a MAF System)

  • Uneven Airflow Upstream of MAF
    • Wrong/incorrectly mounted intake box, missing/incorrect/incorrectly mounted air filter may create an uneven turbulent airflow that will cause MAF sensor to detect wrong amount of intake air flow, skewing fuel trims.
      • 📎 Rich Fuel Trims | Partially Detached Air Filter | 2006 Toyota Tacoma
        •  It had a universal aftermarket tapered air filter box. MAF sensor was about 10 inches downstream. When filter has partially detached from the main hose creating a small opening, it pushed trims +20%. If I remove the filter completely and use my hand to block the intake opening 50%, car almost stalled. It means that turbulent (uneven) airflow didn’t have a chance to even out when reaching MAF, because MAF was too close to air filter.
  • PCV Valve (stuck open)
  • EVAP Purge Solenoid
  • EGR
    • EGR should not affect fuel trims, but it can cause misfire (density misfire) that will by itself create lean mixture.
    • Density misfire
      • It means that there is same mass of air and fuel entering the chamber but air/fuel mixture has different density due to  introduction of inert gasses that dilute it, causing misfire.
      • On a MAF system nothing will change whatsoever (?)
        • 1999 Lincoln Navigator DOHC 5.4l Vin A. EGR commanded open @ idle by grounding EVR solenoid control wire. Truck stalls immediately. The only way I could get it to stay running without manipulating the throttle was to activate the solenoid during the initial start up high cold idle phase. O2s drop dead lean, STFT goes to +45. There does not appear to be a MAP sensor on this engine, MAF only. There is no EGR position sensor.
      • On a speed density system manifold pressure will increase causing the PCM to see an increased engine load which would increase fuel delivery. HOWEVER there is a table in the PCM that compensates for EGR use before any of the EGR gasses even get into the combustion chamber. As a result there will be little to no response from the O2 sensor. Since EGR gasses are inert the gasses alone will have no effect on the O2 sensor. But if you force an EGR valve to open on a speed density system when it shouldn’t be open the change in manifold pressure will create a rich condition and the O2 sensor will respond appropriately.
  • Brake Booster
    • Leaking brake booster or missing O-ring between master cylinder and brake booster.
    • Smoke tester might hold the pressure well and not be able to show the leaking booster.
      • Pinch the hose while watching the fuel trims to confirm.
  • Variable Intake Runner
    • IMRC ? (Ford, Mazda…)
    • DISA Valve (BMW)
  • Dirty Throttle Plate
  • RPM Test
    • Rev the engine to 1500 to 2000 RPM and hold it steady for half a minute or so.  If the fuel trim numbers drops back down to a more normal reading, it confirms the engine has a vacuum leak at idle. This is because vacuum leaks have less of a leaning effect on the fuel mixture as engine speed and load increase.
  • Dipstick Test
    • Take out a dipstick and check for crankcase vacuum with engine running.
    • Troubleshooting PCV
  • Smoke Test
    • IAC with vent might leak, which can be normal, depending on the design.
    • PCV with vent might leak, which can be normal, depending on the design.


II. Exhaust leak (false lean)

  • With exhaust leak, computer will try to compensate for leaner exhaust and will enrich the air/fuel mixture, but since exhaust air leak is not introduced into combustion chamber, it will make car to run richer which will produce misfires.

III. Fuel delivery problem (MAF & Speed Density MAP Systems)

IV. Ignition misfire (false lean)

IV. Compression

  • Bad exhaust valve that allows unburned oxygen into exhaust and fools O2 sensors

V. Sensor Input

  • MAF sensor
  • O2 sensor
  • Temperature Sensors
    • Cold air at 0C is 14% denser than air at 38C, so ECM needs to enrich the fuel mixture on a cold start  and subtract fuel when hot to achieve the proper lambda.  If ECT or IAC doesn’t show a correct temperature, it might cause ECM to always enrich or lean out fuel mixture.
    • ECT (high authority)
    • IAT (low authority)
      • Used in addition to ECT, or as a backup in case of ECT failure.
      • In very cold weather (-30C), if hot engine (93C) to be stopped and restarted, fuel mixture needs to be richer than ECT would indicate.
    • Ambient Temperature Sensor (lowest authority?)


Rich Condition (too much fuel)

I. Fuel

II. Stuck open EGR valve (false rich)

  • especially at idle

III. Restriction in air flow system

  • Extremely dirty air filter or restrictions in air intake system
  • Exhaust restrictions (clogged converter, crushed exhaust pipe or plugged muffler)
    • At idle, no more than 1.25psi (usually)
    • At 2500 rpm shouldn’t be more than 2-3psi
    • If fuel trims on V type engine go in opposite directions, check for restriction in the exhaust.
      • On a MAF System with two banks, the plugged side reads rich and good side reads lean. If the MAF reads 8 g/s, the PCM thinks 4 g/s are going to each bank, but in reality 2 g/s are going to the plugged side and 6 g/s are going to the open side. The PCM wants to add fuel for 4 g/s to each side. That’s why plugged side will be rich, and good side will be lean.
  • Modified intake can cause less airflow that blows directly on the MAF sensor

IV. Boost Air Leak (turbo models)

  • In the non-turbo vehicles the air leak after the MAF sensor would cause unmetered air to be sucked INTO the engine, due to a negative pressure (vacuum) in the air intake system. Hence it’s called a vaccuum leak. This would create a lean condition (too much air).
  • But, since in turbo vehicles the intake air is at the positive pressure (charged/boosted by a turbo), any crack/gap in the air intake system (after the turbo intake inlet hose) will cause air to get OUT of the system.
  •  This will cause a rich condition, because
    • MAF sensor detects X amount of air coming in,
    • ECM calculates amount of fuel needed to achieve stoichiometric fuel/air ratio of 14.7:1 ,
    • yet the upstream O2 sensor detects that combustion chamber ACTUALLY received less air (X – ‘leaked out air’)… or more fuel.


V. Leaking exhaust???

VI. Bad oil

  • 📎 Rich Fuel Trims | Bad Oil | 2005 GMC Envoy 5.3L
    • Had -20% LTFT and -20% STFT on both banks.
    • Oil was really bad. Oil change fixed the problem.

VII. Bad Thermostat

  • Stuck open thermostat. ECM enriches the fuel mixture on a cold start. If thermostat never closes, engine might not reach its operating temperature and ECM will endlessly make a rich fuel mixture.

VIII. ECM Sensors’ Signals

IX. Tuned ECM

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