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Fuel Trims
Posted to Technical Theory Forum on 3/16/2014 21 Replies

What are Fuel Trims ? Fuel trims are a command from the engine computer to modify injection duration which in turn will control fuel/air mixture so the cataylic converter can correctly operate and reduce the majour pollutants. The fuel/air mixture must have small mixture swings from lean to rich for this to happen, lean = reduction in NOx. rich = reduction in HC & CO. The engine computer will look at load sensor signals like air flow meter, throttle position, engine temperature, intake temperature, crankshaft speed, gear selection, A/C demand, power steering pressure e.t.c. The engine computer will calculate all these demands into a single engine load percentage, and will adjust injection duration to meet these demands. Once understood correctly reading fuel trims can save time and give accreate direction for diagnosis, confirm repairs and they are easily accessible its a manditory potocol for OBD systems so the simplist scan tool can read them.

Short Term Fuel Trim "STFT" also called "Integrator" The short term fuel trim is the immediate response from the engine computer to try and keep the O2 sensor switching sightly from rich to lean for cataylic effiency. Only used in closed loop conditions Posative number +0% = rich command (computer increasing injection duration) for a lean condition. Negative number -0% = lean command (computer decreasing injection duration) for a rich condition +/-10% is factory settings

Long Term Fuel Trims "LTFT" also called "adaption valve" and "block learn" After a pre programmed time of the STFT being at set valve/swing the LTFT will adjust to try and bring the STFT as close to +/- 10% as possible LTFT is learned of STFT and is stored in the engine computer "KAM" (Keep Alive Memory). It is used first to know where to start the injection duration. Can be used in both open and closed loop Posative number +0% = rich command (computer increasing injection duration) for a lean condition. Negative number -0% = lean command (computer decreasing injection duration) for a rich condition +/-10% is factory settings

Fuel Trim Operation The engine computer watches the O2 sensor signal voltage and modifys the fuel trims to accomodate the engine load demands. For example a air leak at idle with no loads will cause high oxygen content in the exhaust the O2 sensor will read lean (below 450mv) the engine computer will then adjust he STFT to a postive percentage lets say + 15% (increasing injection duration) to get the O2 sensor swinging from rich to lean again for cataylic effiecny. If after a pre programmed period of time this adjustment in injection duration does not improve the O2 sensor signal voltage the engine computer will increase the LTFT to lets say +15% and the short term will start at its base valve which is 0%. This is because the STFT has only a set swing in the LTFT's percentage. So a 1%LTFT the STFT can only move +/- 30%. So back to our air leak. If after a period of time with the LTFT being at +15% and the STFT has moved to its maximum swing in that LTFT valve lets say +30%, the engine computer will again increase the LTFT to lets say +30% the STFT will start at base valve of 0% in this LTFT % valve. The engine computer contionues to watch the O2 sensor signal voltage and if it still isnt liking what is sees it will again increase STFT to lets say +30%. So now the LTFT is at +30% and the STFT is at +30%. So what is the maximum % a fuel trim can limit too? The maximum fuel trim % is calculated of the physical volume of the catalytic converter. Some vehicle will "max out" or "hit there limit" at 30%some vehcile can go as high as 60%. Every engine rpm and caluclated load has a learned fuel trim is also stored in the engine computers KAM. In open loop condition the engine computer will use its pre programmed or pre mapped fuel trims in these blocks. Some vehicle will use the LTFT in these blocks in open loop status.

How to check fuel trims To correctly check fuel trims you must add the STFT and LTFT together for example a STFT 10%, LTFT 12% is a total fuel trim of 22%, a STFT of -6% LTFT 5% is a total fuel trim of -1%. fuel trims must be checked under all rpm conditions. at idle note the fuel trims, increase to 1500rpm, note the fuel trims, increase to 2500rpm note the fuel trims, e.t.c. Think about the system you are working on and how this can effect your fuel trims, for example if you have a V6 holden commodore and you have rich fuel trims on one bank its is not going to be a air flow meter or low fuel pressure as this would effect both banks. Be a thinking technican not a parts changer.

Common causes of a lean condition low fuel pressure air leaks clogged fuel injectors or injectors not fireing (lean misfire) contaminated fuel IAT sensor problems MAP/Baro or Airflow meter (most commonly hot wire types) Jumped timing belt or chain Exhaust leaks upstream of the O2 sensor (false lean) misfire (extra oxygen entering the exhaust system causing a false lean) Air pump problems (false lean) Oxygen sensor faults (false lean) Jumped timing belt or chain

Common causes of a rich condition Higher than normal fuel pressure leaking/dripping fuel injector EVAP problems ECT & IAT sensor problems (ECT most common on VW & Audi) MAP/BARO & air mass sensor problems (most common MAP/Baro) EGR valve suck open at idle Fuel contaminated oil Head gasket leaking causeing contamination of O2 sensor

Weak Ignition Systems A weak Ignition system can be identifyed on fuel trims, at idle the ignition system is able to ignite fuel air mixture fully and react with the aviable oxygen, under loaded conditions more fuel/air entres the combustition chamber, a weak ignition system can only partially ignite the mixture which will result in unburnt fuel and oxygen entreing the exhuast sytem. The engine compunter will increase injection duration in response to the excessive oxygen in the exhaust system.

Vacuum leaks on MAF engines Vacuum leaks on MAF engines can be easily identifeid as they will have posative fuel trims at idle which get better as rpm increases. Unfortunatly because of the nature a MAP or speed density engine the computer can account of the additional air and the fuel trims will reamin normal.

Michael from New Zealand

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