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LTFT and Fuel Wash

New here with a question: can negative fuel trim cause fuel wash? If negative fuel trim means LESS fuel going into the cylinder, how would that cause fuel wash? My friend argues that if there is a vacuum leak, the computer will adjust to add less fuel . . . and exhaust O2 sensors will read low fuel input and then direct an INCREASE in fuel to compensate, causing fuel wash. I don’t know more than what I read and watch that seems unusual to have the computer issuing contradictory commands (he’s trying to explain fuel wash he sees in a tear down).

No so your next question is moot.

Your friend is wrong. The O2 sensor will read a lean condition and add MORE fuel and throw a code because your MAF reading won’t make sense if it is a big enough leak.

Fuel wash is caused by a no-spark or way too rich condition. The fuel goes IN, the plug doesn’t fire, the fuel washes the lubrication off the rings. Bad things happen.

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In closed loop, that is when the engine is warmed up and operating normally, the O2 sensors will send signals to the computer to fine tune the exact amount of fuel needed to attain the ideal air/fuel ratio of 14.7/1.

The computer has a profile of fuel management stored that came from design and testing at the manufacturer. Of the actual fuel profile differs from the stored profile by a given percentage (around 20% IIRC) then a DTC will be set and the check engine light will come on. A small difference is to be expected and will change as the engine ages.

You will not get fuel wash down unless you start seeing misfire codes. If you don’t have a check engine light, do not be alarmed by a small negative or positive LTFT.