Fuel trim

[borodaylandon]

I have a 1997 328is and I recently ran a code scanner and I got back, fuel trim. Does anyone know what it means because, I cant find it in the forums.

[Butters Stoch]

You at the max of adaption limit. +/- fuel.

[Eric93se]

Change both pre-cat oxygen sensors. If you can get INPA on an XP 32-bit laptop and reset adaptations.

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Also find all your vacuum leaks (smoke test it).

[Mless5]

Exactly why I come to these forums, because of smart people like you - thank you for sharing your knowledge!

You at the max of adaption limit. +/- fuel.

Change both pre-cat oxygen sensors. If you can get INPA on an XP 32-bit laptop and reset adaptations.

- - - Updated - - -

Also find all your vacuum leaks (smoke test it).

[JDStrickland]

When telling us that you got something, it's actually better that you tell us precisely what you got as opposed to some nebulous description that may or may not be accurate.

THE WAY IT WORKS
Air enters the intake, passes the air filter and goes to the MAF, where the density and temperature are measured -- air that is cold and dense needs a different amount of fuel than air that is warm and thin. This information is used to set the fuel injector timing, which actually sets the fuel ratio. The desired ratio is 14.7:1 because the mathematical distribution of atoms divides evenly with the free electrons in the CAT to form oxygen, water, and carbon dioxide, instead of the toxins that would otherwise come out of the exhaust pipe.

The O2 Sensors in front of the CATs measure the contents of the exhaust and the computer uses this information to trim the fuel delivery, basically this is a fine tuning step.

You can have a hole after the MAF in the intake track that opens and closes under various conditions and this gives unmeasured air to the engine that can affect the exhaust that the O2 sensors see. This situation would give a lean condition, generally speaking. The O2 Sensors would see lean, the computer would give more gas and the hole closes so there is too much gas, and this goes back and forth until the computer says that the fuel trim is all over the place.

You can also be having an electrical issue with the MAF itself. The air quality is essentially a constant condition, it changes very slowly and is generally a directional change -- the air gradually changes from cold and dense to warm and thin, or vice versa, as the day wears on. It does not go back and forth. The MAF _can_ fail in a manner that the reports of air quality (density and temp) fluctuate back and forth, causing the computer to constantly change the amount of fuel that is delivered. The O2 Sensors see this chaos and respond that the exhaust has too much fuel, then not enough then too much then not enough. Sometimes the O2 sensors can call for more gas, then more, then more, then less and less and less. The idea is that changes in fuel trim that are correct should be very small changes, not repeated changes over and over and then all of a sudden in a different direction.

It helps to give you diagnostic direction if you tell us what the code is that you are getting.

[truthbtold]

Correct

the 02 sensor does not actually measure oxygen concentration, but rather the difference between the amount of oxygen in the exhaust gas and the amount of oxygen in the air. A rich mixture causes voltage buildup and an oxygen demand. Lean mixture causes low voltage, since there is oxygen excess and causes fuel demand. The goal is optimum mixture for fuel efficiency and emissions.The ECU will increase fuel delivery if a lean condition is detected and the opposite for a rich condition. It complicated but actually simple.

When telling us that you got something, it's actually better that you tell us precisely what you got as opposed to some nebulous description that may or may not be accurate.

THE WAY IT WORKS
Air enters the intake, passes the air filter and goes to the MAF, where the density and temperature are measured -- air that is cold and dense needs a different amount of fuel than air that is warm and thin. This information is used to set the fuel injector timing, which actually sets the fuel ratio. The desired ratio is 14.7:1 because the mathematical distribution of atoms divides evenly with the free electrons in the CAT to form oxygen, water, and carbon dioxide, instead of the toxins that would otherwise come out of the exhaust pipe.

The O2 Sensors in front of the CATs measure the contents of the exhaust and the computer uses this information to trim the fuel delivery, basically this is a fine tuning step.

You can have a hole after the MAF in the intake track that opens and closes under various conditions and this gives unmeasured air to the engine that can affect the exhaust that the O2 sensors see. This situation would give a lean condition, generally speaking. The O2 Sensors would see lean, the computer would give more gas and the hole closes so there is too much gas, and this goes back and forth until the computer says that the fuel trim is all over the place.

You can also be having an electrical issue with the MAF itself. The air quality is essentially a constant condition, it changes very slowly and is generally a directional change -- the air gradually changes from cold and dense to warm and thin, or vice versa, as the day wears on. It does not go back and forth. The MAF _can_ fail in a manner that the reports of air quality (density and temp) fluctuate back and forth, causing the computer to constantly change the amount of fuel that is delivered. The O2 Sensors see this chaos and respond that the exhaust has too much fuel, then not enough then too much then not enough. Sometimes the O2 sensors can call for more gas, then more, then more, then less and less and less. The idea is that changes in fuel trim that are correct should be very small changes, not repeated changes over and over and then all of a sudden in a different direction.

It helps to give you diagnostic direction if you tell us what the code is that you are getting.

[Eric93se]

True for the Zirconia sensor but not for Titania (for newer E36's OBDII 328i, M3).

https://en.wikipedia.org/wiki/Oxygen_sensor