-20% ST Fuel Trims

[CupraTOC]

Morning all,

So I have been doing some live data logging with my OBDeleven and have found that short term fuel trim stays at a steady -20% when the engine is started from cold and the car is stationary at idle.
As soon as I start to drive, this increases to -8 to -10 ish and once the engine starts to warm up, it sits around 0 give or take 1% at both idle and when driving.
Long term trims are pretty much a constant 0 give or take 1% at both idle and when driving.

Do I have a leak or is it to do with the extra fuelling supply when the engine is cold?

 

[Speed-FReek]

I don't claim to be an expert on fuelling so happy for other people to chip in and say otherwise, but from what you have described, it doesn't look to me like you have any fuelling issues.

When the engine is started from cold it will be in Open Loop mode so fuelling will be determined by preset ECU parameters. It only goes into Closed Loop mode once the ECU determines that the engine is hot enough to do so. That is when it then starts using all the sensors to determine the correct AF ratio to provide the engine with.

The fact that your STFT comes back to pretty much zero as soon as the engine has warmed up means that everything is fine from a fuelling perspective, and the fact that the LTFT which is an average over a longer period of time is also pretty much zero as well points to everything being OK from a fuelling perspective. It's when it goes below -10% or above +10% on an engine that is up to full operating temperature that it then warrants further investigation.

 

[Dan FR]

I don't claim to be an expert on fuelling so happy for other people to chip in and say otherwise, but from what you have described, it doesn't look to me like you have any fuelling issues.

When the engine is started from cold it will be in Open Loop mode so fuelling will be determined by preset ECU parameters. It only goes into Closed Loop mode once the ECU determines that the engine is hot enough to do so. That is when it then starts using all the sensors to determine the correct AF ratio to provide the engine with.

The fact that your STFT comes back to pretty much zero as soon as the engine has warmed up means that everything is fine from a fuelling perspective, and the fact that the LTFT which is an average over a longer period of time is also pretty much zero as well points to everything being OK from a fuelling perspective. It's when it goes below -10% or above +10% on an engine that is up to full operating temperature that it then warrants further investigation.

What he said. 'Tis fine

 

[CupraTOC]

Thanks chaps