Fuel trims help - Volvo 5 cyl petrol engine

[SebastienClement]

Hi all,

Trying to get my head around this.

I have a 2.0t 5 cylinder Volvo S60 that is getting a recurring PO171 code, and a mild random misfire at idle (I expect the two are related!)

So far I have ruled out fuel supply issues (does exactly the same running on petrol or LPG), MAF is fine, spark plugs & coil packs changed for new OEM items.

I'm trying to understand if it is an air / vac leak. My code reader is showing the following conditions for the fuel trim

STFT - cycles between 0 & +4 at idle
LTFT - constant +25

Does this mean that the fuel trims are in spec (STFT) and the LTFT is being influenced by a historic reading? I'm presuming that LTFT is simply an average of the STFT over time?

Have I got that right?

[Sub8]

Under the appropriate conditions, the short term trim is gradually learnt into the long term trim.

So you are seeing an overall correction of 29% (25% LTFT + 4% STFT). The system is having to add fuel so under-reading air. Or misreading O2 sensor of course.

[Krikkit]

Sounds like a vacuum leak.

[SebastienClement]

So the LTFT is not a longer term average of the STFT?

[SebastienClement]

Aha I've had a lightbulb moment.

LTFT is the amount of fuel that the car knows it needs to add already & STFT is a smaller second by second adjustment?

[Krikkit]

SebastienClement said:

Aha I've had a lightbulb moment.

LTFT is the amount of fuel that the car knows it needs to add already & STFT is a smaller second by second adjustment?

Correct