'01 540i fuel trim problems

[RockyBalboa-UK]

Hi all,

I'm new here and was wondering if you can help.

I have been experience lumpy idle when cold and cutting out when first started. I've re-tightened the inlet manifold which has helped a lot (no more cutting out). I've checked for vacuum leaks around the intake; O2 sensors are less than 1 year old, maf sensor is new.

I've plugged in a hand-held OBD-II reader to to monitor short and long term fuel trims and get the following:

Took the car for a long run (80+ miles and got):

STFT Bank 1 - 000% -> perfect
STFT Bank 3 - 008% -> bit high, see LTFT

LTFT Bank 1 - 003% -> perfect!
LTFT Bank 3 - 012% -> worrying

STFT Bank 2 - 008% -> bit high, but LTFT is ok!
STFT Bank 4 - 010% -> very worrying, see LTFT

LTFT Bank 2 - 000% -> perfect!
LTFT Bank 4 - 010% -> worrying.

Then took the car, next day, for a short run (3 miles):

STFT Bank 1 - 003% -> not bad but increasing, see LTFT
STFT Bank 3 - 010% -> bit high, see LTFT

LTFT Bank 1 - 007% -> increasing
LTFT Bank 3 - 017% -> worrying

STFT Bank 2 - 000% -> perfect
STFT Bank 4 - 007% -> very worrying, see LTFT

LTFT Bank 2 - 004% -> not bad
LTFT Bank 4 - 015% -> worrying.

I've also discovered that Bank 1, Sensor 1 and Bank 2, Sensor 1 are PRE-CAT. Sensors 2 on both banks are POST-CAT.

So problems on bank 3 and 4, wherever they are.. but I think it basically means pre-O2 sensor adjustment it is running lean (too much air).. what do you reckon?

Thanks.

[marti5]

I had this on my 7 which has the same engine as yours.

1. I changed the PCV valve at the rear of the engine

2. I had oil around the spark plugs, cleaned all this out, 2 new HT stubs, these engines all have this problem due to the slight Kant of the engine, the oil comes from the rocker cover gaskets, but mainly from the washers that hold the covers on.

Its been fine now for around 2 1/2 years.

[RockyBalboa-UK]

Hi

I've changed the crank-case vent-valve at the back of the manifold 2 years ago.

I've also change the valve cover gaskets 1 year ago and there appear to be no oil leaks anywhere near the spark plugs.

[marti5]

Is the MAF genuine BMW as the cheap ones are useless, Ive heard loads of people that had the cheap ones having problems with idle etc, then there's the camshaft sensor if this is playing up,idle will get bad, then there's the crankshaft sensor, then worst case one of your cats is blocked or getting that way, a full diagnostic would show all.

[RockyBalboa-UK]

Hi

MAF sensor is original BOSCH, exactly the same part number.

Full diagnostics have been done and the cam/crank or any other sensors are not showing errors

I am thinking that an air leak is hte most likely cause.

[marti5]

Check the boot with the Maf in they are prone to crack in the ribs.

[RockyBalboa-UK]

Checked that.. all okay.

Going to strip and clean the throttle body and replace the gasket over the weekend. If that fails, I will strip and reseal the inlet manifold.

[marti5]

Yea I would go along with that, good luck, let me know how it goes for future reference.