[mweinst]

I have a 2011 335i and have been trying to find out why my long term fuel trim is 6-7% at idle. It reduces to 2% during cruise, so I suspect a vacuum leak.

Extensive smoke testing reveals no external leaks. However, if I remove the oil filler cap, there is some smoke inside of the valve cover where the cams are. Would this indicate an internal leak in the VC or is it normal to have smoke under there?

If that is normal, what else could cause a higher LTFT at idle? Thanks for any suggestions.

[mweinst]

Not getting a single response. So, either this is a dumb question or nobody knows the answer. Either way, someone let me know.

[Grease23]

I can't answer your second question. Neither question seems dumb t me.

As to the first, I think smoke will or can spread virtually anywhere where there is manifold vacuum when the car is running. That would include inside the valve cover.

[mweinst]

Thanks - I’m still unclear about where the smoke would be getting into the area under the valve cover, though. Does anyone have any more information?

[Sgop335]

are both banks showing 6 to 7%? or may be there is only 1 bank
what fuel is used? its likely e10?

[mweinst]

Yes, there is only one bank on the n55e, and I am using e10 93 octane fuel.

A little more information is that lambda is unstable at idle, varying from 13.8 to 15.0. I’m starting to think that if there is no vacuum leak that I may be looking at needing some fuel injectors. But, I’m not sure how to test them prior to spending for a new set.

[Sgop335]

Any codes? Catless?

[mweinst]

No codes, catless.

[OMEGA412]

I don't think k it's a vacuum leak. Where your cruising it should be getting to much air therefore running with positive fuel trims, under acceleration when u enter boost it would then be too little air causing negative fuel trims. Lean=Positive fuel trims Rich=negative. U can test for vauccum.leaks by pressure testing the intake system with a rented tool from ur locally hardware/autoparts store. Many things can cause lean condition, I saw u mentioned being catless, your o2 sensors could be fouled leading to inaccurate afrs therefore inaccurate fuel trims.

[mweinst]

I actually do have negative fuel trims under boost, and positive trims at idle and steady state cruise. I have smoke tested and pressure tested the entire system and cannot find any leaks.

I suppose the O2 sensors could be fouled, although the rear sensor is almost brand new and the front one is less that 2 years old. I’ve only had the downpipe on for about 6 months.

I have a question, though. Does your rear O2 sensor constantly cycle between rich and lean? My old one (with the stock cat) stayed almost steady, but my new one (with catless downpipe) cycles back and forth, only stabilizing randomly and temporarily. What should it read in numbers?