We've owned our '03 325iT sport wagon for just under two years and for as long as we have owned it, it has on occasion thrown a P0141 or P0144 (lean bank 1/bank2) code, or occasionally both, and I've never been able to nail them down. Even with no stored codes, these two would frequently be pending. Finally the car started to stumble or have a bit of a flat spot, and it continued to deteriorate, but still gave no code other than these two. So I started chasing down vacuum leaks. It had a couple - a crack in the cam cover and a bad O-ring on one of the ports of the balance manifold, but nothing bad enough to account for the driveability problems. Checked the fuel pressure; it was fine. Replaced the MAF; this seemed to help quite a bit but still it had the occasional stumble or hiccup, and the light would come on with P0141/P0144, although not as frequently.
Yesterday I had a really hard time to get it to start while warm, and it wasn't making enough power to even move the car, so after I had it towed home, I went back to basics and slapped the fuel pressure gauge on it. Zero, zip, zilch, nada. The fuel pump had packed it in.
I slapped in a new pump and the car is behaving itself perfectly. No codes either stored, or pending; perfect performance under all conditions.
I guess I had fooled myself into not suspecting the fuel pump sooner, but every time I put a gauge on it, the pressure was within specification. I got almost 500,000 miles on the fuel pump in my E30, so why would I suspect this pump after a measly 160,000? I cut open the pump this morning and found that the commutator had a groove in it several millimeters deep and the brushes had worn down to about nothing, so I have concrete proof that I've finally fixed the problem.
So here's the message: a lean condition could be either too much air or not enough fuel. If my experience helps just one person avoid having to go through all the grief that I have had with this car, it will have been worth it.
Solid advice, thanks for posting. Testing fuel pressure is not too hard, certainly easier than smoke testing for air leaks for the average DIY'er. Not everyone has access to a proper smoke machine, but fuel pressure can be tested easily with an inexpensive purchase or rental.
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