We have a 2000 BMW 323i, with 180,000 miles on it. I’ve been doing a lot of work to get it running properly, including new plugs, and replacing a fuel injector and cleaning the rest. It’s no longer throwing any codes, but when I look at the fuel trim, it’s running very negative (ranging from -12 to -25, depending on RPM) on Bank 1 (cylinders 1-3). The other bank is much better, running between -3 to -6). This would indicate that it’s running fuel rich, but the O2 readings before and after the catalytic converters (one for each bank) are looking normal, and indicating that the cats are in good shape (i.e., flat output on the outlets, indicating that they’re eating the stuff they’re supposed to).

If it was both banks, I’d suspect a fuel overpressure or something, or maybe clogged intake, but it has a new fuel pump and air filter, and in any event, those would cause problems in both banks, not just the one, since they’re common to both. Any ideas?

[Saturday-afternoon update]

Welp, I hope the problem was a bad oxygen sensor on Bank 1, because I broke the wires on it when I pulled it…

[March 31st update]

Well, I replaced the oxygen sensor that I broke, and it’s showing the same thing. -18% at idle. So it wasn’t a sensor issue.

Hard to know what the problem could be, since the two banks differ only on the exhaust side, not the intake. All I can think is a bad fuel injector on one of the three cylinders, but only way to test that would be to swap them out, and it wouldn’t narrow it down to a single one, unless I did them one at a time, which would be a royal PITA, because it’s just as much work to change one as it is to change all of them, given that I’d have to put it back together each time to test it. But maybe I could start with the old ones, and leave the new one (on #1) for last.

[Bumped]

[Friday-afternoon update]

A new weirdness: I was seeing a misfire on cylinders 5 and 6 (bank 2). When I pulled the coils, they came out wet. There was a bunch of water in the spark-plug wells, though 1-4 were dry. I stuffed paper towels down into them, and soaked it up. No idea how it got there. But the car’s running pretty well now. I’ve giving up on the fuel-trim issue for now, though maybe I’ll take it over to my local Bimmer mechanic and ask him what he thinks.