silenceikillyou

My car will sometimes run rich. I thought it was a vacuum problem, and have been trying to find it, but can't find any leaks anywhere. It messed with me because it was only bank 2.

Well I datalogged it today and noticed something strange. The STFT for bank 2 would go down to 0.75 while cruising, all while the LTFT for bank 2 would stay stuck at 1.25. So the two fuel trims on the same bank are completely opposite while cruising. Needless to say, I'm completely confused right now. I'm starting to think it's not a vacuum leak, but something a little more serious. Anybody have any ideas?

Nuke

Are you running a stock or custom tune? It could also be a sensor problem but I'm not savvy enough in that area to have an idea.

silenceikillyou

It's a custom dyno tune. It wasn't a problem for the past 2 years. It just started happening recently.

jpplaw

The two fuel trims should be sort of the "opposite" of each other. The LTFT is the average trim. 1.00 is perfect for each, so if your STFT is .75, your O2 is commanding .25 less fuel, so the PCM takes it away. Hence the LTFT of 1.25.

The real problem is why it says it is running .25 too much fuel. Like Nuke said it could be the O2 sensor itself, or an exhaust leak, air leak after MAF, Air filter gone funky, vacuum leak or even plugs. Your signature doesn't say if you're running a blower, in which case, I would also check the bypass and make sure its working good.

Do you have a WB O2 or vacuum gauge? It would make diagnosing a lot easier. With the WB you could see what the AFR is during open loop, when the O2's aren't kicking in yet. A vacuum or air leak would show a lean AFR on the WB in open loop.

silenceikillyou

I don't have a blower or wideband, but it's not an air leak or vacuum leak. In open loop, throttle response, power, and fuel economy are much better. But as soon as it switches to closed loop, it's more sluggish, power drops, and I'll go from ~18-20 mpg at 65 mph down to ~13-15 mpg at 65 mph.

It went into limp mode last week. I pulled the codes and it had a bank 2 O2 sensor stuck lean code. I decided to change my O2 sensors.

The O2 sensor for bank 1 looked fine, but the O2 sensor for bank 2 was completely black with carbon. Looks like there's a LOT of fuel not getting burned in bank 2. I did several vacuum leak tests using the carb cleaner method and the propane method, but couldn't find anything.

When I run a regular tank of 93, it reads lean, but runs rich, and I'll get some sputtering and random "misfires" on one cylinder (I have a 2002 5.4 F-150. I can tell when it's just one cylinder, or multiple ones from the COP issues I had with it). If I add Marvel Mystery Oil to a tank of 93, it doesn't sputter or miss, and the bank 2 LTFT will actually show changes instead of staying pegged, but still stays high and reaches 1.25 often.

Im starting to think it's an injector thats either very dirty or going bad on one of my cylinders, causing that cylinder to run lean, which tells the car it's running lean. I wish the car had an O2 sensor for each exhaust port instead of just one for the entire bank. That would make this a lot easier.