Different fuel trims per engine bank?
 

Hi Guys -- I got a CEL and it was the random cylinder misfire code, not specific to a single cylinder. It has actually gone on and off a few times over the last several weeks. My car has TPs, post MAF tubes/K&N filters, FI exhaust and a Uprev tune. I haven't driven it hard in several months.

Only real noticeable difference driving is that when the CEL is on the idle is about 300 rpms faster (1000 rpms) then when it is off. Interestingly, when I switched out of the performance tune to the factory tune, the car did studder and seemed to have fuel starvation at around 3000 rpms. I quickly changed it back to performance tune and it drives fine.

Mechanic found that the fuel trims were very different per engine bank. One side is like +18% and the other side is only a few %. He cleaned the filters and the MAFs, checked for vacuum leaks and it still was the same. So he swapped O2 sensors from one bank to the other, thinking it may just be a bad sensor. The fuel trim difference did not change, indicating the O2 sensors are fine. So, we are thinking maybe fuel injectors? Anything else you guys have seen before or can think of?

Thanks!

Get the DTC(s) (probably a P0300; any more?) that are being thrown and a quick web search should point you in the right direction.

Edit: Just go to your favorite search engine and use
DTC <your code>

Wow you are having the identical issue that I just recently had. I got the CEL and the high idle then when I brought it to my local performance shop they found incorrect AFR's between bank 1 and 2. You need to check for leaks at your test pipes. My tuner could not figure out why the car was running like crap until they decided to do a leak test on the exhaust and found the leaks, car runs mint now. Oh and the tuner right away thought it was the O2 sensor as well but it was not. Good luck man.

Well no exhaust leak, no vacuum leaks, still a mystery! Gotta get this sorted out since I have a turbo install coming up soon. Will keep everyone posted so someone else may find this useful in the future as it seems I'm not the only car with these mods and a tune to get this problem when the weather turns cold.

On cold starts my car triggers it, than it will stay away for about 5 or so cold starts than come back on. My tuner said its an easy fix and I think he actually emailed me the fix I just haven't threw it on my car yet via up rev.

Let me try to get your tuners contact info. Maybe my tuner can get some info from him...

Probably a vacuum leak somewhere along the bank with the higher trims.

Check again -- any crack anywhere before or just after the primary O2 can and will cause this problem.

Apparently now the difference in fuel trim can bounce from one side to the other, so I think it has to be some kind of computer gremlin/glitch. They smoke tested the intake and exhaust and only found a small pinhole leak along the welds at the stock exhaust manifold where it mates to the TPs which shouldn't be enough to cause this....

it doesnt take much of an exhaust leak to throw off the o2 sensor... i would start there first.

There;'s usually some difference between each bank and moving the sensor around will move the bias too.

Does the overall magnitude of difference move around randomly? If you clear all the fuel trims and the change from one bank to the next doesn't move with a sensors (either MAF or O2) then it could be an intermittent leak that only shows itself when the pipes are cold -- as they heat up and expand a bit, the leak may "plug" itself.