LTFT not changing for over a year....?

[Truman325i]

I've been doing some work to get a newly acquired e36 m3 with twinscrew supercharger all sorted out and inspected all over. One area of concern is that the LTFT when monitored have not changed over several drives. And in fact, they haven't changed since the previous owner posted the same numbers a while back when he was chasing a CEL. I have even changed air filter brands/size/flow rate which I would have expected to change the LTFT... So two questions:

Is a LTFT of -8.6 that bad?

and why wouldn't it change and adjust?

[tjm3]

Is there any action in the STFT? Look for wild variations in the short term trim going positive OR negative and get the numbers. Can't make a call on your questions without more info.

[Truman325i]

A lot of action on STFT. Idle sits around -1.6 to -4.3. Under throttle we are around +3. I have seen in some cases it go to -14.

It just seems odd, I have never seen the LTFT move and looking at old threads from previous owner, we see the exact same numbers.

[tjm3]

Taking a guess, but with the higher flow injectors needed for the blower I'm thinking the tuner may have found a way to compensate for the injectors by flashing the ECU to adjust the fueling across the board and 'fixing' LTFT at -8.6, a number that would cut the amount of fuel injected by 8.6% overall and allowing the STFT to make the adjustments. USUALLY, a fuel trim that hits +/- 15% a few times would trigger a CEL and DTC. If you're that concered about it, try one of those cheap recording OBD2 interfaces that log all the info into a spreadsheet and look for changes in the LTFT over time. On the other hand, I don't know what limits on LTFT BMW uses and it's possible that the LTFT is continuously pegged at its limit.

[Truman325i]

Did some logging and didn't see any changes in LTFT, but I did note my 02 sensor voltage is taking sometimes 1-3 seconds to switch from low to high.

[marvinstockman]

What physical engine problem are you trying to solve? Does it run crappy?

[Truman325i]

Runs a bit rough upon initial start up/idle. Almost feels like a misfire, but not severe enough to throw any codes currently.

[StarvinMarvin]

Are there any O2 simulators? Did you smoke test for any vacuum or boost leaks? When was the last time O2 sensors were replaced? When were the injectors last cleaned / replaced? Who tuned the car?

I think the comment about the LTFT being set to a fixed position is accurate. And that would be a bandaid fix for something that couldn't be resolved. If it's set at a negative and puling fuel, then the car was running rich to begin with. Maybe injectors weren't sized property or are leaking?

Assuming this is the Eurosport blower. Maybe give them (or the SC manufacturer) a call to see what they think? I'd want to start with the base tune for the blower system, then trouble shoot from there.