Members | Sign In
ScannerDanner Forum > Post your repair questions here
avatar

OEM vs Global Fuel Trims

posted May 06, 2016 16:36:37 by Matt Harradine
I'm looking at the values on a 99 bmw 540 and the additive mixture (idle) is 1-2% on both banks and the multiplicative mixture (slight throttle) is 11-13% on both banks. I am pretty sure these are bmws fuel trims.

In global obd2, I get 13-15% ltft and around 0% stft.

The ltft only gets better with a heavy load, not rpm dependant and doesn't respond like a typical vacuum leak.

I would think the the idle mixture would be high with a vacuum leak too.

Could the difference in numbers have something to do with it being in open loop at idle and decel?

Imbgoing to test the fpr tonight to see if it's stuck open.
page   1
5 replies
avatar
Dylan said May 07, 2016 10:46:13
Could the difference in numbers have something to do with it being in open loop at idle and decel?


Trying to follow here. You should always check your trims in closed loop at idle. Not open loop.

Belgium, Europe
avatar
Matt Harradine said May 07, 2016 11:30:39
Yes, but at idle, decel, and wot, this system appears to use open loop tables.

Fpr test showed that it is working. Pressure drops when vacuum is applied.

To recap, fuel trims in generic mode are high and holding a high rpm in park do not improve them and neither does normal driving. Deep pedal stabs will improve them slightly in park and hand fulls of throttle while driving will too. Load seems to be the key here.

Could this still be a vacuum leak? I've just never seen a leak need so much load to change trims.
avatar
Dylan said May 07, 2016 12:29:49
Thanks Matt. Open loop at decel and WOT are normal. Not familiar with the open loop status at idle.

I would think the the idle mixture would be high with a vacuum leak too.


Normally yes.

Most cases trims will get better at higher rpm. But as you mention with not enough load they don't.
13 - 15 % isn't that high, considering the '10% rule'. But still. How do your 02's look?

The ltft only gets better with a heavy load, not rpm dependant and doesn't respond like a typical vacuum leak.


How much better do they get?
Belgium, Europe
avatar
Dylan said May 07, 2016 12:38:43
Have you checked for vacuum leaks?
Belgium, Europe
avatar
Matt Harradine said May 07, 2016 13:22:01
I only think the open loop idle is happening because the o2 trace totally drops to 0 at idle.

The o2 sensors failed the 100ms switching test so I just replaced them last week. They were oscillating, but not very fast.

They both drop about 3-4%

I couldn't find any leaks with propane or water. Planning on picking up a smoke tool soon though.

Fuel pressure is a few psi low is pretty much the only other data I have right now.
Login below to reply: