Does anyone know what needs to happen for the ECU to switch it's fuel trim map from cruise to idle? Is it related to vacuum somewhere? Or is it strictly based on what the MAF sensor sees?

I had my battery disconnected for a while the other day, and so its been relearning the fuel trims. I have an Accessport to monitor things, and I've noticed that my car is taking a while to switch between the driving fuel trims and the idle one. When I push in the clutch and put the car in neutral, my STFT goes to zero, the RPM's stumble for a few seconds (had the motor actually stall once), and then the STFT climbs to around +25. This alone tells me I've got a vacuum leak somewhere. My idle AFR's, as reported by the Accessport, are consistently high too, around 16, though cold starting, AFR's are close to 14 and then it gradually rises as the engine gets warm.

Just to clarify, idle is smooth once the STFT compensates, but it's just taking a while for it to kick in. Any knowledge on why that is would be greatly appreciated.

 

The ECU maintains three LTFT's and switches between them based on MAF rate.
0-8 g/s
8-20 g/s
20+ g/s

STFT is based solely on MAF, IAT, and O2 sensor data.

If your fuel trims are high, you have a vacuum leak, bad MAF, or bad front O2 sensor.

 

Was hoping you would field this one Notapreppie. You are the expert with this. I'm just the guy telling people to compression test XD.

 

Ah okay, so a vacuum leak would affect the STFT's which in turn will affect the LTFT's, right? My line of thinking was that the ECU should be able to compensate easily for a vacuum leak, but because mine was taking a while to get back to "normal", I thought there must be something else going on. I'm not going to rule other things out completely yet, but I'll certainly be hunting down a vac leak first.

How many drive cycles does your '8 take to relearn it's normal fuel trims?

As more of an update, without doing anything to it over the weekend, the idle has stabilized more. Still a little rough, but the LTFT for idle has increased from 3 to 9, which doesn't allow the engine to stutter as much. It's been between 14 and 20 dive cycles since the reset. I feel like that's too many, but perfectly understandable with a vacuum leak thrown into the mix.

 

You have it generally correct.
The ECM can compensate for a small leak (this is one of the purposes of fuel trims) but not a big one. Small vacuum leaks will disproportionately impact the idle LTFT more than the 20+ g/s LTFT for two primary reasons: first, intake vacuum pressure drops meaning less air gets sucked in through the leak. Second, a 0.5 g/s leak is huge at 5 g/s flow rate but almost nothing at 100 g/s flow rate.

An LTFT of +/- 8% is NBFD. That could just be variances in MAF, IAT, O2 sensor or injector calibrations (or any combination).

The ECM updating LTFT is less about drive cycles and more about time spent at a given MAF rate. Think of the LTFT as a running average of STFT. Except there are 3 LTFT values so if you just let the engine idle, only the 0-8 g/s LTFT will get update. Of course, if you're just trying to get it to idle, then that's what you need to do.

If your MAF rate at warm idle with A/C off is 4.5 - 5.5 g/s then you're probably doing okay.

 

Thanks NAP, good info.

MAF value flckers between 4 and 5 at warm idle, and I've now got 22% LTFT at idle, with the STFT at idle down to around 10%. But the AFR's are right on target, and idle is completely smooth. So this all points to the original culprit of a vacuum leak. I have a feeling last time I pulled off the UIM, it didn't seat correctly.

 

The ECU changes the fuel trims in order to keep actual AFR close to commanded AFR. As long as fuel trims are below +25%, this will be the case. If it goes over +25%, it should throw a lean code.

But, yah, +22% idle LTFT sounds like a decent sized vacuum leak.