[Malteser_wfj]

Hi everyone,

I've been having an issue for a while now where the engine revs will drop to below 500rpm shortly after I've come to a stop in traffic and am in neutral with the clutch engaged.

I've got a log of it here and will try to find and upload a video of the cluster so you can see what happens.

https://datazap.me/u/lemansn55/20240...=2-12-14-15-19

My biggest worry is that oil pressure is dropping down to 28psi momentarily when the engine rpms are at their lowest before rising again.

Initially I thought this was caused by an air leak with my broken charge pipe but that's since been replaced with a metal one and smoke tested with no issues. There aren't any codes either, only that the LTFT shoots up whenever I'm stationary and then goes back down to 5/6 when I take off.


Any suggestions are more than welcome!!

What I've noticed from the log data:

• VANOS intake has a 3-4 degree variance between requested and actual
• AFR goes lean as the revs drop then rise despite STFT increasing
• I'm guessing the timing advance is an attempt to combat the falling rpm below the idle target?

Info about the car: stock engine & tune with the only modification being the charge pipe. 120k miles on 99RON fuel. March 2012 build N55 manual in the UK. No engine codes - used to get Mixture too Lean but that went with the charge pipe.

A full WOT pull log can be seen here to get a larger idea of the engine's health/condition:
https://datazap.me/u/lemansn55/07032...=3-12-18-19-21


Thanks

[wrevilo]

What are LTFT and STFT?

[Malteser_wfj]

Originally Posted by wrevilo

What are LTFT and STFT?

Long Term Fuel Trim & Short Term Fuel Trim.

My understanding of these coming from an E46 M54 is that short term is the immediate reaction/running of the engine from the closed loop lambda control & long term is the learning of the ECU over the course of the engine to adapt and account for any changes to the engine's running.

Say if you had a small air leak for a while the LTFT will increase to make up for this, or a leaky injector would cause the LTFT to decrease or be in the negative.


This may not be accurate for a direct injection engine though as it also seems like the LTFT's shoot up to the 20's when the engine burbles (I get decent burbles as I've got a factory M Performance Exhaust and have the flap open). So the use and application of the LTFT may be different between this and the M54

[wrevilo]

Thanks. I have been having issues with my car that have been based around air leaks and fueling issues that have also given rough idle.

My car is in the garage at the moment having a bunch of bits replaced. The mechanic has reset and relearned Valvetronic adaptations that apparently have helped. Might be worth investigating this?

[wheela]

Originally Posted by Malteser_wfj

Long Term Fuel Trim & Short Term Fuel Trim.

My understanding of these coming from an E46 M54 is that short term is the immediate reaction/running of the engine from the closed loop lambda control & long term is the learning of the ECU over the course of the engine to adapt and account for any changes to the engine's running.

Say if you had a small air leak for a while the LTFT will increase to make up for this, or a leaky injector would cause the LTFT to decrease or be in the negative.


This may not be accurate for a direct injection engine though as it also seems like the LTFT's shoot up to the 20's when the engine burbles (I get decent burbles as I've got a factory M Performance Exhaust and have the flap open). So the use and application of the LTFT may be different between this and the M54

Yep, this is true. My assumption of why LTFT is always jumping around on logs is because the DME is sophisticated to the point where it keeps LTFT values for many different sets of conditions. So as the conditions change, it's constantly indexing between different LTFT's to use the one appropriate for the instantaneous conditions.

Much more sophisticated than my old Mitsubishi that just had just two LTFT's (LTFT low for idle and low load conditions, and LTFT high for mid/high load / WOT).

I guess another potential factor is these DME's have all kinds of check routines that run at different times/ conditions to check performance of the various sensors. I don't know the technical details, but I do know some of these routines intentionally alter AFR by certain amounts to check response - not sure if it's using the trims to make those AFR alterations or not.