M42 AFR problems

[AzzKickr]

I have a 1993 M42B18 with a serious AFR problem. Engine has just been rebuilt but I can't understand where these problems come from. I'd like to get these solved asap as I have a DASC kit underway.

When we dyno-ed it last week after the rebuilt, it shows the engine running very lean at aprox 5300rpm for a short period (200-300rpm) and then suddenly "corrects" itself again to the proper AFR. This is also very noticeable in 1st gear when accelerating hard because it feels like you pull up the handbrake for a fraction of a second, so the car really gets a "bump" during acceleration. Also in 5th gear doing a topspeed run the part from 0-100mph goes really well, but then it goes reeeaaally slow upwards till 110mph and then suddenly the car regains its AFR and accelerates as normal again.

PLEASE does anyone have an idea ? I've tried local tuners here but they don't want to help me as it takes too hard of an effort to find such causes. I've already tried a new lambda sensor (the one in the exhaust, I think you call it oxygen sensor?) but no luck ...

ECU was remapped in the past, but it's now stock again. No other mod currently installed.

PLEEEAAASE !!

[DustenT]

Any check engine lights? How lean? Post your AFR curve.

Sounds normal to me, there is a hp/torque drop around 4500-5000 rpms.

[AzzKickr]

Hi Dusten,

First of all: great ride. Your video comp. of the DASC was one of the reasons for me to go for a DASC (currently selling the Mosselman turbo setup). I was hoping for you to aid me due to your extensive knowledge on both 16valve BMW engines and DASC kits.

No check engine lights of any kind.

I'm going to post results here by a simple drawn image as I have no scanner for the dyno outputs.

This dyno was done on a Superflow:

[DustenT]

This is Chad's stock 318ti (1999 with m44 obd2 motor):



Tell me how the AFR was measured. Was the probe stuck in the end of the tailpipe? If so, there is a little room for error, since outside oxygen can influence the readings. I've seen stock N/A air/fuel ratios get above 15:1, manufactures do this for emissions, but yours doesn't look very smooth.

Check your fuel pressure at idle and at WOT. Change your fuel filter if you haven't yet. Have your injectors cleaned, or run a bunch of injector cleaner through it. If you have a vacuum leak it can cause lower fuel pressure, hook up a vacuum/boost gauge and let me know what your vacuum is at. It should be between 15-20 in/hg, but no less than 15.

[AzzKickr]

Probe was indeed stuck at the end of the tailpipes. I have brand new injectors and new fuel filter. How can I measure my fuel pressure with the stock FPR ?

Thanks in advance !

PS: forgot to mention that the idle is rough as well, after the rebuild. Might have something to do with this ? Perhaps a leak somewhere in the intake and thus causing insufficient vacuum ? Just an idea ...

[DustenT]

Originally Posted by AzzKickr

Probe was indeed stuck at the end of the tailpipes. I have brand new injectors and new fuel filter. How can I measure my fuel pressure with the stock FPR ?

Thanks in advance !

PS: forgot to mention that the idle is rough as well, after the rebuild. Might have something to do with this ? Perhaps a leak somewhere in the intake and thus causing insufficient vacuum ? Just an idea ...

Check for vacuum leaks and intake leaks.