Fuel Cutoff 4300-5600 depending on elevation

[Camper Van Someren]

I bought this racecar very used. Started life as a 325e, and subsequently had M20B25 from a ‘90 325i swapped in.

I noticed right away that it still uses the old harness on the new i motor. The first time I raced it I noticed that it would hit a hard fuel cutoff around 5000 rpm. Figured this was either left over from the eta ecu, or somehow set by the team to increase reliability in endurance racing.

I’ve subsequently raced the car at different tracks and the cutoff seems to vary by elevation. At 1000’ in Phoenix, it cuts at 4300. At High Plains in Denver (5000’) it cuts around 5000. And at my home (7000’ elevation) it cuts around 5600 rpm. Can also get it to rev a little higher with partial throttle. Every time it hits this rpm, it bounces hard just like a factory redline fuel cut. Tach bounces also, then revs back up to the cutoff and repeats. There is no stumble, no backfire and no variability.

After reading lots of threads, by first thought was afm. Also because this seems to be airflow related based on elevation and throttle. Tracks were very worn at the WOT end but adjusting the wiper to run on fresh black track didn’t fix the issue.

My next thought would be fuel pressure, but wanted to get ideas before spending money on fuel pressure tester. I’ve seen a lot about cps spacing also, but since mine uses the old harness, it has the dual cps in the bellhousing and doesn’t seem to be adjustable.

thanks for any input!

[It's Soda Not Pop]

have you tried a new chip in the DME? My car used to be a eta. I have had the car hit fuel cut long time ago when I was in the mountains. After I did a chip swap it never did it again.

[Camper Van Someren]

have you tried a new chip in the DME? My car used to be an eta. I have had the car hit fuel cut long time ago when I was in the mountains. After I did a chip swap it never did it again.

Well my issue gets better (revs higher) in mountains, which makes me think it is the denser air at low elevation triggering the AFM quicker, or (now that the afm fix didn’t work) requiring more fuel at lower rpm? However it is such a hard, consistent cutoff that I still feel it is something electronic rather than fuel pressure.

I do have a spare 325i ecu, harness, and set of sensors. Swapping that on will be my last resort.

[It's Soda Not Pop]

Is your fuel pump stock? if it's a bigger one like Walbro or something. Then the current draw is higher. I just re wired my fuel pump to get power directly from the battery and the signal from the FP relay. My fuel pump was randomly cutting off at times and I would have to wiggle the wire to get back on.

[It's Soda Not Pop]

Or it could be the main relay taking a shit on you. Mine did that too. It would buck at first before it finally died. I had a lot of gremlins in my car that I've had to over come. The car for the most part has been reliable, but it has had some demons to contend with at times.

[82eye]

possible failing fuel pump relay or main relay. could be on the e-car dme still as well and hitting cutoff. if you're on motronic 1.0 with an e harness it won't have the 55 pin connector needed to run an i car dme. if it's 1.1 or higher it should have the 55 pin.