Random sudden RPM-drop at WOT

[oelholm]

Hey all


I’ve been trying to track down an issue with my M20 for quite some time now. Symptoms are as follows:

Whenever I’m driving along everything works great, however, once in a while, if I do a pull at WOT, the RPMs will suddenly drop to near nothing before jumping back up.

This is super uncomfortable as it happens in gear, and you nearly fall out the front window being thrown forward by the sudden jerk – not to mention how it must hurt the engine.

I actually managed to catch it on camera once – I’ll link the video in reply-section.


My E30 has had quite a few issues – most resolved by now – but this is the one thing that bugs me. I’m 99% certain the TPS is fine (checked a few times), the fuel pump is in good working order, spark plugs are new, AFM tested and working, belts changed recently.

I’ve tried googling a lot, but haven’t found anyone with this issue. My guess is a faulty crank position sensor, a ground somewhere or injectors. I’m hoping someone has experienced something like my problem and found the culprit.


Cheers

[oelholm]

Here's a video:

https://youtu.be/4mhAAkC_w1w

You don't really see me jump back and fourth, but you can see the RPM. as soon as the rpms drop I let go of the throttle and clutch, so the jump back up is without me touching anything.

[82eye]

tps and afm will always come back as suspect until ruled out conclusive or replaced. since the issue is not consistent testing them will probably not help. you have to be testing them exactly when it happens, and that is not a likely scenario.

a failing main or dme relay, bad grounds, or a poor fusible link could all contribute to the issue.

what's happening with your tach and instrument cluster when the issue occurs ? is it still showing rpm, or does it drop out or go dead ?

[Hrodebert]

I had a similar issue with throttle cutting out around 4k, turned out the AFM idle bypass screw was turned too far in.

found this when i was trying to troubleshoot my issue

Bucking/Stalling High Gear + High RPM : r/E30 (reddit.com)

[digger]

my guess would be crank sensor or ignition issue

[82eye]

my guess would be crank sensor or ignition issue

yeah cps will do it if failing for sure

[It's Soda Not Pop]

https://www.r3vlimited.com/board/forum/e30-technical-forums/e30-faq-and-diy/333798-diagnostics-tips-tricks-by-jim-levie-e30-jedi-master