E30 M20 engine problems

[almawright]

Hello,
My car runs strong and perfect until it hits 5K RPM. Have you ever tried going above the redline? If you have, you might as well have noticed when the DME cuts off fuel supply and your RPM rushes down. Yep, that's the exact thing we have here except that we have it when the car goes above 5K RPM. We suddenly feel (and hear) the engine going off (at 5200 RPM for example) then it goes on again and the RPM climbs up once again, then OOPS at 6K RPM and engine goes off again.. It might happen two or three times above 5K RPM and it happens at least 90% of the time. When the engine starts again, we can hear a big BANG from the exhaust depending how much we had the throttle open at the time.

What do you guys think might cause this?

[82eye]

have you got an e or an i car? sounds like you are bouncing off the rev limiter. 5300 is the cut off in an e.

a backfire is an indication of incorrect timing and other bad running issues including poor maintenance.

[TobyB]

Having run up against the 128 mph limit in the race car, it's a lot more gentle than OP describes- I too would have suspected fuel pump.
Could also be the fuel pump relay, or the AFM: a misbehaving AFM causes some nasty WOT behavior at around 5k (usually a little lower, but racecar)
There also might be a restriction in the fuel system- a deteriorating fuel cell plugged up the filter, and I would have sworn it was a limiter.
But it was fuel, and that was rougher. Me, I'd put a fuel pressure gauge on it, zip- tie it to the wiper so it stays outside the car but is visible, and go beat on it.

If fuel's rock steady, metering AFM voltage, then scoping CPS signal, although that's not usually such a limiter, more of a 'breakup' in power...

guessing,

t

[digger]

I remember having these issues on my first e30 in the 90s and hearing about many stories of false rev limits since. To me a fuel pump stopping doesn’t act like a rev limit as it’s not as abrupt as the fuel pressure in the lines gradually reduces so the fuel delivery reduces somewhat gradually to the point of misfire rather than stops like injectors stops firing. To me it’s a trigger or ignition issue.

[TobyB]

When the fuel filter plugged up on Jeff'sE30 racecar, it very much felt like an ignition failure.
I'd been dealing with fuel delivery issues on my carbed car that season, and it was doing the 'soft' power cut you describe.

So when I got the 'dit- dit- ditditdit' in his injected car, I was pretty darned sure we had an ignition problem,
and we came in and swapped the DME. No change.
Jeffl then brought me in, changed the element in his racing filter, and off we went with full redline again.
And then we changed the element at every driver change after that, and had no problems.
The pump gas we were running was eating his fuel cell foam, and the chunks were blocking the filter.
We didn't finish as well as we'd hoped in that enduro...

And sitting in the car, foot to the floor, I would have sworn we were losing ignition, not fuel.

Which taught me to just stick a gauge on the darned thing- it takes half an hour,
and then you know for sure. My car had a fuel pressure gauge in it, Jeff's (at that time) didn't.

That was a fun race, even with the ethanol...


t