Engine cuts out at 6000 RPMs

[Tubby]

Update: The mystery cutout started to occur with more regularity. It's doing it now with the fresh rebuild. We've swapped out three engine harnesses, 4 DMEs, almost everything electrical. It doesn't happen every time to its a b!tch to troubleshoot. Never does it while idling or sitting revving, only driving.

Got fed up and decided to either ghost it into a lake or take it to a shop that still works on e30s. Now it does it everytime so should be easier to find. Took the shop about two hours to find it.

A loose piece of metal was migrating around inside the idle control valve. It would short that solenoid to ground robbing all power to the DME which then would think the car was shut off hence the cut out. They found it by hooking up their scope and taking the car for a spin. It started doing it everytime as the chunk finally wedged itself in place. Replaced the idle control valve. Fixed!

By the way, that was just about the only thing we didn't replace. Who would think an idle control valve would cause a high RPM cutout.

We've raced the car now for several races with no cutouts on two different motors.

[Tubby]

We think it was the pistons hitting the previous head. We also had the fuel pump go out last race.

New rebuild runs with no issues so nothing electrical or sensor related.

[Subsqu125]

Hey Tubby, did you ever figure this out and resolve the issue? My teams car is having the same problem and were are looking for some insight. Thanks

[Tubby]

Update:
Cutout continued to get worse and we turned our race into a fix it session disconnecting electrical items that might be causing the cut out. Car handled like a dream and the cut out issue was extremely frustrating.

We threw a belt and overheated the motor. Weekend done.

Taking the motor apart we've got a hole through the #1 cylinder in the head created by the overheating. Head is now junk.

Our machinist pointed out that the pistons were hitting the head on at least two cylinders and that might have been causing the issue.

The bottom end is okay but we are reringing the pistons and rebuilding the head. We are also doing our spare motor from the parts car which was in very good shape. We'll through the new motor in and we'll see what's going on now.

Thanks all for the insight.

[Tubby]

It's been a great source just by searching. People all seem to be nice and not snippy.

Next step is to replace the ignition switch. We bypassed the kill switch and it still cut out so we don't think that is contributing.

Don't quit...

[603Racing]

Will do. Not sure when they plan on racing the shark next.

...and welcome to R3V, by far the best site for e30 info.

[Tubby]

Our plan is to keep the revs higher than the cutout. We did replace the o2 sensor in the flurry to find the issue.

Looking forward to seeing the Shark. Stop by our pits for a beer next event.

[603Racing]

I will probably race the Shark with them one time in 2014.

Perhaps an issue with 02 sensor? Just a shot in the dark there.

Or just keep the revs over 2500, there's no power down there anyway. :)

[Tubby]

Yes it's a chumpcar. We've swapped AFMs with no changes. The TPS was checked but not replaced. That may be next. Thanks for the advice.

I loved the shark. Hopefully, you're still chumping!

[603Racing]

New Chumpcar? Not too many Tubby's from MN that race. :) I used to own the Dark Shark (flat black e24) in Chumpcar. Saw you guys at HPT in the Chumpionship last year.

Does throttle input change how the car runs in that RPM range? Wondering if you might have a flaky AFM or TPS.

[Tubby]

Update:
-Replaced engine harness with a better shape one.
-adjusted CPS distace to toothed gear.
-Used a gopro driving around to see if we have excessive play (nope)
-Used all sorts of jumpers to get better voltage to ECU including capacitors (no change)
-Replaced alternator
-Replaced fusible link
-Replaced blue temperature sensor
-Wire from battery to ECU replace with heavier gage wire

All in all the car idles much smoother with fewer backfires. It still cuts out but is much less frequent and only at lower RPMS. We will race it as is.

Added additional ground to motor - Haven't tested that yet

[Tubby]

Hey guys, thanks again for the help. Here's an update:
-Installed new CPS, no change
-Installed new coil and tried two other coils, no change
-Removed motor and fixed oil leaks (pan and rear seal), no change (just thought I'd add)
-Fuel pressure was 70 (too high), fixed the return line and the fuel pressure stabilized at 40 and the car seemed to run better.
-Swapped DMEs, no change.
-Checked timing with an O-scope, no degradation. We've seen a bad harmonic balancer as well.
-Measured power at the DME and it was 12.1 volts vs. the book value of 13.5 or so. Wired the direct power to DME together with the other power after the kill switch (the engine won't kill wired up this way). The voltage stayed around 12 volts at the DME. Hard wired via jumpers the main relay and fuel pump relay. Those didn't help.

The car under acceleration/load runs flawlessly and only until you reach speed and the RPMs drop between 1500 to 2500 the engine cuts out dropping the tach and loses power for a second. The engine also back fires at this time.

At this point we suspect the harness and possibly the fusible link or kill switch.

Any further insights/tests are welcome. This is driving me to heroin.

[jlevie]

I think the revs just dropped, I don't remember any spastic behavior of the tach. I can't remember if it went to zero or just dropped some.

If the tachometer is reacting, the DME is loosing timing data, or power. A bad fusible link, bad kill switch, or even a bad ground are possibilities. But usually you'll see a flash of the CEL in those cases. If the loss of timing data is brief, you may not see the CEL do anything, though the tachometer (and frequently the econometer) will react.

In addition to what I have already mentioned, on occasion I've seen a bent harmonic balancer cause this and seen (but rarely) a flaky DME be responsible. The DME and/or the CPS are best diagnosed by part swaps. The harness is a bit trickier. You could, of course, swap another harness. But used harnesses can be an iffy thing. I went through three used harness on my first race car fighting this very issue. The fourth worked for a year or so before developing a different fault. The car now has a brand BMW harness. The only way to find out if the timing data coax in the harness is degraded is with an oscilloscope.

[Tubby]

It might be the coil like your maxima. The bad coil in my 325is would cut out at high load and rpm, including turn 12 @ Road Atlanta. Luckily it was just for a second(!) It was worse when warm, which makes sense. My symptoms were not as consistent rpm-wise as you're describing, but it was always WOT at high rpm. It would completely cut off for a second or two and then go :pback to full power.

After the CPS, we'll try a new coil as well. One at a time and hopefully we'll get it sorted.

[Tubby]

Thanks guys. We're ordering up a CPS and we'll try that first. The fuel system is a frankenstein setup using the stock 16.6 gal tank but has a fuel filter in the engine bay.

I think the revs just dropped, I don't remember any spastic behavior of the tach. I can't remember if it went to zero or just dropped some.

I'll throw in a CPS and we'll beat on it. Next race is in October so we don't want this issue there.