Engine cuts out at 6000 RPMs

[Tubby]

Hey guys,
I have a racecar that when warmed up and mid race would begin cutting out pretty consistently at 6K RPMs. If I shifted before that, it would not cut out and it seems to do it more often when the engine was warm.

The motor would lose power for a second then continue as if nothing happened. It reminded me of when I would lose power driving my old Maxima when a coil pack would short to the block.

Is this fuel related, electrical or something else like a vacuum leak?

I appreciate any insights. It's a newly built stock motor and all the ignition stuff is brand new (spark plugs, wires, distributor cap, rotor, etc.).

Thanks,
Tubby

[jlevie]

The first think I'd suspect would be the CPS, followed by the engine harness.

[Tubby]

The first think I'd suspect would be the CPS, followed by the engine harness.

I'll order up a new CPS (crank position sensor).

Any suggestions on checking out the engine harness? I can check connections and obvious stuff. Short of putting a different one in, any advice?

Many thanks for the reply,
Tubby

[redlightpete]

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.

[TobyB]

If you still have the stock dash, what does the tach do when it happens?

If it suddenly drops to zero, you either have a loss of timing information (Jim's CPS)
and if it does it spastically, it's worth checking the power to the ECU.
If it stays up, then start looking at fuel pressure (something you should be ready to do anyways)
I once drove a fast car (long ago) that had a fuel cell coming apart. It felt EXACTLY like
we were losing the ECU, but it was a fuel pickup problem.

These can be really frustrating. Battery disconnect switches have done it, loose grounds
on the fuel pump, yadda yadda yadda. You almost have to bust out science to fix it...

hth

t

[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.

[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.

[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]

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.

[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

[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]

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]

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]

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]

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

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