Oddjob

Track/Race event:

Stock configured engine management - DME/KLR, AFM, CV in use, Cup 100 okt chip set.

Under high load, upper rpm range in 4th and 5th gear, didn’t show up much in 3rd – car has a minor miss. Not a dramatic stumble or boost drop, the sensation is a slight but distinct/noticeable tugging.

Not DME overboost protection, not the KLR limp mode.

Problem showed up on Friday test & tune day, and kept getting a little worse thru the day. I swapped the coil, distributer cap, and a couple plug wires, and one plug that was bad. Distr rotor looked fine. Checked fuel pressure, ok. Problem seemed to go away and car ran ok on Saturday, but same symptoms came back on Sunday morning during warm-up and first lap of the second sprint race, so I parked it.

Maybe fuel or maybe ignition. Potentially dropping just one cylinder (?). I don’t think it was smoking (black).

Plugs from the Sat-Sun runs look ok, no indication that one cylinder is running different than another.

Wondering if this symptom is similar to something someone else has seen, and what the fix was. Issue is that is that it only shows up at track/race conditions. I can’t get it to replicate on the street at anything close to legal speeds. So will be difficult to confirm it’s fixed before the next time I drag the car to the track.

What symptoms has anyone had with a bad fuel injector or bad fuel injector harness/wire?

I don’t think this is the knock system retarding timing as I believe that to be smooth, not a tugging sensation (?).

Thanks

Humboldtgrin

I would suspect the spark plug wires or spark plugs. At high loads is when then will show there weakness first.

vette951s

I had a similar problem on my old street/track Turbo S but the miss was more dramatic and only above 5000-5500 rpm. It drove me nuts finding the problem. Turned out to be too cold a heat range spark plug (that was recommended by Jon Milledge!). This was a long time ago and I can't remember what heat range they were.

Good luck! John

Chris White

Yes, check the plugs!

951and944S

Man, these kind of problems suck. You lose paid for track time, get home, go over everything and hope you have it covered but never know until you're back on track.

I had a random misfire once eat me up in a 500 mile enduro and one one of the stops (4 drivers), finally pinned it down to the cutoff switch that I had wired to interrupt coil.

Waited on pitwall at Sebring one year after a random misfire ended my son's last sprint at TWS and caught the problem in a warmup with a DME in hand.

You really need a second person helping hand when you come off track so you can try something and go right back out to test a potential fix.

What about a collapsing cross over...?

I love the turbo cars but give an NA all the way at the track.

Good luck Jim.

T

Paulyy

change plugs, then leads,

That's all you can do really. You've swapped the dizzy unless they're both bad. clean the contacts?

Lastly coil. swap that with another one.

Mach 5

Maybe a slight exhaust leak upstream of the wastegate.

Roger

jerome951

Ugh, Jim.. Good luck.

I had an almost similar issue. Occasional hard ignition cutout under high-boost/rpm conditions (running stock engine mgmt). Cycle the key and problem would go away for that session. Got more frequent over time.
Replaced everything in the ignition system with no improvement. Started monitoring (using an indicator light) electrical flow between various inputs/outputs to the DME & KLR. Something was causing the ignition signal to the coil to cut sporadically. Spent better part of a year troubleshooting.
Spoke w/ a mechanic friend who suggested replacing the entire engine harness. That solved the issue. A wire somewhere must have worn and was dropping a key signal.

The fact you moved some ignition system wiring around that temporarily resolved the issue could point to something.

topley

Please educate me on how a pre wastegate exhaust leak can cause a missfire. I'm trying to track down if I have a leak (other symptoms... but I'd love to know what to look for)
Thanks!

fejjj

I had a similar problem and it turned out to be the coil.

Humboldtgrin

O2 sensor will sense extra O2 in the exhaust from a pre O2 sensor exhaust leak, try to compensate, rich misfire may occur due to over compensation.

Oddjob

Where will the extra oxygen come from with an exhaust leak?

Oddjob

Boost level was fine, I didn't sense a problem with that. First thought was fuel, but because the miss went away for a day then came back, I am leaning towards ignition/electrical.

Will be replacing all the ignition pieces (rotor, coil, wires, plugs, etc) and probably some fuel delivery parts too just to cover as many bases as possible - and these are the easy fixes. I was more worried of a potential break in the wiring harness which would be harder to find/diagnose and fix.

Humboldtgrin

From the leak. The exhaut gasses go both ways believe it or not but only at idle or decelerating . So the outside oxygen content is around 20.5% and when it enters the exhaust stream in between the cylinder pulses it will give a missreading to the O2 sensor. Have you ever heard a car backfire? That's due to a leak in the exhaust upstream of the tail pipe which adds O2 to unburned fuel which causes the fuel to burn in the exhaust pipe creating a bang. And it will happen under vacuum not boost. So when you let off the throttle in a turbo and have a leak it could go bang all the same depending on how bad the leak is and where the leak is located.

Cloud9...68

Wait until it gets dark, and have the car in a dark place, like a garage (with the door open of course), and start the engine. Let it fully warm up. Then, stare at the plug wires for any sign of external sparks. I had a similar problem years ago with my 968, and it turned out to be the plug wires. I did the run-it-in-the-dark test, and almost gave up after about 15 minutes, then saw a spark, and then another, and ultimately a mini-fireworks show. Replaced the wires, and the problem was solved. Good luck.