Random Misfire popping up during endurance race costing us the win.... Need ideas on where to look

Hello everyone. The car is a lucky dog endurance car. 1.8 vvti engine in an NA miata. The car has run wonderfully and even garnered first-place finishes. However, there is a misfire that is forming and the logs don't seem to show anything. I do see a fuel cut trigger but that's only showing when the throttle is closed it seems. Can someone take a look at the logs I shared below and let me know if you see anything obvious? The car will start to randomly stutter all throughout the rev range until the ECU is reset and that fixes the issue most of the time. Fuel and oil pressure look okay. The car is on a Link G4X Ecu. I can happily pay for some time for an extended review if needed. Thanks.

https://drive.google.com/drive/folders/1CvoFcXfYqQbNK49eCu4-OdlfzZZ-uuBt?usp=sharing

If it's random, I'd be looking at wiring and connections early in the checking process, as it sounds like an intermittent OC or SC?

Pranav,

% fuel and spark cut are 0 and fuel pressure is good, so that does narrow it down a bit.

Please add triggering, diagnostic, VVT, lambda target items to your log list. My entire diag template showed blank when opening your log except for RPM and voltage, so there's a lot of data not being collected.

Despite fuel pressure being appropriate, CL trims are +15 in places at steady state so there may be some work to do on the tune, but it may also be related to a mechanical or electrical fault. Additional data will help narrow things down.

Pranav,

I have identified a part of the log which seems to show the issue. It happened couple of times in the log but it's pretty much the same.

We can definitely see that the Lambda getting a leaner, then becoming erratic which indeed indicate a missfire. The throttle was stable when it started, then you closed it slightly before flooring it then release it and flooring it again, then the issue seemed to be resolved and all is fine for a while.

MAP, Fuel pressure, injector pulsewidth, ignition timing is matching the throttle movement, the RPM is becoming erratic after missfire is happening, so the ECU isn't commanding a change in fueling or ignition that could lead to the issue.

Unfortunately, intermitent troubles aren't easy to diagnose, but the first thing to do is trying to replicate the issue. If you're unable to do so, a visual inspection is the next steps like Gord mentioned. Trying to trace back what was done to the car prior to the issue happening is also important in the diagnostic process. Swapping parts for known good ones and testing on track again is a common practice, but can lead to a waste of time if it doesn't fix the issue! Finally, make sure you don't have a "tunnel vision" focusing on only one thing.

Here's a couple things I would check :

-Log doesn't show the VVT data, I would make sure it matches target.

-Battery voltage is consistent at the ECU, but it could be erratic at the injectors or coils.

-Sticking or clogged injectors

-Bad Coils

-Checking connectors and wiring for damaged or recessed terminals.

-If you have access to an oscilloscope, checking that the commanded PW and actual PW of the injector matches.

-Change spark plugs.

So here was my thought on the issue and I hope it will helps!

Frank

Thank you so much for all the replies. I will take all these notes and check them one by one. The help is much appreciated.

Don't forget the "wiggle test" - with the engine running give the various cables and looms a wiggle, or shake, as that may help isolate a possible connection problem.