Massive hesitation and stumbling, cold start/idle, after last autocross.

[CJ10]

Hey all,

After the last autocross, well, the next day, my Miata started running really poorly. At cold start/idle there is a very aggressive stumble. The engine also does not rev until about 1/4 throttle, even then, very misfire-like - almost stalling. Once things are warm it is totally fine. Before I go throwing parts at it or spending a ton of cash on labor to find the issue... any ideas where to start besides the basic spark plugs, etc? Similar experiences? Hoping I can amateur-mechanic my way out of this. I haven't looked at it closely yet, waiting for the weekend. No CEL.

I found one of the two coil packs at the rear of the motor was loose, tightened it and no difference. Nothing else feels or looks loose, no spark arcing, frayed wiring, etc. Car was perfect the day before. Last tune up was only 10k miles ago.

[— kung fu jesus —]

Vacuum leak, failing sensor, loose ground. I wouldn't throw parts at it until diagnosing the health of your coils with a multimeter, either.

[Pyr0monk3y]

I had similar issues caused by an ECU coolant temp sensor. I think it's on the back of the head. I replaced mine, but in hindsight the sensor itself might have been fine. The connector was very corroded.

The O2 sensor is another possibility. I'm sure the exhaust gets much hotter during autocross than normal driving. This may have prompted a failure. Unfortunately, diagnosing a bad O2 sensor is hard to do without trial and error.

[CJ10]

Thanks guys. *edit - Duh, I do have a multimeter. Haven't used it since, literally, 2004. Any tips/how-to's, kfj?

Off to the garage to tinker...

[— kung fu jesus —]

There are a few writeups out there, I don't remember the proceedure off the top of my head. I second Andy's comments. The OBD1 cars will take a lot of sensor variance before they throw codes. I would bet it's going to be a relatively easy fix, when you find it. Your issues also had me thinking water temp sensor. If it is, the car is running rich, I bet.

There are a lot of diagnostic procedures in the factory service manual, which are available for free online with a little searching. Worth keeping the ones for your car on your computer/phone/cloud.

Good luck!

[CJ10]

Thanks brother. I'll do some digging.