I took the Miata for a drive last weekend, and on the way home I noticed that the car would break up and lose power once it started to make ~5psi of boost. It worried me, but I was able to make it the 30mins home as long as I kept it out of boost.
While I was driving back, I had plenty of time to think about the issue. The car drove perfectly fine at cruising speeds, so I knew it wasn’t anything too catastrophic like loss of compression or a bent valve. AFRs looked fine. EGTs looked consistent for the first 3 cylinder (I only have thermocouples in the first 3 runners right now). I started to convince myself that it must be ignition related. I just recently switched over to a Toyota coil-on-plug setup early last year, maybe something went hay wire. I also hadn’t changed spark plugs since probably last race season, which was 2019 believe it or not (it’s 2021).
I got home pretty late, so I parked the car and let it sit for the night. I ordered a new set of BKR7E spark plugs, thinking it’s probably a good time to replace the plugs anyways.
The next day, I pulled the plugs and found that cylinder 4 was pretty cruddy looking. I’m not sure what the cause of it is. I know cylinder 4 has always been ~5psi lower compression than the rest, but maybe there’s something else wrong with that cylinder now. I really should get the fourth EGT sensor hooked up to collect more data on that cylinder…
I checked the gaps on the plugs and found that cylinder 4 was a little larger than the rest.
| Cylinder # | Gap |
|---|---|
| 1 | 0.025 |
| 2 | 0.025 |
| 3 | 0.026 |
| 4 | 0.030 |
I’m not sure if the discrepancy in gaps is the cause or the effect of the problem. I always gap my plugs to 0.028” before installation, so I would expect them to all be around there. Maybe cylinder 4 is running hotter than the rest, and the tip is actually getting eaten away? It’s hard to tell from visual inspection.
When the new set of plugs arrived, I decided to gap them to 0.030” because the internet says a COP setup should run a little wider gap than OEM coils. I installed the new plugs and the car drove fine!
Going forward, I’ll have to keep an eye on cylinder 4. Like I said, I hope to get the last EGT sensor installed, so I can monitor cylinder 4. I just need to reroute the heater core hard line since it runs right under the exhaust manifold and blocks putting a sensor in the last runner.
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