Over past week I have being doing my first engine rebuild. It's a 99 Miata with NB1 head and NB2 block. Using NB1 head gasket. After it was done, I ended up with the same issue that caused the problems that lead to the rebuild.
Over a year ago, I overheated the engine and replaced head gasket. Not long after, 3rd cylinder started misfiring. I found out that issue was coolant leaking into it. That also explained a mystery of disappearing coolant and antifreeze scent of the exhaust bouquet. I removed the spark plug and found that it was missing half of ceramic on the spark side. I thought that I winged it and everything was good.
It was good indeed, until recently. Roughly couple weeks ago I got oil smoke out of exhaust. I took off head and found that piston was pitted and cylinder wall was scratched. At that point full rebuild was in order. Got a shop to bore and hone cylinders, machine surfaces, machine and hone rods and mains, replace rod bushings, polish crank and test valves. Valves in 3rd cylinder seemed to leak a bit due to scuffs from ceramic debris so I got them lapped. I replaced the cylinders with rings, water pump, thermostat, and bearings. Car came with ARP head studs so no need for head bolts. Got a full gasket kit that came very handy. Everything looked promising
On Monday I started up the car. It worked great until Tuesday morning. Misfires started again and threw P0303 code. I checked compression and got these results (front to back): 185, 185, 180, 187. Compression looked good even though no cat was supervising the procedure. While doing the test, I noticed that 3rd cylinder was spitting coolant. On closer inspection I found coolant vapor coming out of the spark plug opening.
I never had coolant in the oil or vice versa. At this point my only culprit is a crack in the head.
Did anyone have similar problem and can give advice? Am I overlooking something?
I appreciate all the feedback that will help me get to the bottom of this.