You guys want to come in here and tell me I don't need a head gasket?
 

Because I'm pretty sure I do need a head gasket.

I bought a blown up 99 Miata with the intention of putting a motor in it then putting some boost to it. I picked up a motor from a local junkyard that does a lot of Miata stuff.

I went ahead and drilled the oil pan for a turbo return, did a timing belt, replaced most gaskets, etc.

Put the motor in a few weeks ago and it ran totally fine, but I had damaged the splines on the clutch disk because I'm apparently a cave man.

I dropped the transmission again and replaced the damaged clutch disk. Suddenly I had a hard blinking ass check engine light miss on number 4. I did all the normal shit, and then all the abnormal shit because it didn't make any sense. I ended up replacing or trying a spare spark plug, plug wire, coil pack, injector, rail, and injector harness. Compression is 195 or so in that cylinder.

Finally I just decided to let it run for a while and see if I could figure anything out. I noticed the "condensation" in my exhaust never burned off.

Here's my theory: On first start, I had no cooling system installed, I just fired it up to verify it ran, then killed it within 15 seconds or so because no cooling. That's what screwed me up. Because I had no cooling, I had no coolant to leak into 4 and make it miss. While waiting on the clutch disk, I thought I was done with the engine so I went ahead and installed the radiators and fans and filled it up with coolant. Once I cranked it again with coolant in it, it started missing because it was leaking into cylinder 4.

The only thing I'm still stumped on is how it still has good compression in that cylinder with a head gasket that by all appearances seems to be blown into the coolant passage. Anyone have any thoughts?

Combustion creates water. Moisture in the exhaust doesn't really tell you much.

It takes a pretty serious head gasket issue to create a dead miss on a cylinder. I'd be looking somewhere other than head gasket if compression is good.

Yeah I kinda feel the same way, that's why it's not apart yet, but I can't think of any other explanation. It's smoking pretty badly but not as badly as I would think to be drowning out a cylinder, but the cams and valves looked fine when I had the valve cover off and literally every other part is new or known good. Wire, plug, coil pack, injector, injector harness. Plus none of those explain the burning coolant.

I've also spent the last weekend searching here at Miata.net for "Burning coolant" and all variations of that and pretty much every one comes down to head gasket. I did find a few threads on an intake manifold coolant leak, so I might pull the plenum off and see if there's coolant in the intake runner, but that aside I'm honestly kinda stumped.

Order a borescope that attaches to your phone and take a look.

Have you tried running it with the spark plug out and seeing what's coming out?

I actually have a scope. I will check that out. I haven't thought of running it with no plug in 4, but I'm reasonably certain it's burning coolant. My coolant is low and the smell of coolant burning is pretty distinct.

Pull and inspect your spark plugs.

Did that, they all look fine except number 4 smells like coolant.

That's actually why I thought maybe injector because #4 didn't smell like fuel like the others. I was thinking maybe I had a bad injector or connection in the injector harness because the injector harness looked trashed. I had the fuel rail from my original motor, along with the injectors and harness, so I swapped all of that over. Still hard miss on 4 and burning coolant.

I also swapped 3 and 4 to see if the miss would follow it just to make sure, number 4 misses no matter which spark plug is in it.

Spark plug looks reasonably decent, smelled like coolant.

Attachment 227812

I'm gonna keep looking around with the bore scope, but nothing looks that weird too far. I can see some coolant puddling in the cylinder. I'm gonna throw the other spark plugs on and fire it up with the spark plug out in a second. I'll grab my GoPro and make a video.

I think this accidentally fixed it! It was a bad spark plug wire!

Before you all call me a total idiot, it was actually a bad brand new NGK plug wire AND a bad plug wire off my 95 Miata.The 95 Miata was running fine with no misfires or check engine lights. The new NGK was damaged at the coil boot and shorting to the coil bracket and the one off the 95 was arcing through the tube that passes through the head/valve cover.

Best I can figure, my unburned "coolant" was just gasoline, combined with the fact that I have a coolant leak at the front block off plate for my SuperMiata reroute. I never would have guessed that TWO plug wires were bad, so I ruled that shit out. Now it's hitting on cylinder four again and hopefully the smoke will clear out once I get the coolant leak fixed and can drive it a little.

Attachment 227811

notice the spark from #4?

Okay I'm running out of ideas. The plug wire seemed to improve it, and it stopped smoking, but I still have a cylinder 4 misfire.

I've done:

Coil packs
Spark Plugs
Plug Wires (Three different #4 wires!)
Injectors
Injector/coil pack wiring harness
fuel rail
Compression test (190psi)

I'm going to attempt to drive it around the block and see if it gets better/worse but this is a "new" motor and clutch so i have to take it reasonably easy since the clutch is still in the break in period. Obviously not going to beat on it if the CEL is flashing at me either.

I did a timing belt on it before I put the motor in, but everything looks to be lined up, not a tooth off, and I don't see how that would only affect #4 anyway.

I'm kinda just guessing shit now. Maybe Crank Sensor? I'm running out of ideas on what can cause a single cylinder misfire.

Stop throwing parts at it and diagnose it.

Does it miss at idle? Is it a dead miss, or an occasional miss?

If it's an occasional miss at idle, let it idle and unplug things to see what causes it to change. You can also check to make sure you've got power at the injector and plug wire.

If that doesn't work, you can try swaptronics - swap injectors between cylinders and see if the misfire follows. same with plug, wire.

When you say you swapped coils, did you put in a new coil, or a used one? NB1 coils are notoriously bad. Swapping to another used coil may not be any proof that part isn't failed.

Good luck.

It does miss at idle. Bad enough for a flashing check engine light at idle, but seems somewhat intermittent as it doesn't always flash at idle. I haven't driven it at all, this is idle in neutral, so it's clearly load independent.

I totally did the "throw parts at it" thing with the coil pack, just because it's such a common failure. When that didn't work I checked for spark and I have it (though later learned the new NGK plug wire was arcing out of the plastic tube, but I'll get there). I didn't think I had fuel because #4 spark plug didn't smell like fuel to me but is getting a signal at the connector. That prompted me to switch the rail, injectors, and harness with some I had on hand. That changed nothing. At that point I was thinking head gasket and made this thread. I went in to run it without a spark plug in it to see if it would blow coolant out the spark plug hole, and that's when I noticed the bad plug wire arcing to the valve cover from the "tube" part. I swapped the #4 plug wire yet again with another one and it seemed to help, now instead of a dead ass miss at all times, it seems to do fine at high idle, but still misses noticeably at low idle (850rpms or so).

I'm kinda running out of things that would cause a miss on just one cylinder, as I'm now fairly sure I have spark, fuel, and compression on cylinder 4. I was thinking crank sensor because the first time I turned the motor over, I had not secured the crank sensor harness and the wires got pulled behind the crank pulley a little. They appear fine honestly, but a bad crank sensor/harness COULD cause a single cylinder miss, though I would think it would get worse with RPM, not better.

The code is and has always been P0304 cylinder 4 misfire, but a new development is P0106 for the map sensor. I just found that before I left my shop, so I haven't had time to investigate it yet.

Some other info that might be pertinent: 99 Miata 5 speed. Bought it with a blown up motor. Bought a used motor from a local miata junkyard. Did timing belt and most gaskets while it was out. Completely stock at this point other than an FM stage 1 clutch happy meal. Deleted EGR with FM block off plate/cap.

Oh and I have swapped the spark plug from 4 to 3 and vice versa, no change. I also replaced the injectors, no change. It makes me feel like it’s a mechanical issue with cylinder 4 but it has good compression and doesn’t make any crazy noises and looks fine in there using a borescope.

If it’s arcing out of the “tube” to the valve cover the wire is definitely not bad. It’s also likely got a strong coil. The valve cover is grounded to the motor just like the plug in the head would be, so the spark is still finding it’s way to ground like it should. Double and triple check all your plug wires are fully seated onto the coils.

When you say compression is good, what do you mean? And how does it compare to the other 3?

Have you cleared the code and had it return?

I'm honestly super confused by this car, and despite this thread, I'm not a noob at working on cars.

Today I cleared the codes and said "screw it" and just drove it. The smoke went away and it hasn't missed anymore. The best I can figure is it had a bunch of coolant in the exhaust from the original engine which did legitimately have a blown head gasket, it was shooting coolant out of #3 when I cranked it over without spark plugs. I'm gonna keep driving it a little and see if the miss every comes back or if it stays good for a while.