Head Gasket?
 

So the Miata overheated about a month ago (heater hose slipped off the back of the head), and around the same time started running funny. The car runs normally, except that it doesn't want to idle and sounds...off...during decel. There is no smoke, and both the coolant and oil look fine, but cylinders 2/3's spark plugs look different than 1/4.

I did a compression test a few months ago, and got 200-205 across the board. I did another test the other night and got 203,180,180,205 which leads me to believe that I have a leak between 2 and 3, but after adding some oil down the cylinder it bumped up to 205 for both.

The only other thing that could cause my symptoms, besides the lower compression, would be a dying coilpack but I just replaced it last Feb.


(sorry for spelling/gayness, posting from my phone)

IIRC when the test rises higher after a wet test then it's the rings.

Leakdown>Compression test

Yeah, I thought the same thing, but you have to understand that the pressures between 2 and 3 were nuts on the same.

I need to try a leak down test the next time the car is running well enough to make it to the shop (and therefor air compressor). I am really, really hoping that it is not rings, because as a poor college student that would be a fairly bad thing. Also, the price on the M-tuned rods are just about right, and it would be unfortunate to get that far into the motor and leave it stock...

I hear ya, college student here building a car..

Those are the same symptoms that lead me to refresh my head and throw on a new head gasket to prepare for boost. Several months later and a bunch of work thrown down the toilet, I fixed the real issue which was the rings. I was sure it was the head gasket because I had ~ 185,160,160,185. Surely those 2 middle cylinders were leaking across the head gasket... they weren't. If oil bumps up the compression, its the rings. You are correct about doing the pistons. I kick myself in the ass every time I think about how I left the stock rods in. "Oh, I'm only going to shoot for ~200whp, it will be fine" Then everyone starts dropping stock rods on the road at not much more than 200. Do the rods!!!