Overheated badly now compression is low
 

Today I was doing a series of datalogs and going for short drives. On my final datalog I noticed the smell of burning coolant. I looked down at the gauge cluster and notice that the coolant temp was pegged to the right. :eek: I was only 30 seconds away from my house so I decide to continue to my house before immediately shutting the engine off. When I got home I plugged in a OBDII scanner, the coolant temp was reading at 146. I cant believe mazda didn't put a simple check-gauges/overheating light on the gauge cluster. It probably would have saved my engine and many others. I just did a compression test, here are the results.

10 months ago pre-trubo compression was 160-170 psi on all 4.

Now
cylinder #1 135
cylinder #2 130
cylinder #3 135
cylinder #4 139

Is my engine trashed?

No but your head gasket probably is.

You may have lost ring tension and scuffed the pistons against the cylinder walls.

You don't need an idiot light because you would have kept driving it the rest of the way home anyway. It's your car, but I wouldn't have done it that way with mine.

146C?

Headgast is toast at minimum. Possibly warped head, possibly more damage like six said above.

Pull the head and look

I don't think the extra 30 seconds really mattered at that point. When I got back home their was already coolant on my garage so it must have been leaking before i even left. Chances are that it was overheated for quite some time. After I started the car back up to pull into my garage the AFRs were super lean and now i'm thinking that I cooked one or more of my new FF injectors . Going to pull the head and see how bad things are. If the piston rings are toast will I see obvious signs of scoring?

Wasn't easy but I was able to get the head pulled today. I put a straight edge on the head and it looks warped, about a 0.5mm gap in the middle. The cylinder walls look ok though, just a couple lines but noting bad looking. Anything else I should be checking?


https://cimg3.ibsrv.net/gimg/www.mia...c305a88190.jpg

https://cimg7.ibsrv.net/gimg/www.mia...079fdb81f5.jpg

https://cimg8.ibsrv.net/gimg/www.mia...91b49c769b.jpg

Really rough rule of thumb:

can you feel the vertical score marks with your fingernail?

If no, it's probably ok.
if yes, it will likely need a light hone at best, a bore at worst.




The head will obviously need to be skimmed flat.

Most of the vertical scores cannot be felt with a finger nail, only a few can but just barely. I talked to a mechanic today and showed him the pictures. He said it looks normal and not to worry about it. :dunno:

Are you NA?

I think its worth a try to flatten the head, put a new gasket on and drive. What does this cost? 120$?
Otherwise i would build it the right way, if there is any intention going turbo in the future...

I am already turbod with some supporting mods. Was making close to 200whp before it overheated. Would be nice to build it right but sadly I don't have the tools or money right now.

Going to be taking the head to a machine shop tomorrow. Hopefully it is repairable.

This is my guess too.

0.5mm is what 0.020 (20 thousandths)? I want to say the limit for a miata head is 0.002 (2 thou) measured diagonally IAW the FSM.

If the head is warped 0.020"...make sure you pull the cams and check for journal issues. With it that warped, you could have junked a cam journal and at that point you'd be way better off finding a new head. Dr.

If the head is warped .020 it is a paperweight

I've already taken the head to a machine shop and had them do a full rebuild. The warp was no where near 0.5mm and was small enough that just a resurface would have fixed it. I have all the stuff to put it back together but I have not had the time to do it. Plus it is currently over 100 degrees here in phoenix right now so i'm probably going to wait until it cools off a bit.