Zero compression cylinder 4
 

Hey guys.. Finally finished putting back together my miata.

Ran it for about two weeks. Noticed my AFR's began getting rich. Car started idling like crap and was sitting down at 12 AFR at idle. Pulled spark plug 4 and the plug was completely blown up. As in, the tip was disintegrated. It didnt look like the piston made contact. Thing is, there is ZERO compression in cylinder 4 and only around 90 psi on cylinder 3....

Where do I go from here? Time to tear the head off? I've got forged pistons and rods and arp headstuds in this build... Felt like I was pretty conservative at about 15-18psi max.

Whats the chance its a stuck valve or something? I'm really hoping the spark plug self destructing didn't crack the piston.

Ignition timing @ 15-18psi will determine how conservative you are. Not necessarily the amount of boost.

Sounds like you could of had serious detonation that melted the spark plug and destroyed your rings. And if that's the case, your pistons is likely fried as well.

A leak down test will tell you where the compression is going but my money is on piston damage.

/\ I'm with that guy

Well fuck boys. I guess I'll know when I tear the head off. I'd like to not yank the engine until I'm sure that's it.. How hard is it to yank the head off while it's still in the car? I'm so fried working on cars right now. It might sit for a couple weeks honestly. Its got brand new tires that were delivered the day the engine blew sitting in front of it LOL.

Why are you ignoring a 15 minute leak down test?

What will you do if you pull the head and can't find a problem with a visual inspection?

Wouldn't a leak down test just confirm that I need to take the head off? I'll look into it, I've never done one before if I'm being completely honest. I didn't think you could "point" to the issue by using a leakdown test. I'll see if a local place has a kit I can borrow

A leak down does point you in a direction. If you confirm compression is escaping through the bottom end you can probably skip pulling the head and pull the engine instead. If you have broken ring lands and you just pull the head you won't see an issue. If you do a leak down and find compression leaking past a valve you may find a clearance problem and pull nothing more than the valve cover.

Testers are less than $100.
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Again, he's right. And I would never bother taking apart a miata engine still in the car. Such a pain and your back will hate you.

If something failed in a built engine, I’d be pulling the engine regardless of cause, to tear down and inspect bearings, oil pump, etc. it’s worth it for peace of mind. If anything fails after you “just” replace the head, you’ll be wondering if it’s from your head gasket work or the original failure.

This.

You can kill a piston at 8psi if your timing is borked.

I would use the comp testers hose to run 30-40psi to the cylinder in question. Bring it up to TDC on the comp stroke, trans in 5th with the e-brake on and wheels chocked. Then listen to the intake, oil fill in VC and the exhaust. This will let you know if it's an intake valve, piston or exhaust valve. If it is a valve pull the VC and check the valve lash. If the air is coming from a valve pull the head and if it is coming from the VC/crankcase pull the engine. Make sure you find the root cause before running the engine again. Good Luck.

Quote:

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Originally Posted by LeoNA (Post 1625552)

How well it is put together is a factor.

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Miata engines, BP, are one of the easiest to work on, removing the CH is not that difficult. It is a RWD car, way easier to access all sides with enough room to wrench.

Autozone lends a borescope so at least you can check the pistons, but if there is zero compression, you are in for a teardown.

​​​​​good luck