Expert: Dan Liddy - 6/29/2006

Question
Is it possible to have a blown/leaking head gasket, not have any oil in the coolant, not have any coolant in the oil, and have deteriorating compression in the #7 cylinder, to the point where the #7 cylinder would miss when cold, then fire normally after 10-15 minutes of driving (engine warmed up to normal operating temp)...then over time take steadily longer for it to eventually fire, and now, not fire at all, with no compression in that cylinder? It looks to me, and I cannot be 100% certain, but it looks like there is some slight oil/residue off the back of the #7 cylinder (facing the passenger compartment), starting at the head gasket and leaking down ever so slightly.  Thanks in advance...

Adam

Answer
Hi Adam:

Compression is vital to the cylinder firing, along with a fuel mixture and a spark plug that's working. No compression, no fire .

The three most common things that cause a reduction and subsequent loss of compression are , blown head gaskets, burnt valves, and broken or worn piston rings.   The fact that neither your oil nor your coolant are contaminated means very little depending on where the leak might be (if it's the gasket)  That contamination helps only to confirm a diagnosis of a leaking gasket.  

Once you remove the cylinder head to effect repairs, any burned valves will be obvions, and you should re grind them anyway since you have the disassembly already done. Do both banks even if they don't need it yet.

It would, however, be important to know for sure that it is not a broken piston ring in advance.  The way to do that is to remove the oil filler cap, and start the motor , listen at the filler cam and if you hear a periodic whoosh at the filler cap it is compression leaking into the crankcase.
To confirm that diagnosis, then add a little oil through the #7 spark plug hole and re-check the compression. If it improves, the problem is at least partially your rings.

One quick question for you , has the motor been overheated recently (like before this problem started ) ?  If so, it's almost certainly the gasket. You'll probably also have to have the head re-surfaced.

The oil stain you mention near that cylinder may or may not mean anything but those things are not dependable signs for diagnosing a problem.

Finally, I'm guessing from what you say, that it's a blown gasket, but you should plan on doing a valve job while you have it apart anyway, it just makes sense to only do all that labor once, rather than having to do it over again within the next year or two.

If it does turn out to be a broken or worn piston ring, you are in for a major overhaul, there's no way around it.

Good luck !!

Dan Liddy
Sarasota, Florida