Hi All, got a 01 Disco with 145k. I've got a misfire due to low compression (about 30 psi) on adjacent cylinders 1 and 3. What is most likely cause of this? I have no oil/coolant leakage, no overheating. I've read adjacent cylinders with low compression means an internal head gasket failure, how true is this?

 

Drain the oil. If it looks milky, it's got coolant in it. That would be an internal head gasket leak, right folks?

 

Most likely will be a blown head gasket, when you pulled the plugs how did they look? Do you have a borescope or a good flashlight to see into the cylinder? If the #3 piston is shiny clean you might have a cracked block as there is no coolant passages near there.

 

Checked the oil, looks clean and not milky at all. I pulled the plugs on 1 and 3 and found them blackened with dark soot all around them, smells like gas but not too sure if oil is mixed with it. I don't have a scope but was able to see the inside of cylinder 1 with a flashlight, looks blackened too, probably a lot of carbon build up as well. I suspect cylinder 3 will look similar. Does this look like a valve job too?

 

There a various reasons for low compression, piston rings, cracked pistons, cracked block, cracked head, blown head gasket, fissure between bores, valves letting by etc etc. It's eliminating the reasons that is difficult. Test for exhaust gas in the coolant radiator first, there are kits available. There is a very good chance it's a blown head gasket so check that out first.

 

add a few drops of oil and repeat the compression test, if your numbers go up you need rings, if not it is a topend problem

 

Typically the most likely cause of such low compression on adjacent cylinders is the failure of the head gasket in the thin portion which seals them from each other. You could confirm this by taking out the spark plug on both cylinders, bringing one up to tdc, and blowing compressed air into the plug hole. If the gasket is bad you should be able to hear/feel it come out to the adjacent cylinder. Really the test is not needed, whatever it is you need to take the head off and the gasket failure will be obvious. If its not then you could inspect further (valves next, then rings), but most likely the head gasket has failed.

 

Also, the failure of the head gasket in that area would not cause a coolant leak because it is not near a water jacket