A couple weeks ago I posted about my '89 5.0 F-150 (107,000 miles) which has been running poorly since I ran it without the oil cap.

I decided to start with a tune up and go from there. While performing a compression test, I found that cylinders 1-7 were in the 150-180 psi range; kinda high; perhaps carbon deposits. Unfortunately, cylinder 8 gave me no reading on my compression gauge. #8 will blow my finger out when sticking it in the spark plug hole, it just doesn't register on the gauge.

Now I need to figure what is causing the lack of compression. I'm hoping it's just a head gasket.

The engine is not making any unusual mechanical noises; it just sounds like it's missing on a cylinder. Because of this, I'm ruling out a bad valve, but I suppose a bad valve seat could be the culprit. I have not noticed any mixing of oil in my coolant and vice versa.

Do you guys have any suggestions as to how to determine if the cause is a ring, valve/seat, or gasket short of taking it apart?

Thanks,
Kyle Kruszewski

 

The only way to determine where the problem is without taking the engine apart is to do a air leak test. this is done by injecting air into the cylinder & listen to where the air is leaking out. If its coming out the exhaust pipe, its a exhaust valve, intake manifold, its a intake valve, crankcase, probally a burnt piston or bad piston rings. Good Luck!

 

Did you do a wet compression test by pouring oil in the cylinder?

You could consider doing a used oil analysis.