Six of the eight cylinders tested 245-248 PSI, number eight was 211 lbs., and adjacently number six was 210 lbs. Squirted a teaspoon of oil into 6 and 8 and both jumped to 225 PSI. Am I experiencing weak piston rings in those two cylinders? Valve seals or guides? Has my HG been compromised between the #6 and #8 cylinder? Spark plug check revealed oil on both plugs in question.

Will a leakdown test help find the problem or is it a guessing game from here?

 

The leakdown (also known as a differential compression) test will tell you where the leak is. Just listen in the intake, exhaust and oil cap. You should be able to hear air leaking from one of those.

You are holding the throttle blades wide open during the compression test, right?

 

Yes, the throttle blades were held wide open. Personally, I think there is a breach in the HG between the #6 and #8 holes, allowing some of the compression to leak. Is there a way to check for this or will I have to pull the head?

 

If the head gasket was blown, doubt it would be holding that much pressure, but stranger things have happened I guess. More likely rings if the compression came up with the addition of the oil.

I don't suppose 6 and 8 were the last two you tested (batt flattening)?

For a more informative compression check, see post 2.

 

I never tried it but, I suppose you could rig up an adapter so you could pressurize say cyl#8 and connect your compression gauge to cyl#6 and see if any pressure transfers. You would need to loosen or remove the rocker arms so the valves would stay closed.. You could also remove the radiator cap and listen for gurgling indicating pressure is leaking into cooling sys.

Be advised, the crank could easily turn up to a half revolution (with substantial force) when the cyl is pressurized. Plan accordingly..

BTW, squirting oil in a good cyl will raise the press seen on the compression gauge. The amount of increase will depend on how well the rings have seated.

 

A blown head gasket or crack head will show up in the cooling system as a steady stream of bubbles in the rad once the stat opens.Valve seals are going to smoke on a cold start-up.
Compression tests and leakdown tests are only indicators of probable causes as any other tests you can run.That can be like a dog chasing it's tail without getting anything productive done in the repair.At the end of the day,it has to come apart for a close inspection.