Compression revisited

[Chero77]

A few weeks ago, I posted the compression readings on my 401 with 114,000 miles (so far as I know).

A recap of readings: Readings are dry/wet, where wet means three squirts of oil in the cyinder just prior to the test. All readings were taken with the engine warm.

Left Bank Cylinders:

1) 120/120 3) 173/180 5) 115/130 7) 130/150

Right Bank Cylinders

2) 135/135 4) 170/180 6) 160/175 8) 90/120 ???

Plugs get oily on cylinders 3, 4, and 6.

Other info, engine oil pressure is good (60 psi cold, 30 psi warm idle).

So, I need opinions on what happend in cylinder 8? It reads 90 psi. Is it likely, that this is due to a broken ring, blown head gasket or cracked piston? Or is it more likely that the bore is just plain worn? Will a leak down test, identify the cause?

Its sort of a deliemma, if its a broken ring, blown head gasket, or broken piston, I am inclined to do a partial tear down myself and have the heads rebuilt. However, if the bore is excessivly worn, I'll haul it into a rebuild shop (someday).

Since, I have taken this project this far, the idea of doing nothing just sort of bugs me.

[joe]

Those numbers are pretty suspect? Even a brand new motor wont read 175-180 lbs.
Either your gauge is junk or your testing method is flawed. Try it again with a diifferent gauge and take your time. Then do a wet test with some oil squirted in to each cyl. If the comp comes up it's the rings that are bad, if not the valves are leaking(rings may still also be bad though).

[godzilla]

Could have dropped too much oil in that cyl. Even on the big diesels (MACK, CUMMINS< etc...) we only put about 3 DROPS of oil in each cylinder on the wet test.

[joe]

I missed the part where you've done the wet test already. Yeah maybe 3 drops will work on a dsl with 20+:1 compression but on a gas motor I'd get a tsp-tbls in there if you want a reliable reading.

[Chero77]

I will run the test again, but I think the gauge is accurate. The cylinders with high compression have oily plugs so carbon build up is likely. Also, I've checked #8, twice. If the gauge was bad, I would expect to get different readings each time. But, I get the same readings which leads me to believe its accurate.

However, cylinder 8 is hard to get to because of the heater box, which I removed today. I suppose there is some possiblity I didn't screw the gauge in far enough for the o-ring to seal when I checked it before. But that won't be a problem now. I'll rerun the test tomorrow with a loaner gauge from a parts store, but I don't expect to see much of a change.

[porchpiggy]

Sounds to me like you have a somewhat worn engine that could very well last quite a few more miles. Does it run good, have decent power? How about blow by, when you run it with the oil cap off does it blow a little or a lot of smoke out?, or if you can figure a way try it with a little bit of load on the engine, like on a hill or a little bit of power braking (be very careful of course). If your engine does not use excessive oil or show other signs of heavy wear I would keep going. You can tell a lot from a compression test, but it's only part of the equation. Now, keep in mind I don't consider myself an engine expert by any means, but I think this is worth considering.