Compression check issues


(Greg) #1

So I pulled my plugs last night, and thought I’d give the rings a bit of lubrication with marvel mystery oil. I put about 1 oz down each cylinder and let it sit overnight.

I turned the engine a bit this morning, and then hooked up my gauge on #1A. It’s going off the charts! Within 3 cranks, it’s already at 300 psi. Either my gauge is busted, or can too much oil still sitting in cylinder do this? I thought it would have seeped thru by now.

88 V12, cold engine.


(Greg) #2

Ok, ive cranked the engine a good few times to get oil out, and now the pressure reading is stabalizing at 255psi. At first, it was well over 300psi. I guess I’ve never done a wet test on what I presume are excellent condition ring seals.


(Greg) #3

Ok, all cylinders but two were 225-300 psi with variable amounts of oil still in there.

But 3A and 6B would not get over 180psi, even with more oil added. Yet in October, their dry/cold readings were 185 and 190. Would carbon build up on exhaust valves do this?

Other cylinders that were 225-300 wet were 185-195 cold/dry in Oct. Isn’t that a huge jump?

Will do warm/dry compression test one day once I get engine running properly for accurate results.


(phillip keeter) #4

Just a few ounces of MMO in the fuel tank with each fill up will provide all the lubrication you need. In the old days, a guy could slowly pour MMO down the carb into a running engine until it began to smoke for a good lubrication as well.
Phillip


(Ed Sowell) #5

Any liquid that you put through the spark plug hole doesn’t run down into the cylinder will raise the compression readings significantly since there’s a smaller volume left for the air pushed up by the piston.

When I asked how the compression was after the head rebuild, the shop that did the R/R sent be videos of the tests. I can’t remember the numbers but they were far above what a brand new preHE engine would show in a proper test.


(phillip keeter) #6

That would be a good trick for a dishonest car dealer to use when trying to sell a car with a bad engine.


(Greg) #7

but as soon as you start it, blue smoke would come pouring out the exhaust.


(phillip keeter) #8

Of course the engine would not need to be started for a compression test. Maybe it wouldn’t be that easy to fool a prospective buyer. Something to look out for though.


(Michael Garcia) #9

Greg,
did you get a reading
prior to pouring?


(Greg) #10

My last dry readings were cold in Oct, but good, all 180-195.

I’d like to do a proper test warm/dry, once i get engine running well. I have no a/c. I can now remove throttle and plugs in 30 minutes, so can do a good test.

I put way too much oil into cylinders, I had read 1-2 tablespoons, but that was wrong. One squirt is all you need for wet test. Friend told me, with the amount of oil I put in, if I had cranked with plugs in, I would have blown the head off. I stupidly thought the oil would seep past compression rings overnight. I was wrong.