compression test

[geunther]

I have a stock 1979 350 (LS9 code) with extremly low original miles. What should I expect from a compression test. I observed about 55 to 60 psi.

The results seem low to me. My guage is fairly new and think I am doing it right.

On a wet test there was not much difference. I installed new guide seals prior to the test because the truck had sat for so long but now it seems to smoke more than when I started.

[Z16]

That is low. Should be closer to 90-100psi. Valve seals won't cure much. If a guide is worn out, you have to realize it's worn out. Best bet is a leakdown test to determine where the compression is going, past the guides or rings

[geunther]

Currently the engine has 15,300 original miles. I bought it with 8600 miles and put the other 6700 miles on myself. Even at 8600 miles there was blue smoke. I don't imagine that it's wore out even if it was driven hard. I bought from the original owner and I know it was taken care of. Obviously it sat for considerable periods though.

I performed a leak down test on 1 cylinder. It showed about 7% leakage. I heard air flow into the crankcase but dont know how much is normal. I also heard a smidge at the exhaust, but couln't tell if I was just hearing the flow of air that was going into the crankcase. I could't feel any flow at the exhaust, I could just could hear the air flow.

I've considered the possibility of stuck rings but I wouldn't guess that they would all be stuck, and all cylinders show similar results on compression testing.

When I replaced the guide seals I checked the guids for excessive play and didn't note any.

The spark plugs all appear dry, but there is most definately blue smoke. The smoke gets worse after an idle period, it kinda loads up. At highway speed you can not see any smoke

[ed455]

I would say that there are stuck rings, cylinder corrosion, or something of the sort. With compression that low, I'm a little surprised it even runs at all. This is an example where low miles can spell trouble. Sat around with rust forming in the cylinders. Very possible that the rings never seated in the first place due to not having a real break-in period. With miles that low, the PO probably took it home when he bought it and it was only used for short runs, thus never getting a good break-in.

[geunther]

When I bought the truck it smoked a little so I removed the heads, boiled them, lapped the valves, and installed new seals. There was no corrosion in the cylinders. No damage and were in great shape. No ridges but a little build up, probably from not being run very much and on short trips at low rpm.

It seems to smoke a little worse now so I can't help to think that i did something wrong.

Of course it could be stuck rings, but would all 8 be stuck and show similar compression?

I appreciate the help and agree with the probable problems but from what I've checked out, nothing yet fits as to what the problem is.

[jhow66]

On the compression test did you have all the plugs out-did you have the throttle wide open when cranking-did you crank until gauge stopped? How did you install the "O" rings on valves (steps)?

[geunther]

Compression test: I held open the choke plates with screw driver handle, held open the throttle with foot. All spark plugs were removed. Cranked until guage stopped climbing . Then performed wet. No cylinders showed significant improvement, all stayed about same or about 3 psi max difference between wet and dry.

On intake valves I installed umbrella seals, on exhaust I only installed little black round seal around bottom grove on stem: I installed valve, then installed spring, compressed the spring, installed keys, released spring, then insured seal was still in place. I admit it was kind of hard to see through the spring. (It is possible that some of the seals are in wrong position (slipped out of groove) on exhaust only, could this be the source of my woes?) I wouldn't think this would cause blue smoke becuase the guide is under pressure not vacuum when exhaust valve is open. In other words, the mechanics of the engine would force oil up the guide not suck it into the chamber, even if seal was not installed.



If it wasn't for the blue smoke I would guess that I had a bad compression tester guage.

[Green Machine]

Valves set too tight ????

[geunther]

Im gonna check out the valve adjustment Saturday. If they are too tight, would that cause blue smoke?

[BurnoutNova]

how does it run? does it seem to have normal power, or does it seem to weak?

i would think with low compression like that it would'nt run very good. a compression test is sometimes called a go-no-go test, its either there or it isnt. it doesnt really tell you what the issue is if its low, like 60 psi seems to be.

do a cylinder leak down test and see where the leak is at, either the rings, headgaskets, valve guides or seats... but i would say if the 60psi is constant throughout all 8 cylinders the headgskets and valve seat(s) probobly arent the issue.

[geunther]

Actually it runs pretty well. From driving you can not even tell that there is a power issue. It seems pretty normal, 4bbl kicks in, accelerates well, sounds smooth. (just smokes dang it) The picture is when the heads were off for a cleaning and guide seals. You cant see cylinder wall very well, but can see the shiny reflection on top of the piston.

[jhow66]

Here is how "O" ring seals should be installed:
1--install valve in head
2--install spring shim (if used)
3--install spring
4--install spring retainer
5--compress spring
6--Install "O" ring
7--install keepers
8--release spring compressor
If you put the "O" ring on before you install spring retainer, it will push the ring out of groove when you compress spring.
Also install "O" rings on intakes even if you have the umbrella seals.
If seals were installed wrong it will smoke even if guides are not worn.

[geunther]

Thanks jhow66. Today I examined my handiwork, and that is exactly how I installed them. (incorrectly). They were all chewed up and in the wrong position. Im on my way to the parts house now.

Hopefully this will be the fix. Thanks all for the pointers.

one last question? The umbrell seals just fit on the valve stem, not in the bottom grove correct?

[Z16]

Yes the umbrella seals go on the valve stem and overtop the valve guide that sticks thru the top of the head

[kaycee]

WOW your right next door if I would of known about this I would of came by and helped u out yesterday (sat.)I would be very cautious of a machine shop in Cheyeene had 1 really stick it to me last year.PM me maybe we can get together and bs

[geunther]

Well I spent Saturday installing (correctly) the valve seals. Blue smoke is gone.

On the intake I installed positive seals that attach to the valve guide by spring pressure, as well as the factory O ring seals.

On the exhaust I installed only the factory O ring seals.

Now back to the compression test. I believe I experienced a combination of symptoms. The blue smoke was cause by incorrectly installed valve stem seals, and low consistent compression was cuased by p.o.s. compression tester.

Thanks again all.