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Low Compression

USNA1969

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1972 Big Block/4-Speed
No. 1 cylinder has a compression of 130, the no. 3 cylinder has 135 and the rest are between 149 and 151. The valves are not bent or burned and the head gasket is intact. The cylinder walls and pistions appear to be ok.

Before I pull the pistons, is there anything else besides cracked or broken rings that would cause low compression?
 

Vettehead Mikey

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Aug 26, 2003
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Not that far from Ottawa
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1973 Coupe
Just as a started, excessively worn lobe(s) on the cam, collapsed lifter, bent pushrod, incorrect adjustment on lifter.

A high of 151 and a low of 130 is not that bad. I'd leave it alone if performance is not too bad and it's not burning oil.
 

USNA1969

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1972 Big Block/4-Speed
Thanks. The push rods are straight, but I haven't checked out the lifters. How do I tell if they're bad or out of adjustment?

I don't see smoke, but I go through a quart of oil every 250-300 miles.
 

Evolution1980

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I don't see smoke, but I go through a quart of oil every 250-300 miles.
Sounds a bit high... how many miles you have on your engine? ...I'm gonna guess around 68K or so?
Are you finding oil anywhere or does everything seem dry? Seems strange that with that much loss, you'd either being seeing it from the exhaust in terms of smoke, or somewhere under the engine... :confused
 

USNA1969

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Maryland
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1972 Big Block/4-Speed
You're right on - 72K. There's some oil on the bell housing and on the sides and bottom of the engine, but my driveway is clean. At least I don't see any appreciable oil stains.
Interestingly, several of the exhaust manifold bolts near the Nos. 6 and 8 cylinder came out with oil dripping off of them. Not so for the head bolts.
 

Evolution1980

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You're right on - 72K. There's some oil on the bell housing and on the sides and bottom of the engine, but my driveway is clean.
Unfortunately, the oil you're seeing is relatively normal for a SBC. You are probably leaking a little bit from the valve covers and from the rear of the engine. Even my newer ZZ4 is leaking from the rear (which bothers me). However, that alone shouldn't be eating up a qt every 300 miles.
My best guess (not "educated opinion" :D) is that you have some pretty worn rings and that you are getting a lot of "blow-by" in the cylinders or exhaust valves. I had this as an issue on my L48 when I finally got around to taking it out of the car. Again, it's not 'unusual', but I think the amount of oil that you are losing is.

Maybe Vettehead Mikey or JohnZ could give you better clues to finding the source of your engine's appetite for oil
 

craig32

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Sep 30, 2004
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482
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PA
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1981 repainted black coupe
Generally, two consecutive cylinders with low compression is consistent with a blown head gasket between them. When you say a quart of oil every 250-300 miles, do you mean every two weeks or twice a summer? What you have described sounds like valve cover gaskets, the heat of the engine will burn off oil before it hits the ground. The bell housing isn't as hot, so you will see it there. Good luck!

Craig
 

Koop

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Mar 16, 2005
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Kalifornia
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YELLOW 69 modified GM Crate ZZ383, LS2 TBSS
Valve guide seals been checked?

If I leave my 8.2 Eldo sit for a month between uses it seems to burn a quart then it stops. This could be my imagination.

I sure wouldn't open my motor up for that small of an amount of consumption or 130 pounds(unless it's slow).
 
Joined
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'67 Marina Blue Convertible
Was the compression test done with the engine warmed-up and the throttle blocked wide-open?
 
J

JOE BYRNE

Guest
I use a quart about every 400 or so. I have 87,000 miles and run Edelbock heads, manifold and headers. I have a very small rear main leak and valve covers are dry. I put new seals in it last year and very little change. My compression is about 130 to 150 all the way around. My plugs are dry and it does seem to smoke...( Ihave even put a mirror on the wall in the garage on the floor and reved it with little noticed other than a little black that smells rich than anything else...I do get some black build up in the exhaust tips). Im not worrying about it since it seems to run fine. I put about 1,500 miles a summer on it and add the oil when needed.
 
T

tonyvdb

Guest
I would bet on the head gasket leaking. If it was really blown then you would know it, and compression would be lower. Rings could also be worn because it shouldn't use that much oil if it's not leaking somewhere. My 70 with a 350-350 doesn't use enough in a summer to add any and I put 2500 to 3000 miles a summer. Heck I hardly ever check it:bash. What do you use for oil, that might make a little difference. A good brand of plain old 10-30 work great, remember it's just on old Chevy engine.:boogie
 
J

JOE BYRNE

Guest
I just use 10-30. Oil pressure at idle about 30 to 35. Runs at 180 no matter how hot it gets during the summer
 

USNA1969

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Maryland
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1972 Big Block/4-Speed
Thanks for the advice, guys.

The #1 and #3 exhaust valves had a gummy buildup on the backside which, the machine shop tells me, came from worn guides. The rest of the exhaust valves were better but still had some carbob on the backside. The intake valves had white stuff on the backside.

I did the compression test with the engine warm, but I didn't open the throttle - I just turned it over 5 times per cylinder. I used the 4th cycle reading but did a 5th cycle to see if the compression continued to rise after the 4th, which it didn't. Also, I did the test twice, once with the adjacent spark plugs removed.
 

Koop

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YELLOW 69 modified GM Crate ZZ383, LS2 TBSS
Thanks for the advice, guys.

The #1 and #3 exhaust valves had a gummy buildup on the backside which, the machine shop tells me, came from worn guides. The rest of the exhaust valves were better but still had some carbob on the backside. The intake valves had white stuff on the backside.

I did the compression test with the engine warm, but I didn't open the throttle - I just turned it over 5 times per cylinder. I used the 4th cycle reading but did a 5th cycle to see if the compression continued to rise after the 4th, which it didn't. Also, I did the test twice, once with the adjacent spark plugs removed.

Those worn guides could be your oil consumption issue. I used umbrella shields in one of the old motors in my Vette when the plugs started fouling (stop laughing), they worked okay and kept the car alive for many more years. I drive my car into the ground and then rebuild it.

How is it you guys have these 70,000 mile cars? My car's odo has turned over 3x and the odo didn't work at all for a decade and when it did work it was intermittent at best. Then I guess mine only shows 80 some thousand if I don't count the roll overs. :L
 

Evolution1980

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How is it you guys have these 70,000 mile cars?
Some of us have this rather annoying thing called "weather" that we have to deal with. Or, won't deal with.
If I could drive my car year round, I would. Hence one of the reasons I'm heading southwest (relative to my present location).
 

USNA1969

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Sep 12, 2005
Messages
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Location
Maryland
Corvette
1972 Big Block/4-Speed
Those worn guides could be your oil consumption issue. I used umbrella shields in one of the old motors in my Vette when the plugs started fouling (stop laughing), they worked okay and kept the car alive for many more years. I drive my car into the ground and then rebuild it.

How is it you guys have these 70,000 mile cars? My car's odo has turned over 3x and the odo didn't work at all for a decade and when it did work it was intermittent at best. Then I guess mine only shows 80 some thousand if I don't count the roll overs. :L

I'm with Evolution. I don't drive it in the winter. In fact, I've put less than 5k on it in the 18 months I've had it.
 

Koop

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Kalifornia
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YELLOW 69 modified GM Crate ZZ383, LS2 TBSS
We have weather in So. Cal! We must've had a 1/4 inch of rain last month alone!
 

Evolution1980

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Vette79

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Jan 23, 2007
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Millersville, MD
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1979 L-48 Black Coupe
63K on my 79 Vette. Suffer during the winter, have fun with it the rest of the year. Could use a little Southern Cali weather!
 

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