LS3 low compression cylinder 3
Thread Starter
10 Second Club
Joined: Jan 2010
Posts: 2,317
Likes: 20
From: Hurley, VA
LS3 low compression cylinder 3 Went to tune a friends 10 SS and got a misfire and knock retard. Said it had the miss for a long time. Well long story short I’m getting 175psi on cylinder 1 and 50s on cylinder 3. I tried oil in the cylinder and still in the 50s. I’m thinking it’s in the head. Anybody got any opinions or another way I can’t think of to verify.
Thread Starter
10 Second Club
Joined: Jan 2010
Posts: 2,317
Likes: 20
From: Hurley, VA
Well I hooked air to the cylinder and couldn’t seem to feel air coming through the intake or the header but you can definitely hear the air going straight through. I can take my hand and lay it between the valve springs and feel air hitting my hand but can’t tell if it’s coming from the intake or exhaust. And both ports held gas with no leakage. Almost like it’s coming through the guide but it would have to bypass the seat for that to happen. How did they hold fuel? I had both rockers removed.
Staging Lane
Joined: Jul 2017
Posts: 81
Likes: 12
Did you get a reading of % leakdown, or did you simply apply shop air pressure to the cylinder? Was the piston at TDC?
I'm guessing the air you're feeling on your hand is air that leaked past the rings into the crankcase, and is exiting the oil-drainback holes in the head.
If you can fill the ports with liquid (with rockers off), then pressurize the cylinder and NOT blow the liquid back out of the ports, then you can't blame your 50psi compression number on a bent or burned valve.
You've got a broken ring land or a scuffed cylinder. I guess it could also be possible the HG is blown just right to cause similar, but I doubt it. Either way, that head needs to come off.
Report back on your findings!
I'm guessing the air you're feeling on your hand is air that leaked past the rings into the crankcase, and is exiting the oil-drainback holes in the head.
If you can fill the ports with liquid (with rockers off), then pressurize the cylinder and NOT blow the liquid back out of the ports, then you can't blame your 50psi compression number on a bent or burned valve.
You've got a broken ring land or a scuffed cylinder. I guess it could also be possible the HG is blown just right to cause similar, but I doubt it. Either way, that head needs to come off.
Report back on your findings!
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Verdad Gallardo Thread Starter
10 Second Club
Joined: Jan 2010
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From: Hurley, VA
Head is all ready off. Piston looked good. The fuel was put in with the head off the car with hopes of well that’s obviously the problem. Don’t have a leak down gauge so it just got air.
Staging Lane
Joined: Jul 2017
Posts: 81
Likes: 12
Sorry I can't help, but I'm totally confused on what diagnostics have been performed and in what order.
Buy or build a leakdown tester. Whenever engine health is in question, use it the "normal way", pressurizing each cylinder at TDC of compression stroke. Find the source of leakage. Sometimes the flame from a cigarette lighter or a smoke generator (lit cigarette works well for this) is helpful in finding air leaks.
Buy or build a leakdown tester. Whenever engine health is in question, use it the "normal way", pressurizing each cylinder at TDC of compression stroke. Find the source of leakage. Sometimes the flame from a cigarette lighter or a smoke generator (lit cigarette works well for this) is helpful in finding air leaks.