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I have an LS6 from a 2002 Z06. It was partially apart when I got it, so I've never had it together to run. No intake, no electronics, and it is sitting on a lifting table. I added a starter and spun the engine for a dry (and cold) compression test. 180+ in 7 cylinders. Cylinder 4 was at 90 psi, both dry, and with a few squirts of oil. All of the valve springs appear intact, including the ones on cylinder 4.
I have a borescope on the way. Any tips on what I should be looking for, and any other tests I could try before pulling the head? I should take a video of the valvetrain, at least.
I'll update once the borescope arrives. Since I usually post in the C3 Forum, and C3 option combination records do not exist, do I need to mention that this engine is destined for my very rare, 1 of 53,807, 1979 Corvette? It is 1 of only 2606 with an Oyster interior, if that helps.
I don't have anything to directly help you out, just some general thoughts:
Might be worth trying a leakdown test to see where the loss of compression is going.
Yellow valve springs on 2002 LS6 are a known/common failure point. While you're in there it's probably wise to replace them.
Thanks! I ordered a leak-down tester. From my own observations, it seems the exhaust valve isn't closing fully (not the intake, as mentioned above), but that will be another good test.
If I end up pulling valves, I have a set of the better springs ready to go.
My guess would be a bent valve or carbon build up. If you can rotate the engine to where both valves should be closed and put compressed air in the spark plug, see if the air is going into the crank case (some will always), intake, or exhaust port.
If it is carbon or something in the way, you can try taking the rocker off and smacking the valve tip with a rubber mallet to let it open and close quickly. It might knock something off.
My guess would be a bent valve or carbon build up. If you can rotate the engine to where both valves should be closed and put compressed air in the spark plug, see if the air is going into the crank case (some will always), intake, or exhaust port.
If it is carbon or something in the way, you can try taking the rocker off and smacking the valve tip with a rubber mallet to let it open and close quickly. It might knock something off.
Thanks for the tips!
It is almost certainly the exhaust valve leaking. I'll re-test when the leakdown tester arrives, and I retrieve my air compressor.
The intake, and exhaust, are off right now. Is there a danger of spraying both valves (or all 16 valves) with SeaFoam? I'm assuming I'll have to change the oil afterward, but will it wash down the cylinders? I can't run the engine except to spin it with the starter.
MetalMan is right - a leakdown test will reveal the issue. Hopefully it's a valve issue - pull the head, deal with the problematic valve, throw on a new head gasket, put the head back on, and you're in business.
I would think pulling the valve cover would be cheaper and quicker (not waiting wise) than the leak down test. A loose rocker with the piston at TDC on compression tells all, does it not?
I agree that there is something wrong with the valve train. I would pull the heads and eye ball the valves in that cylinder since your engine is out already. I think you need to do that to fix the issue anyways.
I would think pulling the valve cover would be cheaper and quicker (not waiting wise) than the leak down test. A loose rocker with the piston at TDC on compression tells all, does it not?
I agree that pulling the valve cover and looking at the valves is cheap, and won't hurt anything, but we don't know that it's a valve issue. IF you do the leakdown test - you will be listening for air escaping. If air escapes through one of the valves - we know there is a problem with that particular valve, but a significant amount of air may also be escaping thru the crankcase - which is indicative of bad rings. I'd kinda like to know that before I pull things apart. It would really be annoying to fix a valve issue, reinstall the motor, and find that you still have low compression due to a ring issue.... Unlikley - YES. Impossible - Hardly.
Does not the wet compression test result tell us that it is not a ring issue?
I had hoped so. With a few squirts of oil, the compression in that cylinder went down, from 100 to 90 psi. I'm still getting two exhaust pulses, but shining a light in the spark plug hole shows no light.
I'll try again with the borescope, and there's no reason NOT to do a leakdown test. But I'm betting the head is coming off...
Does not the wet compression test result tell us that it is not a ring issue?
It's not 100%... If a compression ring is broken - oil isn't gonna make a difference. And just because oil is squirted into the cylinder - doesn't mean it will end up were the issue is. Think if the piston is at or near BDC, you shoot some oil in - by the time it runs down the cylinder - you probably don't have much oil to seal up a bad ring seal.
The heads and valvetrain stuff are cheap the motor is already dissembled there is no good reason not to remove the head, service the valvetrain, clean up the pistons, perform inspection. While heads are off look at lifters also. Look for dirt, debris, sand, grit, any of that in the oil supply and motor turns to trash, materials embedded to bearings, ruining the lifters.
I was able to use my borescope and leakdown tester today.
First, there is nothing to leakdown test. The exhaust valve does not seal. When air pressure is applied, there is a constant leak to be heard through the exhaust port (and not through the intake port, or the oil dipstick hole).
Looking at that valve with the borescope shows some orange debris on one side. The last image shows me sliding a zip-tie through the gap after I opened the valve by spinning the engine a bit. Perhaps I could pick it off with the correct tool, or clean it with SeaFoam if the engine were running, but that's just wasted effort. Sorry about the photos, the borescope does not focus as closely as it should.
So, I'm thinking both heads come off and I clean the valves, and hope that fixes it.
Any suggest on head gaskets? Hopefully that's all I need to replace.
Low compression in Cylinder 4, 2002 LS6
