C3 Tech/Performance V8 Technical Info, Internal Engine, External Engine, Basic Tech and Maintenance for the C3 Corvette

Calling all mechanics!

Old 04-26-2019, 12:08 AM
Kirk H
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Default Calling all mechanics!

A while ago, I was concerned with a ticking sound, lower compression on #3 (by 15 to 20%), and slightly unsteady vacuum reading at idle (13 inches or so) on '68 L-36. It started fine, ran strong for the most part, but idled a little rougher than it should. I finally got around to doing a leak down test after not driving for over a year and was slightly surprised to see the cylinder holding only 30%! 70% was leaking out the crankcase and some of that I could hear through the exhaust pipe as well. The cylinders on either side were at 9% at 12% loss. Should my next move be pulling the oil pan and looking for broken ring parts or pulling the head? I used an endoscope but did not see any hole in the top of the piston. The motor was rebuilt in 1990 and has about 20,000 miles on it. Forged TRW pistons and stainless steel valves were used and the only non stock change was using a cam with 110 lobe centers with same lift and slightly longer duration exhaust. I would have thought a massive difference in leak down percent would have caused more dramatic symptoms but I have not come across this before. On the motor prior to its rebuild, a ring land busted off, snapped the oil pump shaft, and I was burning blue smoke like crazy. No blue smoke or oil fouled plugs, but that cylinder plug was running on the black side.
Thanks for any suggestions on how to proceed.
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Old 04-26-2019, 05:32 AM
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Pull the head, remove the oil pan and push out that piston. You will see what you need to know, ring, piston, cylinder wall condition. T
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Old 04-26-2019, 07:41 AM
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Sounds likely to be a ring problem. Could have broken one.
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Old 04-26-2019, 08:21 AM
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sounds like you will be rebuilding your motor
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Old 04-26-2019, 05:08 PM
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I would do a wet compression test just to confirm the compression is getting by the rings. If the rings are good the compression should only come up slightly.
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Old 04-27-2019, 05:53 AM
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Why are you so sure it is leaking into the crankcase? The valves could be leaking off, too. For the cylinders to seal well, the valves AND rings need to be doing their job. You might only have burnt carbon deposits preventing a couple of valves from closing completely. Worth diagnosing completely before going for a 'ring job'.
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Old 04-27-2019, 07:10 AM
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pull head. lay it on it's side, either ports up. fill ports with water. watch valves for water leakage. flip over test other 4 valves. now pull pan and push out pistons. IF you can handle these big-*** heads without hurting fenders. replacing pistons in place is not a huge issue. it also stops you from taking the block to a machine shop and saying..."Here. I will pick it up in about 3000 dollars."

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Old 04-27-2019, 02:46 PM
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Leaking out the exhaust pipe is an exhaust valve.

You say its ticking too? Any reason to suspect a bent valve from floating the valves?

I'd go after a valve problem first.

Some confirmation of that may be to do a compression test on that cylinder. First dry then oiled, if there is a big difference in compression then I'd be looking at rings, if only a little difference or none then a valve.
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Old 04-28-2019, 01:12 AM
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I should have mentioned I pulled off the intake manifold and was able to cover and uncover the intake port on the head heard zero effect on sound. I put a huge cloth over the lifter valley and it muffled a lot of the sound. I did hear a small amount of air in at the end of the exhaust pipe, so I know some was escaping the exhaust valve. I had also ran a compression test on the engine prior to pulling off the intake and oil dramatically increased the compression reading.
Thanks for your input- it is very much appreciated!
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Old 04-28-2019, 01:18 AM
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Thanks for your input. I did pull the pushrods and they were not bent (as they were before my rebuild years ago). The lifters seemed ok and the lift was very close to tolerance. I also did do a wet compression test and it did significantly raise the pressure in the cylinder. I was hoping the ticking was something a bit more simple since I did not notice any blue smoke or excessive consumption. It appears as if I'm out of luck.
Thanks for all the responses- this is really an awesome site and has many friendly members!
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