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Faunsdale, AL | I’ll second everyone else that says you’re worrying too much about the leak down test. Compression is going to tell the story on the effective cylinder condition. What you’re looking for is mainly the balance/difference between cylinders. Go ahead and block throttle open if you want to or leave it at idle…….just do all 8 cylinders under the same conditions.
The ones that show up low, hook up the leak down test to each one (with ignition OFF) turn on air flow and rotate engine by hand with ratchet on crankshaft pulley bolt until the pressure on the cylinder you’re hooked to starts to rise. That will be when the valves are closed and piston is coming up to TDC. When you get close to TDC, the engine will stop trying to turn backwards and you should be able to leave it stopped on or about on TDC and check for leaks back into intake, out exhaust or into crankcase. If you rotate too far and pass TDC, the air pressure will push piston down and if you’re using a ratchet it’ll spin it without hurting anything. If you’re using a wrench or a bar on the socket, watch out it doesn’t pinch you as the engine rotates! If you go past TDC and engine rotates, just turn it on around and come back up to TDC again.
The leak down tester has been used to diagnose and condemn aircraft engine cylinders for many years but with the cost of replacement cylinders and the lead time to get replacements and cost to have them installed, there is more interest these days in avoiding replacing them if at all possible. Another reason to avoid unnecessary cylinder work is there’s worrying statistical evidence of higher than normal chance of engine failure following cylinder replacement! In an airplane, it doesn’t just leave you stranded on the side of the road!
There’s pretty good evidence that aircraft engine cylinders that fail the leak down text spec from wear, not burned valve or broken piston etc are capable of providing very close to full power of a new one. One of the manufacturers actually tested an engine with the piston rings left off and showed that it was capable of around 90% of expected power. So I think the lesson for automotive folks is don’t embrace leak down testing to the point of having to meet an exact spec. Instead use it as a diagnostic tool to support a diagnosis of bad valves, blown head gasket, broken piston etc
Compression testing is not too much different. If you’re hung up on exact numbers,something like a weak starter or battery can result in condemning an engine and unnecessary expense. If the valves have been ground or the head has been milled or block decked, cylinders bored oversize etc, the compression test results will be slightly different not to mention the gauge might not read perfectly accurately.
It’s the balance between cylinders that’s most important, and even if you show up some low ones, you need to confirm the problem by spark plug analysis, putting some oil in to seal up rings, leak down test etc. It’s only a diagnostic tool.
I’ll say I really think you’re more likely looking at the wrong thing for the problem you describe. There’s replacement HEI distributors available for your engine (and many others too) that are a plug and play solution to ignition problems.
Edited by ccjersey 12/12/2024 13:50
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How to do a compression test and leak down test on Chevy small block quesions.
