The Ford Barn

rotorwrench · 04-05-2020, 08:38 AM

Leakage past the exhaust valve could be a damaged valve or seat face but it could also just be some carbon keeping the valve from seating all the way. The rings & cylinder walls likely have a good bit of wear for the overall low compression readings.

I do regular leak down checks on the aircraft engines. We use 80 psi input with the pistons at TDC on compression stroke. The Lycoming service instruction lists 65 psi minimum reading with no more than 15 psi difference between cylinder readings. If I have a low one, I always run the engine and pull some power with it before rechecking the pressures. This will usually clear carbon from valve seat surfaces and allow compression rings that might have ring gap alignment to move out of that alignment. If results are the same, it's time to make repairs.

On an auto engine, it's not as critical that things be up to that level of safety. If it still runs on all four and doesn't use too much oil then it's still operable for a time. If it would double as a mosquito fogger then it might be time for an overhaul.

04-05-2020, 08:38 AM	#5
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 17,410	Re: Comments on these leak-down test results Leakage past the exhaust valve could be a damaged valve or seat face but it could also just be some carbon keeping the valve from seating all the way. The rings & cylinder walls likely have a good bit of wear for the overall low compression readings. I do regular leak down checks on the aircraft engines. We use 80 psi input with the pistons at TDC on compression stroke. The Lycoming service instruction lists 65 psi minimum reading with no more than 15 psi difference between cylinder readings. If I have a low one, I always run the engine and pull some power with it before rechecking the pressures. This will usually clear carbon from valve seat surfaces and allow compression rings that might have ring gap alignment to move out of that alignment. If results are the same, it's time to make repairs. On an auto engine, it's not as critical that things be up to that level of safety. If it still runs on all four and doesn't use too much oil then it's still operable for a time. If it would double as a mosquito fogger then it might be time for an overhaul.