Thread: Is pressurzing cyclinder a good substitute for leak down test?

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I'm still trying to limp along my old 283. It's got about 100k on a 1982 rebuild and has lots of blowby, after sitting in storage for 8 years. A gut at Napa suggested a leak down test, to see if it was worth putting new rubber valve caps on the valves, to stop oil from dripping into the valve.

A good comp leak down tester looks to be $75 or more, and the cheapo at HF is $40. I read that the HF one doesn't work that well. While reading about the testers, I read that one diagnostic test is to listen to where the air escapes after you put air in the cylinder. I bought a $7.00 hose that screws into the spark plug hole on one end, and connects to air compressor on the other end.

I pulled all plugs and found TDC and screwed in the air hose. At 25 psi, air was steadily leaking out of the cylinder. It was coming out of the hole in the top of the head where I think oil drains down (if looking at left side of engine, the top left of the left head). Also, air was coming out of the dip stick. I felt no air coming from next cylinder or the valves, and no bubbles in radiator. I heard no air coming from carb, and could hear a hiss from the exhaust pipe, but I think that was just transferred sound, as no actual air seemed to come out of exhaust pipe.

I did the same check on cylinders 3 and 5 on the left head with identical results.

I'm not much of a motor guy, but this seems to tell me that the valves on 1,3,5 are sealing, but that air is escaping from around the cylinder rings. So, nothing I do to the valves will stop the blowby - to stop blowby I need to at least replace the rings.

If that's the case, I'll stop trying to do a quick fix on this motor and drive it as is til I can get a different motor. Is my guess that the rings are shot reasonable?

Thanks for any input.

yuppers, rings are gone. Not unusual for an engine that has been sitting for awhile to develop a glaze on the cylinder that will not allow the rings to seat properly..

I fully agree with Dave and ad that if you hear air out of the tailpipe the exhaust valves are leaking also, you may not be able to feel it but if you can hear it they are leaking.

No---------pressurizing a cylinder isn't a good subsitute for a cylinder leakage test-----a cylinder leakage test puts in a smaller, well regulated flow/pressure so the flow can be measured as a percentage AND let you listen? for where it is going--just hooking an unregulated flow/pressure to the cylinder with a hose will rotate the motor by blowing the piston down away from tdc and the air will blow out exhaust or intake because of valve opening----------Its not accurate in anyway-----