IO-360 (180hp) #1 cylinder at 5/80

[bjohnson1234]

Originally Posted by WA85

Instead of all the conjecture about how and what your compression test indicates, possibly consult Lycoming's service instruction SI 1191A. While not regulatory, Lycoming probably has a better understanding of their engine design than most other conjectures.

Do your research - Lycoming and TCM cylinder compression tests have significantly different processes.

Again, not regulatory, but Lycoming and AC 43.13B have similar compression test processes which recommend that if a static compression is below 60 psi, further investigation should be considered. 5 / 80 psi is a dead give away for a massive leak somewhere. Possibly a valve is burned or crack in the cylinder. Leaky rings show up as air hissing out the oil dip stick or breather, air out the exhaust or intake. Spray the cylinder head / barrel with soapy water solution to look for cracks in the head / barrel.

We have a lot more data now and I think what it has shown is that some of these recommendations were not evidence-based. Is 5/80 a problem that shouldn't be ignored? Absolutely, but I want engine maintenance to be science, not dogma. Consider TBO for example. If you want to increase your odds of a catastrophic failure then by all means overhaul an engine that is running well and shows no signs of major problems. On its face it seems like a conservative recommendation and in a perfect world it might be, but we have plenty of data now to show that the first 200 hours on a new or overhauled engine have the highest risk of catastrophic failure. Every time you take something apart you also increase the risk that the person doing the assembly/disassembly will make a mistake. Lycoming is unlikely to change their recommended practices because those changes require FAA approval.

[sblack]

I don't know what is causing this problem, but I do know that if there is a leak at an exhaust flange it will erode the interface. I would expect if there is a leak at valve it would also erode the valve seat. If there is a stuck ring, because the engine has not been run regularly then it might free up, or it might not. My point is that there is "potential" for something that will damage the engine, but it might not be anything serious. I would certainly keep investigating.

I think Mike Busch is saying that the compression test can show a bad result which can mean either something trivial or something major - you just don't know, so don't go ripping off a jug and undoing through-bolts just based on that. He certainly isn't saying don't worry, be happy. Fly for a few more hours, borescope, check the filter for any metal, redo the compression test, do a very careful visual inspection around every possible leak location looking for soot etc. Listen for where the air is leaking out - it's gotta go somewhere. Eventually something will turn up or it will all go back to normal. But you probably don't want to head out over to Russia like Vlad until you have satisfied yourself that you know what it is. Please let us know.

[tim2542]

With 5/80 you have a horrendous leak that ought to be obvious and easily located in the intake, exhaust or crankcase vent. With those numbers there will be a torrent of wind coming out somewhere. Pretty simple to find with those kind of numbers. How about the possibility you are on the valve overlap position instead of TDC?
Tim Andres

[lr172]

Originally Posted by tim2542

With 5/80 you have a horrendous leak that ought to be obvious and easily located in the intake, exhaust or crankcase vent. With those numbers there will be a torrent of wind coming out somewhere.
Tim Andres

The air goes through an .040" orifice. There is never that much air flowing. Dial up your unit to 80 psi and hold your thumb over the end to produce 5/80. You can't even feel the air moving once 5" from the exit. It is far from a "torrent of wind."

[bjohnson1234]

Originally Posted by lr172

The air goes through an .040" orifice. There is never that much air flowing. Dial up your unit to 80 psi and hold your thumb over the end to produce 5/80. You can't even feel the air moving once 5" from the exit. It is far from a "torrent of wind."

I think you have that backwards. 75 psi is leaking, not 5 psi.

[lr172]

Originally Posted by bjohnson1234

I think you have that backwards. 75 psi is leaking, not 5 psi.

5/80 means 5 PSI on the post-orrifice gauge and 80 PSI on the source gauge. Yes, the 5 PSI reading is 75 PSI less than the measurement with no leakage at all. That is what I was referring to in my post. Eventhough it is very large movement of measurement on the instrument, it doesn't translate to very large movement of air.

I suggested setting the source gauge to 80 PSI with an open end. The second gauge will show 0. Apply a bit of resistance to the air outlet and the second gauge will start going up. Stop when it reaches 5 (the OP's reading) and feel how much air is flowing. You will find it is not significant and definitely not a torrent.