Stuck valve?

[172driver]

Here's one for you A&P/systems guys out there. I had a power loss last night with a student after descending from 4K to 2K. He levelled off, added power and the engine began to shudder. Full power gave us about 2000 RPM with a terrible vibration, enough to make it to the nearest airport. Seemed to run better with a very lean mixture. We were in a C172 with the O360-H2AD, a model which seems to be having this problem quite often lately. The maintenance guys diagnosed it as a stuck valve.

My questions are these. Which valve usually sticks, intake or exhaust? Does it stick open or closed? Why do they stick, particularly in this engine? What kind of service is needed afterward? Is it enough simply to redo the guide and the seat or does the engine need to be torn down and inspected? What kind of service/inspections can be done beforehand to prevent this, if any? How about pilot technique...how does it contribute to the problem? We were in a powered descent, 2300 RPM, 500 fpm, probably not shock-cooling it.

It didn't seem like a terrible emergency, as emergencies go, but it could've become one...night over the forest. However, I do a lot of night and IMC flying, and I'm curious about the cause and possible solutions to this all too common problem. Thanks for your help.

 

[350DRIVER]

The worst thing about single engine aircraft is that you have just one engine. I make sure prior to launching into low IMC with students in single engine (training) aircraft is that their has been absolutely no mx problems or write ups pertaining to powerplant or anything else that could make for a bad day. I have heard of this happening due to other reasons...


I have seen problems occur when the students make abrupt throttle & mixture changes as well as not clearing the engine every so often when practicing simulated engine failures.
A 172R by any chance and what was the year.??

3 5 0

 

[172driver]

1982 172P

 

[A Squared]

Either valve can stick, although it is usually the exhaust valve. They can only stick open, they can't stick closed, as valve opening is positive (unless the valve train is damaged) It's better to have the exhaust valve stuck, as that mostly just affects one cylinder. A stuck intake valve could open the combustion chamber to the intake manifold, letting the fuel/air to the other cylinders ignite, which would really affect performance <g>

If hte valve wasn't stuck far enough open to allow it to contact the piston, it's a fairly simple fix. A good mechanic can drop the valve, ream the valve guide and re-install the valve without removing the cylinder. Takes a little more than an hour. If the piston struck the valve, you're probably looking at a cylinder change.

regards