A puzzling compression problem

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming.

Q: My engine, which now has 300 hours on it, is an O-290-D2B which had a new-limits field overhaul. It has settled-in as a nice smooth-running, economical and reliable powerplant with plenty of power. It gets about 7-8 hours per qt. (Aeroshell 100) at 6.5 gph. All EGTs and CHTs are normal and it always has a clean oil filter element (it is cut and checked at each oil change, 50 hours or six months), and has a healthy oil analysis report (no anomalies or unusual artifacts). However, I’ve noticed the compression going down at each annual on all cylinders and a recent pre-annual check-up has them in the low 60s (over 80) with a static compression test. It also takes a bit of “prop-rocking” to get it to settle in. I can hear some blow-by in the dip-stick tube. It seems uncanny that all cylinders would have this problem, so I’m a bit puzzled.

This year at Sentimental Journey in Lock Haven, Pa., I was talking to an engine guy from Canada and he said the O-290 was a great engine but had a tendency to break the top piston rings (sometimes with very little time in service) and usually the only early evidence would be low compression on a static test.

I’m not sure what the best course of action might be as it seems like a tough call to pull all the cylinders on a great running engine that seems otherwise healthy in all respects. The cylinders were re-worked at overhaul by a big outfit in Texas including dye penetrant check, all new valve guides and seats, Cerminil process, new valves and pistons/rings, some welding, machined exhaust ports/flange, new ex. port studs, new rocker boss bushings, etc.

Any ideas or advice is greatly appreciated.

ROBY DENMAN

A: Reviewing the information you provided, I can’t think of anything that may be a contributing factor to the compression check readings you’re getting. Just for the heck of it, I want to be certain we’re on the same page as to how you are taking your compression checks. You mentioned that your readings are in the low 60s over 80, so I know you’re doing a differential type compression check, which is the industry standard. The only thing you didn’t mention was whether the compression check was taken with the engine cold or at close to normal engine operating temperature. For the most accurate results, the engine should be at normal operating temperatures.

You mentioned you can hear the blow-by coming from the oil filler tube, which would indicate air being passed by the piston rings. The fact that you’ve put 300 hours on this engine since its overhaul in 2005 certainly tells us that extended periods of inactivity is not a factor in this case. There is always a chance that this could be a result of a stuck piston ring, but again, with your choice of engine oil and change frequency, I’d have my doubts. With regard to the conversation you had at Sentimental Journey about the O-290 having a tendency to break the top piston ring, I can’t recall ever hearing that before, but that’s not to say it doesn’t happen. Let’s assume you do have a broken ring. Since your compression check numbers are nearly all the same, should we be led to believe each cylinder has a broken ring? I would think not. However, if you want to pursue this theory, I’d suggest having your maintenance facility perform a thorough borescope inspection of each cylinder. If a piston ring has broken, there is a likely possibility that it will leave score marks on the cylinder wall and should be visible through the borescope inspection. Another possibility, which in your case I also have serious doubts, would be a stuck piston ring. Here again, having this occur in every cylinder would be a rare situation.

My only suggestion at this point would be to conduct a good hot differential compression check and record your findings. If you have one cylinder that exhibits a lower reading than the others, then this would be my candidate for the borescope inspection. If there is no indication of anything being amiss, then I’d probably continue the aircraft in service and check the compression again in about 10 hours or so. Once the compression reading falls below 60 over 80, further inspection and corrective action should be taken.

Another option you have is to take the cylinder with the lowest compression and remove it for a closer inspection, but this costs money so I wouldn’t do this until you see a reading below 60/80. If you do end up removing a cylinder and, of course, depending on what you find, be certain to hone the cylinder and install new piston rings when you reinstall it. Failure to do this could lead to an oil consumption problem on this cylinder and you could end up having to remove it again to correct this problem.

I don’t feel I’ve really offered too much here except a commonsense approach with some troubleshooting using the information you provided, but let’s hope it’ll lead to a satisfactory conclusion. All of your other maintenance practices are very good and, no doubt, better than the norm, for which you are to be congratulated and I encourage you to continue these in the future.

Send your questions to: AskPaul@GeneralAviationNews.com.