Low compression on one cylinder on new engine

[Tango Mike]

YIO-360-M1B factory new installed by builder (not me) in June 2012. When I bought the airplane in September 2013, it had mineral oil for the break-in period, and I switched to Phillips X-C 20W50 at about 40 hours. Oil consumption had stabilized, cool cylinders, operation normal, as it has been since I bought the airplane, with no significant differences in cylinder parameters as viewed on the G3X engine monitoring system. The engine now has about 70 hours.

Today for my first annual, with the assistance of an experienced A&P, we found the following compression numbers on a cold engine:#1 78/80, #2 45/80, #3 78/80, #4 76/80.

With air hooked up and the #2 cylinder under pressure, we can hear air escaping from the oil filler tube. When we checked that same cylinder with a spark plug installed and rotated the prop through a compression stroke, we can hear air in the filler tube as well, but in comparison with that same test on one of the other cylinders, it didn't sound that much different. Ran out of time today or we would have repeated the compression test on a warm engine.

I'm going to call Lycoming in the morning to discuss this with them, but the A&P suggested that the #2 cylinder might not be fully broken in yet.

I'm curious if anyone has run into something similar and has suggestions about troubleshooting.

Thanks in advance for any help.

Tosh

[JonJay]

Run it and see if it comes back to normal. The rings may have aligned. They move around all the time.
If it does not, boroscope the cylinder. It will be very obvious if it has glazed. I seriously doubt it since your other parameters appear normal.
Those cylinders should have broken in many hours ago....

Check should be done on a warm engine.

[DanBaier]

The initial ring seating happens fast. On the last engine I put together, it was flown right from the shop with no test stand time (to be debated in another thread). The point is, the temps sharply dropped at something like only a half hour. Afterward, it seemed fully broken in at around ten hours. (Lyc O-360-A with nitride cylinders.) Bottom line, with the hours you have, it should not be leaking past the rings unless you got lucky and the ring gaps are aligned. If a look with a borescope doesn't reveal any obvious problems, flying it for a bit should relieve the alignment. If not, the jug probably should come off.

Dan

[tim2542]

Are you sure you don't have any leakage from the valves? Open the throttle when listening for leakage. It seems much more likely you have something stuck to a valve seat, a loose valve seat, or hopefully just insufficient valve lash, like the hydraulic assembly did not get bleed down correctly before the lash was checked, or it was otherwise done wrong.
Run it, recheck it. That kind of leakage should be easy to find, and not likely to be the result of incorrect or insufficient break-in unless you broke a ring or holed a piston. But you said the leakage through the case vent seem equal among the 4.
Other thoughts, but you are sure the test was done on the compression stroke and not 180 away where both valves will be slightly open?
Good luck, be interested in hearing the verdict.
Tim

Edit....if the hyd assy was not bleed down the error would be too much lash, not too tight.

[Jesse]

As others have suggested, I would run it and recheck it. The rings could be aligned and you are blowing air into the case. Seeing only 45 seems low, but not impossible. Do you hear any air through the exhaust? If it runs smoothly, run it up before you shut down. Check it warm.

Definitely not an un-broken-in cylinder. I have checked compression on cylinders never run and get in the 70's.

My guess, rings aligned and running it will solve the problem.

[jrs14855]

I had a 540 with new Lycoming cylinders that had maybe 15-20 hours since overhaul. It was running good but not perfect. After a great deal of troubleshooting we found low compression on one cylinder. Pulled the cylinder, it was slightly out of round. The reason was never satisfactorily explained. The overhaul shop was able to hone the cylinder round and it ran fine after that. I had never heard of such a thing before and not since.

[turbo]

it is amazing how easy it is to check your comps by just turning it over with the prop. a 75 or better with give you quite a bit of resistance. now that you know this it should be easy to keep and eye on that pesky cylinder. let us know when it comes back up.

[Tango Mike]

Thanks for all the info.

The air was very definitely leaking out of the oil filler tube and not the out of the induction or exhaust. I could hear it and feel it against my palm with the dipstick removed.

We did the check cold because this A&P and I have always done that first on two other RVs (both with about 500-600 hrs SMOH), using the philosophy that if they check good cold, there's no reason to do them hot. For about six years now, those cold comps have averaged about 75-76, so I was expecting that or better on a new engine. Color me shocked.

I've got a copy of TCMSB03-3, which The Savvy Aviator Mike Busch has labeled as one of the best resource documents for differential pressure test and borescope procedures.

The plan for tomorrow is first to borescope the leaking cylinder and one of the good ones to see if there is any visible difference. If there's nothing obvious, we'll warm it up and check again. If that doesn't result in a better test, I'm going to do a lot more troubleshooting before removing the cylinder, including a test flight to record CHTs and EGTs. If all four cylinders are showing normal temps, I think I'll follow Busch's advice to fly the airplane. From everything I've read/heard, the days of being in a hurry to pull a cylinder off are long gone.

I'll post results as soon as I have some. Thanks again for the excellent comments.

Tosh

[Tango Mike]

Prior to this annual, I would have never believed that a differential pressure test on a cold engine could have resulted in three cylinders testing at 78, 76, and 78, and one cylinder at 40. Or that after a 10 minute ground run, the low cylinder would test at 77. It appears that maintaining airplanes, like flying them, is always a learning process.

One tidbit passed on by a Lycoming rep this morning is that if you suspect excessive leakage might be due to ring gap alignment, flying the airplane rather than conducting a ground run can be a more positive way of getting the rings to rotate. When the engine is driving the prop (like in climb or cruise), the pressure on the rings is much different than when the prop is driving the engine (like in a descent), and the latter condition is more conducive to ring rotation.

My apologies for cluttering up the forum with what turned out to be a non-event. If I had been able to ground run the engine the day I found the low pressure, I probably wouldn't have posted anything about it. But as it happens, the replies I received that evening eased my concerns, and I really appreciate the cool heads of experience among the members.

Tosh