High CHT # 3 cylinder

[John Wedge]

24-180 with 245 hrs new Lycoming cylinders, 1375 SMOH, 30 minutes since annual. EI UGB-16 EGT & CHT installed about 130 hrs ago with in well CHT probe - not spark plug ring. During annual plugs cleaned, were burning good, exhaust system removed for repair of crack and replaced with new gaskets etc. Nothing else removed from engine. Exam of exhaust indicated have been operating a little on lean side during cruise. Extensive pre flt in which all baffels / cowling in place. During 20 minute test flt after annual I had 18 point list to check on systems. #3 CHT always hottest and I monitor it on takeoffs and climbs. Normal has been 390 to 398 even during July-August. (399 is highest I have ever seen it) Test flt takeoff & climb - WOT 2700 rpm for 1 thousand feet, OAT about 94 F and #3 was normal at 398 at Vy. After all checks, landed, topped off fuel and departed for home. On takeoff and climb again WOT 2700 for 1000 ft at Vy and then climb at 25/25 at 110 mph indicated to 3,500 ft. #3 CHT went up to 413 deg. then back to normal of about 350 at cruise. Next day flew traffic pattern 3 T&G with normal CHT however when power pulled back to 16 inch on base engine developed vibration. Landed did runup and lean and it cleared. Flew 2.5 yesterday afternoon and engine run good, #3 up to 399 on takeoff 90 deg OAT, flew night at 79 deg OAT and on 3rd takeoff climb at 95 mph #3 goes up to 433 deg. EGTs seemed to be near normal for past on #3 however did not monitor them that close. Is something going on in #3 cylinder or something changed from annual work? Your thoughts, experience, advice appreciated.

[John Johnson]

High John,

I've been struggling with high CHT's on my #3 & 4 cylinders (O-540: see elsewhere in this forum) but your problem sounds different to me. The thing that concerns me is the inconsistency of your CHT "fevers" and the episode of engine roughness. Since your baffles are good, I don't see how it could be an airflow problem. The place I'd check first: Magneto timing needs to be exact - I found mine about 2.5 degrees retarded, and resetting to 25 (as nearly exactly as my mechanic could make it) seems to have brought all my CHT's down about 10-15 degrees (though still not down enough). Advanced timing is even worse, as the detonation margin drops way off with advanced timing, even sometimes if advanced only 1 to 1.5 degrees. If your timing's advanced at all, I'd get it set back.

If the timing's perfect, it might be worth opening or (re-opening if that was done at annual) the magnetos themselves to be sure there's nothing malicious going on in there and no evidence of arcing or such.

The next concern I might have would be intermittent sticking of a valve in #3. Usually it's exhaust valve sticking that causes intermittent roughness, but usually that causes the CHT to drop and EGT to rise. Did you notice what those two parameters were doing while the engine was rough? A sticking intake valve, much less common, will cause roughness and high CHT, I understand, and is a much more serious problem from what I've heard - possibly leading to prompt engine failure if not resolved. It would worth a quick borescope to be sure there's no obvious evidence of valve damage or burning, and shouldn't be at all expensive to do.

Of course uneven fuel flow will cause uneven CHT's and EGT's, and all our non-fuel injected Lycomings have some of that, but it's hard to see how that could suddenly cause a single CHT to go to 433. And I don't see how the annual inspection could be a cause unless they mis-timed it. Sounds to me like some sort of problem related directly to #3 cylinder or it's ignition sytem.

I'm no mechanic (you could guess that) but I've spent a lot effort lately researching my high CHT problems, and the above are some of the things I've learned lately. Hope it helps, and let me know what you find out - it might help my problem as well!

Regards,
John Johnson

[John Wedge]

John,
Thanks for your reply. I have been reading your problems also but did not know enough to make a suggestion. Now I got this and am stumped. It was my first thought following the rough engine was a stuck valve but the run up seemed to clear that and no problem with roughness since, flew it again today with no roughness. Still could be valve, I had that happen before (no prior indicators until BAM) which resulted in the new cylinders - I was lucky then even being low I was within 3 miles of airport and it made it there. Today two of us flew so I could cycle through the cylinders checking CHT and EGT for comparison. #3 went up to 410 while climbing to 3,500 and all others were in what was normal in past and EGT on #3 stayed within 10 degs of #4 EGT. I have went into the trouble shooting manual for this UDG-16 and it covered things like you mentioned - fuel flow, timing, valves etc however my "symptoms" don't make sense with their info. You have good point about intake valve - I was mainly looking at their info for exhaust which did not fit this problem - I am going back to look more into an intake valve problem. I am going to borescope it also. Thanks again. John W.

[John Johnson]

[Zach Grant L1011jock]

[John Wedge]