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Been having gradually increasing CHT on my #4 cylinder over the past two weeks.  Last flight resulted in a immediate return to the airport at minimum throttle to maintain CHT below max limit* (just barely and in a slight descent while setting up for landing.)  *Downside of ECU is that we have no mixture to cool things down with.

All this was after trying to isolate/resolve the issue by verifying/troubleshooting probes and a new injector with no improvements along with the 100 hr engine service. There had been no change to baffling or cowl and I was puzzled.  What was going on?!?

This morning I pulled the cowls and cylinder head ram airbox (again) and did a differential leak check.  Obvious air leak on top of the left side cylinder bank along with a 80/15 diff test numbers on the #4. but good on the #2 (80/76)..did a soap bubble spray with slightly pressurized cylinder to isolate the source (multiple leaks around the head and cylinder interface were discovered)  The biggest surprise to me was when I was pulling the other spark plug on the #4 cylinder to borescope things. That's when I noticed the crack across the plug hole!  I had also noted a very slight looseness/wobble on this particular plug during my 100 hr, but simply retorqued after inspecting the plug.  Cleaned things up a bit and there was certainly a distinct discontinuity across the cracks when using a pick.  I'm hoping the cylinder is in good condition and I can simply replace the head assembly.   The cylinders are new with 139 hrs SMOH, but the cylinder heads are original with approx 295 hrs.  The worst part is missing this year's International Sea Plane Fly-In at Greenville, Maine because of a maintenance isse.  I guess there is aways next year!

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"Wow" that was a good catch by pilot / builder. I'm thinking its time to consider a repower with a more traditional "proven" aircraft engine. If you lose confidence in the engine the choice is simple.  

Sometimes I think I'm getting closer to seriously considering crossing that bridge Bob!  The cracked lug on the early model cylinder with the resultant oil leak I can accept as being part of the "early adopter" crowd.  The oil filter backing off during cruise - well I will attribute that to poor harmonics on the Catto prop/engine combo and not being aware/proactive enough to safety the filter per standard practice on GA aircraft - though the UL mx manual does not call out for having it safety wired!  I have a harder time on having a cylinder head crack after less than 300 hrs.  Though, even the major manufacturers have had issues with a variety of cylinders, etc. with numerous SBs and ADs.  I will say that I am glad to have CHT and EGT monitoring on all cylinders!  Paying attention to things helps with avoiding some lousy outcomes.

Backside view of cylinder head crack after removal from cylinder.  Cylinder walls look good and so does the piston.

David. Years ago I had a Turbo Arrow and while climbing out of the airport I had a cylinder temperature rise to the point it blew a hole in the cylinder. I was about 20 miles from the airport, pulled the throttle back, full rich, kept the cowl flaps open and and initiated a dive. All to no avail. Before long there was smoke in the cabin from oil into the exhaust. 

Upon tear down of the engine the opinion was that an injector became partially plugged leaning that cylinder to the point the temperature continued to rise. Who knows how that would happen. 

Is it possible the same thing happened in your case?

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