High #4 CHT following overhaul

[Charles]

My Lycoming engine was overhauled 2 years ago. Aside from intermittent rough running which took a while to diagnose and eventually fix by replacing a defective ignition harness, the engine works incredibly well but the #4 CHT is higher than the others by 50-70F. The shop was diligent about addressing the issue: after confirming the accuracy of the sensors, swapping spark plugs and injectors, checking baffle seals, etc., they went so far as to replace the entire cylinder with a new one, which made no difference, and then changing the camshaft, a huge job which also made no difference. They claim the engine runs perfectly in the test rig with no abnormal CHTs.

They've ruled out timing issues because the other cylinders are normal. Also ruled out mixture issues because injectors were tested, swapped, etc., and because the #4 EGT peaks at the same mixture as the other cylinders when leaning.

I am reaching out to the collective to get suggestions on what else to try. One thing I suspect was overlooked is the seating of the hard baffles, which we're going to look at. Any other suggestions?

[MarkA]

Have you run a GAMI lean of peak test https://gami.com/gamijectors/leantest.php to see how balanced your fuel flow is? Since you already checked the injectors, one possibility to consider is the fuel manifold may be causing a fuel flow difference between cylinders.

[Charles]

Yes, we ran a lean test. All cylinder EGTs peak within 0.2gph of fuel flow while leaning. The high #4 CHT is observed at all mixture levels including full rich. Would you agree that this eliminates fuel flow imbalance as a potential cause?

As an update, a close inspection of the baffles yesterday revealed no anomalies. We'll be running a ground test without the cowling to see if the issue is related to cooling airflow, the theory being that if all temps are equal without the cowling, then the issue is with the cooling, not the engine (and vice versa).

[BuzzB]

Had something quite similar with a new (factory rebuilt zero time) engine. Ultimately turned out to be the CHT sensor itself. Try swapping the sensors for hottest and coolest cylinders and see if that explains what you’ve been observing.

[Charles]

Yeah, we did that. We even tested the sensor by dipping it in hot oil and comparing with a thermometer.

[BuzzB]

What are your typical CHT's while in cruise? Please include your MP, RPM, and FF settings, as well as ALT and OAT.

[Charles]

Here are a few recent measurements, all at peak EGT:

2000 ft. 2400rpm, 24.1", 12C OAT, 9.2 gph: 389 377 370 439
5000 ft., 2400rpm, 24.1", 3C OAT, 9.1 gph: 360 360 346 423
6500 ft., 2400rpm, ~23", 6C OAT, 8.8 gph: 356 356 335 418

[blsewardjr]

Charles- I'm not a mechanic just a DA40 owner who has had to deal with his fair share of mechanical issues so take this for what it's worth. Your CHT pattern looks a lot like the pattern when breaking in a single new cylinder, although much more extreme. As I understand it, high CHTs while breaking in a cylinder are caused by friction between the piston rings and the cylinder wall and decrease once the ridges in the cylinder are worn down. The fact that your first #4 had the same number of hours as the rest of cylinders mitigates against a theory that a normal break-in period is the current issue, i.e., you would have had high CHTs in all four cylinders at that point. Similarly, since you presumably replaced both the cylinder and the piston rings when you got your second #4 cylinder, a defect in either that persists across two separate cylinders/rings seems implausible. However, could it be that the #4cylinder is somehow not getting sufficient lubricating oil, thus creating higher than normal friction and CHTs? Oil is also a coolant, although I don't know to what degree that matters for cylinders. I don't know if or how an individual cylinder can get less oil than the other cylinders, but if it can that might be the solution. Hope you can solve this soon. Bernie

[BuzzB]

Charles - I also went through a long (and expensive) journey when I got a new engine that exhibited one cylinder (like you, #4) that ran too hot. Baffles checked and re-checked - no joy. My coolest vs hottest CHT delta was about 40F. Yours appears to be double that! You said they've told you all is nominal running on a test stand - this certainly does make it sound like an insufficient air flow problem, even though you've had that checked. For what it's worth, here are a couple of things to consider, based on my experience. May not help solve the root cause, but should help get your #4 CHTs below 400F:

1. The measurements you supplied were all running right at peak EGT. What do you see when you run substantially ROP, or even better, slightly (say 15F) LOP? Moving away from peak EGT should lower all your CHTs, and hopefully get #4 below 400F. The fact that all four cylinders peak within 0.2 gph implies your engine will run very nicely LOP
2. Especially running LOP, try running "over square" i.e. push your MP up to 25" and reduce your RPM to 2200. See this article for more on that... https://www.aopa.org/news-and-media/all ... ver-square
3. Lastly, I recommend signing up with Mike Busch's Savvy Aviation. It'll be a minor expense compared to what you've already incurred. You then send them your engine monitor data, and they have really top notch independent experts look at the data and offer their suggestions on what you and your mechanic should focus on next.

[OMB]

My engine was recently overhauled. My CHT#1 runs higher than the rest. First it was running 40F hotter. After going through all the tests, I found during a 2000 rpm ground run with the top cowling off all CHTs were about the same temperature. When the top cowling was back on the CHT#1 was much higher then the rest. So, I had the flexible baffles changed. Now CHT#1 runs 50F hotter. I seen the Savvy Aviation YouTube videos on leaning and have taken the GAMI advanced piston engine online course. I suggest getting the free online book “Engine Management 101” by Bill Ross from Superior Air Parts. It has a lot of good information. My technique is to rotate at 60kts, then climbout and retract the flaps at 90kts. At 1000ft I set 2400rpm and 22in and continue a slow climb. At 3000ft MSL I set full throttle. I found at 22in MP during the climb stops the CHT from rising. At 3000ft MSL or above at full throttle my CHT does not increase. Then, at cruise, I set the shaded cruise power setting from the flight manual. These are the recommended power settings from Lycoming. As Bill Ross says, it keeps you out of the red box. I use 55% power. It gets me 120kts TAS at 7gph. My CHT#1 stays below 400F at cruise and below 420F during climb. It is a technique and not a substitute for the approved aircraft flight manual.