Ask Paul: What else can I do to discover why my EGTs and CHTs are climbing?

A Question for the General Aviation News Engines Expert, Paul McBride: My IO-360B1A is in a 1993 Glasair II-S FT, with a standard Hartzell blended airfoil prop. This engine was overhauled around 1999 by Western Skyways. It has performed well over the years and has provided many hours of enjoyable, consistent flying. It has about 850 hours on it.

It is “babied.” No shock cooling, no silliness.

I have a Bendix mag on the left, and a Lightspeed electronic ignition (with a Hall module) on the right. The Bendix mag was overhauled two years ago. All of these units have been running well. I run at 24.5 squared, usually using 10.1 GPH, about 40° rich of peak, which seems to be my sweet spot. Yes, I do have GAMIjectors, which helped even out EGTs some years ago. I do not run this engine lean of peak.

About one to two years ago, I started noticing my EGTs and CHTs climbing slowly. Normally, my EGTs would be about 1380 all across, and CHTs would be in the 380s…very comfy and consistent. There has been no change to the airframe/cowling/baffles/inlets. I do not have cowl flaps.

A few months ago, on climbout, my EDM700 monitor went bonkers…CHT’s on #4 suddenly climbed to almost 500, and then #1 started to climb to about 450. Everything was flashing. Oddly, the engine was running fine.

Return to airport with “maximum clamping factor,” reduce power, enrich mixture, and land. We did some troubleshooting, and I finally ended up sending my EDM back to JPI, and they found a bad “MUX” board, which they replaced. All grounds, wiring, probes were good. All wiring checked extensively.

Next flight, some mild improvement, but CHTs have remained in the low 400s (415, 420, 410, etc). EGTs seem to be in the low 1400s. I am NOT comfortable with these numbers/changes, as they ARE a change from normal. Compressions are all in the mid 70s.

Here’s what we’ve done (in our quest to see why my CHTs are higher):

• Replaced the Bendix fuel servo with a new Precision Airmotive unit (the original unit was too old).
• Replaced all four intake seals and hoses.
• Cleaned out all four exhaust guides/valves (only #4 had some mild gunk in it).
• Overhauled the fuel divider/spider. Also did the babyfood jar test, and all were pretty much even.
• GAMIjectors cleaned properly.
• All new plugs.
• Rechecked all exhaust pipes and seals/flanges (#1 was always leaking a little exhaust).
• Borescoped all four, and everything looks ok.
• Removed the magneto, installed new flange seal.

I watch blogs and YouTube videos, and ask questions. My mechanic and I are stumped. He is very experienced, and is an FAA A&P examiner, and worked for a well respected overhaul shop for many years.

I keep reading where Lycoming says we could reduce CHTs if we retard the timing to around 20 BTDC, which we did the other day, and then appropriately adjusted the electronic ignition (I’ve always run this engine at 25 BTDC with the magneto, and follow Lightspeed’s instruction for its Hall module, and it has always run quite well).

We followed the instructions. I did some taxi tests yesterday, and oddly, my CHTs in #3 and 4 went to 385 and 390…on the ground! And when I came back to the hangar, the top cowling felt hotter than it normally would. Yes, excessive taxiing sure isn’t good for an engine…but I would expect this after a flight…not some taxiing.

I also notice this: I normally will use 10.1-10.2 GPH in cruise, to achieve the previous EGTs/CHTs. I am now finding I am using close to 11 GPH (on my Shadin Miniflow) just to achieve my 40° rich of peak settings in all four, which is very odd. Yes, we did fill both tanks and then flew and then refilled, and other than 0.3-0.4 gallon difference, there was no unusual fuel loss.

I am bummed, to say the least. I hesitate to fly with these numbers. I’m thinking my mechanic (who has been very gracious to help me work on an experimental) is starting to get frayed.

I doubt my electronic ignition is bad. But I keep thinking…is this a fuel issue? Mixture? We’ve checked fuel flows and mixture settings. One comment (from a blog) said “your engine is tired and needs an overhaul.” Hmmm.

Others said I was causing detonation by running 40° rich of peak…hmmm to that too. I would rather run a bit rich of peak for engine longevity (despite a little extra fuel use), rather than run it as lean as possible.

Thoughts? Comments? Ideas? Suggestions? Things we should do/look for, maybe that we haven’t done yet? We talked about corrosion on the cam lobes, and haven’t gone down that road yet, but since all cylinders are hotter, is this a system issue, rather than a mechanical engine issue?

Any help is very much appreciated. Thanks.

Isaac, Omaha, Nebraska

Paul’s Answer: Isaac, I’ve read and reread the information you provided and appreciate the details in which you described your situation.

It appears that your operating procedures are right on the mark, and doesn’t raise any concern, therefore keep on doing what you’ve been doing.

However, I’m going to be honest and tell you that I don’t have an answer for you. It appears to me that you’ve done almost everything I would have recommended — and probably more — with negative results.

The increase in fuel flow that you mentioned was probably the result of trying to use more fuel to cool the engine, which apparently had little effect.

The only thing that came to mind was the possibility that the engine mount rubbers may have exceeded their life and have begun to sag. The result is less airflow through your cowling, which would drive the CHTs up.

I’d suggest you closely inspect the mount rubbers and if they’ve never been replaced since the aircraft was built, then replace them. It may come as a surprise that the engine mount rubbers are probably the most overlooked item when it comes to troubleshooting when the CHTs begin to climb when nothing else has changed with regard to engine operation.

I’m not certain this will fix the problem, Isaac, but it’s the only thing that I can think of that may be the cause of what you’re seeing. And, it’s one thing that you didn’t mention doing during all of the troubleshooting you’ve done.

I hope for you and your patient mechanic this might resolve the issue.

And a solution was found

From Isacc: Turns out it was this: The engine timing was way too advanced on the electronic ignition side. The magneto side was perfect. The Lightspeed electronic ignition needs an auto timing light up against the flywheel…and it was 40° BTDC! Adjusted, and now things are better.