Diagnosing high EGTs - a long sad story

[jbell2355]

In early October, I signed a purchase agreement on a beautiful RV7A built by a physician here in Michigan. He had his CFI friend fly the plane up to my neck of the woods for a pre-buy inspection. I got the green light from the A&P and sent the wire transfer ? the plane was mine!

The CFI and I hopped into the plane for a few landings and then I flew him back home solo. I recall during that first run-up that the engine seemed to run rough on one mag, but the CFI didn?t seem too concerned and it was my first time in the plane, so I didn?t know what was normal. On my 2 hour return flight home, all alone in a new plane, I started getting a high EGT annunciation on cylinder #4. This was disconcerting for me, having never had this kind of engine monitoring info and being new to the aircraft.

Back home, I consulted with the builder who told me that he hadn?t had this issue before. At first it seemed it had to be related to the pre-buy inspection, but what could the mechanic have done that would cause this? We did a little troubleshooting and decided it was probably a bad EGT probe. I flew the plane regularly and found that the annunciation was mostly an issue on climbout and that with proper leaning (or lack thereof), I could mostly manage the high EGT. Cylinder head temps did not appear out of line.

I flew the plane for 25 hours and brought it back to the pre-buy mechanic for a supervised oil change. I uncowled the plane and was shocked to see that the #4 high tension lead was disconnected from the upper plug on cylinder #4 (Lightspeed Ignition). The only good news was that we now knew the source of the problem. The mechanic was very nonchalant about the fact that he left the plug unplugged, and didn?t seem bothered by it at all. I wasn?t very happy, except for the fact that we thought we had solved the problem.

I flew the plane home after the oil change, excited to finally be rid of that irritating flashing red light, but it was still there! For the next few months, I swapped EGT probes, installed a new one, had them tested, checked for wiring problems, and spent a lot of time messing around trying to diagnose the problem. I was about to send the GRT EIS monitor back to be tested, when I shared some engine monitor info with Doug Hamerton at Aero Sport Power (the engine builder). He felt that, based on the data, I was leaning #4 lean of peak and that?s how/why the temp was going back in line with the other cylinders which were leaned ROP.

At this point I had put 80 hours on the engine. I had the mechanic on my field take a look and he told me I had a leaking exhaust valve, so I had him pull the cylinder. Based on the amount of carbon buildup, we decided to pull all the cylinders and have them looked at by Doug.

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Aero Sport did their thing and a few weeks later the cylinders were remounted. 2 new pistons, 1 new #4 cylinder and the rest just cleaned up/painted. Once it was all put back together, I fired her up and did the ground run procedures.

Then I flew it, and still had the **** EGT problem!

This isn't the end of the story, but I?m going to stop the story here for a day or two. You should know that we did subsequently find and fix the problem. I?m curious whether anyone would have saved me all this hassle I've been through if I would have asked sooner.

And Klaus, you?re not allowed to answer.

So...what happened here?

[Kahuna]

Bad coil???

[snopercod]

I can't quite see from the photos. Is your engine carbureted?

[jbell2355]

Snopercod, yes, it is carbeurated. It's an O-360.

Kahuna, you hit the nail on the head! Where were you $5,000 ago? Neither mechanic I worked with thought of a bad coil. Neither knew that running the ignition without a plug would burn up the coil.

#4 was running rich after the pre-buy inspection. I assume this was because only 1 plug was firing. Eventually, it started running lean - because enough fouling of the exhaust valve caused a leak on the intake stroke. This was puzzling to me at the time, but now it makes perfect sense.

Anyhow, I hope this post saves someone else a few bucks down the road!

So, next question...does the pre-buy mechanic have any liability here?

[Kahuna]

Coils fail for a number of reasons. High resistance is the usual culprit. The coils can fail one side (one lead) or both side (both leads same coil) and can often be intermittent. The check for it is pretty easy. When you have a high EGT and a suspect coil, do an ignition check. Boom there it is. AND, since you mentioned that the lead was off for some period of time, it certainly would have failed an ignition check at run up.

[Kahuna]

Originally Posted by jbell2355

So, next question...does the pre-buy mechanic have any liability here?

And FWIW, while the mechanic may have failed to put the lead back on, you got in the plane and flew it many times for 25 hours and certainly failed to deal with the ground run ignition check failure. Your systems were telling you there was a problem. During run up, while rough, the engine monitor would have clearly showed the problem, which cylinder, and which ignition. A lead off would have been very rough, enough so that even without an engine monitor, it should have been back to the hangar for diagnosis. Even taking into consideration that you were unfamiliar with that plane, any engine would have shook and should have been an immediate red flag to any pilot.

[Carl Froehlich]

This is a worthwhile thread for everyone on recognizing when the engine is telling you there is a problem.

With any igntion, mag checks are best done by observing EGT change. No matter what igntion you are running, turn off one side and all EGTs should go up. If they don?t, one or more cylinders are not firing properly. Park the plane and find out what is wrong. Do not rely on RPM drop alone.

Lightspeed coils have, in my experinece, failed on high resistance as discussed in previous posts. Yours failed because you had a plug disconnect. I suggest more common mode is a plug lead going bad. You need to verify lead resistance at every inspection.

Verify spark plug resistance in range at each inspection as well when you clean them (aircraft plugs) or replace them (EI auto plugs).

Do not ignore when the engine is talking to you. Believe your instruments until you prove them wrong.

Carl

[Noah]

Great advice by Carl and Kahuna.

At EVERY ignition check before EVERY takeoff, look for all EGTs to rise by 20 or 30 degrees when you switch from both to one ignition, as well as a slight drop in RPM. When you switch back to both, look for all EGTs to fall back to where they were and for the RPM to rise slightly to where it was. Do this first on one ignition, then on the other. If ALL EGTs DON'T CHANGE as described, decowl and find out why - because you are operating without the redundancy of dual ignitions at that point - and you are one ignition failure away from a forced landing.

One other suggestion is post on this forum when you have a problem, you will get a lot of good suggestions by very seasoned, knowledgeable, and helpful individuals. Would have saved you a few $k in this case - from your description that was an easy diagnosis, even over the internet.

[Caveman]

Originally Posted by jbell2355

So, next question...does the pre-buy mechanic have any liability here?

Do a VAF search for automotive spark plug wires popping off by themselves due to trapped air pressure.

[rockwoodrv9]

Originally Posted by Noah

Great advice by Carl and Kahuna.

At EVERY ignition check before EVERY takeoff, look for all EGTs to rise by 20 or 30 degrees when you switch from both to one ignition, as well as a slight drop in RPM.

This is new info to me. Thank you.

I am flying a 172 now and the only thing I have been taught to look at is RPM drop.

The plane does not have the G3X system I have with the sensors on nearly everyplace on the engine so I guess that is all it has. It is amazing the info we have available now that wasn't even thought of even 10 years ago. The trick is to learn it all.