High EGT Question

[Keith Lane]

I have a question to post on behalf of a friend who is not a member regarding leaning and EGT's.
It's a 1946 Ercoupe with a Continental 85 (A "Super 85" from Don's Dream Machines in Griffin, GA)
200 hours ago, this engine was overhauled and upgraded per a STC owned by Don Swords.
It has all new cylinder assemblies, new cam and was , except for case and crank, essentially a new engine.
At 100 hours, one cylinder lost all compression, and it was found to have collapsed compression rings. 50 hours later another cylinder had a stuck exhaust valve, requiring removal and repair. The rings showed signs of beginning to "carbon up" and the remaining cylinders were pulled for inspection. Don's assesment was that the engine was running too rich and an four probe EGT was installed. The EGT unit has one indicator and a switch to view each cylinder's EGT at a time. The probes were installed at 4 inches from the exhaust port, the manufacturer recommended 4 to 8 inches from the port, so it is at the minimun distance called for.

Now here's the problem.

Ever since the probes were installed, the EGT has consistently read 1400 degrees at full rich. Seems high to me and to the owner as well. The switch was replaced to eliminate it as a culprit (it did read 1450 degrees before that). At 2500 feet msl, you can lean it till it reads 1550 degrees indicated, and further than that, it starts to stumble as though it is just starting to run lean, just as I was taught to lean a C-152 during primary training. Back to full rich and it's back to 1400 degrees indicated.
Considering that this is a pretty simple system, does it seem likely that the engine could really be running that hot? It's on 100LL, and the carb has been pulled as a troubleshooting measure and "gone through" for good measure. They did find that the mixture plate had a small "burr" on it and it was repaired. My own inclination is that the instrumentation is at fault, but I'm biased because I do instrumentation for a living. This is really driving the owner crazy because the engine runs fine but he's afraid of causing future problems.
Can increased compression, as this engine has, cause higher EGT's? These seem really high to me. There are no CHT probes on the engine, so I cannot tell if it is burning itself up or not.
If anybody here has a suggestion or idea, I'd welcome it.
Thanks, Keith

 

[Dave Siciliano]

I have a question to post on behalf of a friend who is not a member regarding leaning and EGT's.
It's a 1946 Ercoupe with a Continental 85 (A "Super 85" from Don's Dream Machines in Griffin, GA)
200 hours ago, this engine was overhauled and upgraded per a STC owned by Don Swords.
It has all new cylinder assemblies, new cam and was , except for case and crank, essentially a new engine.
At 100 hours, one cylinder lost all compression, and it was found to have collapsed compression rings. 50 hours later another cylinder had a stuck exhaust valve, requiring removal and repair. The rings showed signs of beginning to "carbon up" and the remaining cylinders were pulled for inspection. Don's assesment was that the engine was running too rich and an four probe EGT was installed. The EGT unit has one indicator and a switch to view each cylinder's EGT at a time. The probes were installed at 4 inches from the exhaust port, the manufacturer recommended 4 to 8 inches from the port, so it is at the minimun distance called for.

Now here's the problem.

Ever since the probes were installed, the EGT has consistently read 1400 degrees at full rich. Seems high to me and to the owner as well. The switch was replaced to eliminate it as a culprit (it did read 1450 degrees before that). At 2500 feet msl, you can lean it till it reads 1550 degrees indicated, and further than that, it starts to stumble as though it is just starting to run lean, just as I was taught to lean a C-152 during primary training. Back to full rich and it's back to 1400 degrees indicated.
Considering that this is a pretty simple system, does it seem likely that the engine could really be running that hot? It's on 100LL, and the carb has been pulled as a troubleshooting measure and "gone through" for good measure. They did find that the mixture plate had a small "burr" on it and it was repaired. My own inclination is that the instrumentation is at fault, but I'm biased because I do instrumentation for a living. This is really driving the owner crazy because the engine runs fine but he's afraid of causing future problems.
Can increased compression, as this engine has, cause higher EGT's? These seem really high to me. There are no CHT probes on the engine, so I cannot tell if it is burning itself up or not.
If anybody here has a suggestion or idea, I'd welcome it.
Thanks, Keith

Keith:

I don't know that plane or engine well, but am pretty familiar with how the JPI works. There are two different probes and if someone has the instrument set for one type and you are using the other, you can get high readings. I'd check that first.

That being said, EGT readings are more relative numbers than anything. CHTs are much more important. The temp of the EGT probe depends on a lot of things, such as, location of the probe.

We only use EGTs to monitor relative readings, detect peak to enrichen or lean properly and look for a change in trends. The temps you're reporting don't seem disturbing to me. Have you discussed this with the manufacturer? I did have something done once to the JPI where the mechanic changed the type probe and got unusually high readings across the board. When that was changed, things were normal again.

Hope this helps.

Best,

Dave

 

[Lance F]

As Dave accurately stated, EGT are really only useful as a relative measurement. MANY variables affect the absolute number. That being said, your friend's numbers seem to be reasonable. It sound like peak is about 1550 dF. If so, than 1400 dF is 150 dF rich of peak. At 2500' and wide open throttle this is a reasonably safe place to be. I just looked at the data of a recent flight of mine. I was at 11000' IIRC and my EGTs were 1426, 1417, 1423, and 1443. This was about 5 dF lean of peak.
Oh, the light bulb just came on. This is Sam's plane. Duh. I'd worry more about Don's STC and its implementation than the EGT instrument.

 

[gismo]

150F ROP at full power does seem lean to me. For the engines I'm familiar with, full rich and full power is more like 250-300F ROP. 1550 F at peak EGT also seems a bit high if the probes are 4" from the exahust port. If your probes are closer than 3" they might be inside the "flame" or burning end gas but at 4" that seems unlikely unless the valve timing is off.

 

[Let'sgoflying!]

lance whatever happened to your ignition woes on the.....C85? O-200?
(hate losing track of those mysteries)

 

[gismo]

lance whatever happened to your ignition woes on the.....C85? O-200?
(hate losing track of those mysteries)

Funny you should ask. I just flew the plane yesterday afternoon and it's fixed! Turned out to be a leak in the intake manifold for the problem cylinder.

 

[Let'sgoflying!]

leak in the intake manifold for the problem cylinder.

whaaa? Why all the 'electrical' symptoms??? Wasn't it firing good on one particular plug, one mag only? How did you finally do the diagnostics, or did you stumble upon a popped hose?