EGT gage oddity; stuck valve, other??

Folks, I'm hoping you can help me diagnose an odd EGT gage behavior.

Background: 1965 Aircoupe with Continental C90-16F engine (has vacuum
pump). Plane based in N. California so freezing temps not a factor
(pre-heat not needed nor used). Has both a CHT and EGT on different
cylinders, with the two-needle combined gage in the cockpit. The CHT
works fine, the EGT was working fine until 2-3 months ago.

Now, the EGT fairly consistently acts like this: it reads "normal" on
cold start, taxi, runup and takeoff. Climbout it reads around 1200
deg F. like it always did. Then, after a few minutes, it starts to
fall, and within maybe a minute or two, the needle quivers at the
bottom of the scale. It never recovers from that; but at next flight
a week or two later the same behavior happens. I never notice any
change in engine performance or sound.

Would a stuck exhaust value cause this? But wouldn't I notice
something from the engine? What else could cause this repeating
strange behavior? Maybe it's something heat related?

Thanks,
Bob

In article ,
Bob Fry wrote:


Now, the EGT fairly consistently acts like this: it reads "normal" on
cold start, taxi, runup and takeoff. Climbout it reads around 1200
deg F. like it always did. Then, after a few minutes, it starts to
fall, and within maybe a minute or two, the needle quivers at the
bottom of the scale. It never recovers from that; but at next flight
a week or two later the same behavior happens. I never notice any
change in engine performance or sound.

Would a stuck exhaust value cause this? But wouldn't I notice
something from the engine? What else could cause this repeating
strange behavior? Maybe it's something heat related?

seems to be a failed probed, or wire from the probe. Can you get
another probe and see if it still behaves this way? Also, check
the wire from the probe to the gage.

--
Bob Noel
looking for a sig the lawyers will like

In article ,
Bob Fry wrote:

Folks, I'm hoping you can help me diagnose an odd EGT gage behavior.

Background: 1965 Aircoupe with Continental C90-16F engine (has vacuum
pump). Plane based in N. California so freezing temps not a factor
(pre-heat not needed nor used). Has both a CHT and EGT on different
cylinders, with the two-needle combined gage in the cockpit. The CHT
works fine, the EGT was working fine until 2-3 months ago.

Now, the EGT fairly consistently acts like this: it reads "normal" on
cold start, taxi, runup and takeoff. Climbout it reads around 1200
deg F. like it always did. Then, after a few minutes, it starts to
fall, and within maybe a minute or two, the needle quivers at the
bottom of the scale. It never recovers from that; but at next flight
a week or two later the same behavior happens. I never notice any
change in engine performance or sound.

Would a stuck exhaust value cause this? But wouldn't I notice
something from the engine? What else could cause this repeating
strange behavior? Maybe it's something heat related?

Thanks,
Bob

A bad EGT probe.

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html

Make sure your wiring and connections are OK before replacing the probe.

I had a loose wire behind the panel.

-------------------------------------------------------------
Now, the EGT fairly consistently acts like this: it reads "normal" on
cold start, taxi, runup and takeoff. Climbout it reads around 1200
deg F. like it always did. Then, after a few minutes, it starts to
fall, and within maybe a minute or two, the needle quivers at the
bottom of the scale. It never recovers from that; but at next flight
a week or two later the same behavior happens.

Make sure the probe wire isn't shorting out to anything.

Bob Fry wrote:
snippo

Would a stuck exhaust value cause this? But wouldn't I notice
something from the engine? What else could cause this repeating
strange behavior? Maybe it's something heat related?

Thanks,
Bob


Almost certainly a bad probe...
--

-Gord.
(use gordon in email)

You have an electrical problem with the probe, wire to the probe or
the gauge. When something gets hot the resistance goes up and the
gage reading falls. Most likely a bad crimp somewhere on one of the
electrical connections but could be on any of the connectors or even
the gauge itself.

On 10 Jan 2005 06:56:36 -0800, Bob Fry
wrote:

Folks, I'm hoping you can help me diagnose an odd EGT gage behavior.

Background: 1965 Aircoupe with Continental C90-16F engine (has vacuum
pump). Plane based in N. California so freezing temps not a factor
(pre-heat not needed nor used). Has both a CHT and EGT on different
cylinders, with the two-needle combined gage in the cockpit. The CHT
works fine, the EGT was working fine until 2-3 months ago.

Now, the EGT fairly consistently acts like this: it reads "normal" on
cold start, taxi, runup and takeoff. Climbout it reads around 1200
deg F. like it always did. Then, after a few minutes, it starts to
fall, and within maybe a minute or two, the needle quivers at the
bottom of the scale. It never recovers from that; but at next flight
a week or two later the same behavior happens. I never notice any
change in engine performance or sound.

Would a stuck exhaust value cause this? But wouldn't I notice
something from the engine? What else could cause this repeating
strange behavior? Maybe it's something heat related?

Thanks,
Bob

(John_F) wrote:

You have an electrical problem with the probe, wire to the probe or
the gauge. When something gets hot the resistance goes up and the
gage reading falls. Most likely a bad crimp somewhere on one of the
electrical connections but could be on any of the connectors or even
the gauge itself.


I still think it's the probe, if it was a bad connection in the
leads then you wouldn't likely get the same effects time after
time...heat can do that to the -probe- though, my money's on the
probe.










On 10 Jan 2005 06:56:36 -0800, Bob Fry
wrote:

Folks, I'm hoping you can help me diagnose an odd EGT gage behavior.

Background: 1965 Aircoupe with Continental C90-16F engine (has vacuum
pump). Plane based in N. California so freezing temps not a factor
(pre-heat not needed nor used). Has both a CHT and EGT on different
cylinders, with the two-needle combined gage in the cockpit. The CHT
works fine, the EGT was working fine until 2-3 months ago.

Now, the EGT fairly consistently acts like this: it reads "normal" on
cold start, taxi, runup and takeoff. Climbout it reads around 1200
deg F. like it always did. Then, after a few minutes, it starts to
fall, and within maybe a minute or two, the needle quivers at the
bottom of the scale. It never recovers from that; but at next flight
a week or two later the same behavior happens. I never notice any
change in engine performance or sound.

Would a stuck exhaust value cause this? But wouldn't I notice
something from the engine? What else could cause this repeating
strange behavior? Maybe it's something heat related?

Thanks,
Bob

--

-Gord.
(use gordon in email)

On 10-Jan-2005, Bob Fry wrote:

Would a stuck exhaust value cause this? But wouldn't I notice
something from the engine? What else could cause this repeating
strange behavior? Maybe it's something heat related?


It's certainly not a problem with the cylinder. If there were any ignition
at all the EGT would be well off of the peg, and if not you would certainly
know if from vibration and loss of power. Check he EGT probe, the gauge,
and the connections between the two.

--
-Elliott Drucker