Twin Comanche CHT

[Carl Thuringer]

In a 1966 Twin Comanche we recently replaced inoperative CHT gauges with over hauled gauges that have been converted over to Rochester probes by Lockhaven. We are having a really tough time with the left engine, on take-off it is climbing to around 450 deg F or just about red line. After leveling off it does come back to normal readings. We have tried everything including checking mag timing, changing probes, and ensuring that the reading is correct. The baffling appears to be in good shape. Any suggestions as to what might be causing this 200hr engine's CHT to spike so high are greatly appreciated.

[N3322G]

Carl,

My first thought would be to examine the probe and its installation - maybe swap it with the right engine. I'm assuming that Air Parts did calibrate it with the gauge so I don't know if this will help or not? You might also look closely at the exhaust stack fitting nearby.

Does replacing inop CHT gauges mean that you don't know if there was a problem with CHT before the 'new' gauges were installed? If this is the case, it will be harder to chase down the problem.

If you run the engines at full power in level flight, does the high reading come back?

Is there any other indication that the the left engine is running hot? Higher oil consumption? Higher EGTs? Higher Oil temp? Higher fuel flow to keep the engine running smoothly? Any yaw?

Any advice from Lockhaven ?

BTW had a similar problem with a JPI oil temp probe. On the ground the right engine only would read 1310 degrees. The probe install was the problem.

Have also had newly installed baffles on O/H'd engines to be a source of problems. Cooling in take-off config can generate problems. Perhaps try to re-create problem at altitude will point to what area of problem resolution to pursue.

This is a shotgun answer generated by various experiences on our Twin over 44 years. Hope one of the thoughts helps.

[Carl Thuringer]

Thank you for all of the information.

1. Tried swapping the probe around, showed same
2. The old CHT gauges were not functioning when we purchased the bird. So we did not know what the engines were running at before
3. Will try running the engine at full power in level flight, I am assuming that the reading will come back, but I will check it out.
4. All other indications are normal, the oil pressure reads a little bit higher on the left than the right. The left "hot" engine does burn more fuel than the left. Which puzzles me.
5. From what we can tell the reading is correct. I have not been in-touch with Lockhaven, but I will check with the A&P that is working on it.
6. As far as proper installation, anything that I should be looking for?
7. I have suspected baffling as well, after reading through all of the Flyer blogs it seems like that is a big culprit. Is there anything that I should be looking for when inspecting the baffling.

I did notice that the alternate air system was very stiff and not operating properly, can anything with the alternate air source have anything to do with this?

Thank you for your advice Kristin! I am at the point where I am thinking about taking this bird down to Heritage Aero in Rockford to see if they can resolve the problem.

[Carl Thuringer]

Thank you for all of the information.

1. Tried swapping the probe around, showed same
2. The old CHT gauges were not functioning when we purchased the bird. So we did not know what the engines were running at before
3. Will try running the engine at full power in level flight, I am assuming that the reading will come back, but I will check it out.
4. All other indications are normal, the oil pressure reads a little bit higher on the left than the right. The left "hot" engine does burn more fuel than the left. Which puzzles me.
5. From what we can tell the reading is correct. I have not been in-touch with Lockhaven, but I will check with the A&P that is working on it.
6. As far as proper installation, anything that I should be looking for?
7. I have suspected baffling as well, after reading through all of the Flyer blogs it seems like that is a big culprit. Is there anything that I should be looking for when inspecting the baffling.

I did notice that the alternate air system was very stiff and not operating properly, can anything with the alternate air source have anything to do with this?

Thank you for your advice Kristin! I am at the point where I am thinking about taking this bird down to Heritage Aero in Rockford to see if they can resolve the problem.

[Carl Thuringer]

Sorry Pat. You responded to me. You two post so many answers to peoples questions that I got mixed up.

[N3322G]

Carl,

No problem - I was flattered or, my sense of humor is good, so 'all we women Twin Comanche owners look alike' ... I have the advantage of the pleasure of several Kristin visits - she is quite a bit taller so it is very funny to me. I remain envious that she can stand on the ground aft of the flap and un-lock her passenger door ) whereas, I'm lucky to see over the panel when parked.

wrt to the alt air door operation. Two thoughts - first and most important, Alt air door operation, adjustment and condition is critical as more than one Twin has had an engine failure by ingesting parts of it. Our Twin had an aborted departure due to a alt air door failure. There's a good description of it in the service manual and how it should be adjusted. Once had a not-my-regular shop tell me there were zero discrepancies on an annual - it was still on jacks with cowls off, I walked over and thanks to reading the Forums and the Service Manual (online under Tech Tab on Home page here), looked at the alt air doors which had not been properly adjusted. Webco has parts if this needs attention as both of ours did at last annual.

Second thought - I got frustrated that several control cables were not operating as I'd like. Tried several fixes and finally, just replaced all 10 that run from the engine to the cockpit.

On baffles. LoPresti educated me on them in 2005. They filled gaps of an 1/8 of an inch in various places with engine-rated caulk and guessed that the engines would run 20 degrees cooler. They were right, JPI showed engines ran 20 degrees cooler after their work. In 2009ish I copied the longer teflon baffles from other Twin owners photos and had a fifty degree temp drop in normal operations as measured by JPI. Some Forum folks scoffed at the results until Kristin did one engine and not the other and she also got 50 degree improvement. So baffles can be a big deal.

Oh, if you have WOW cowls, shops very often re-assemble then so the inlet baffle flaps are bent backwards and this forces cooling air around vs through the engine. Even experienced Comanche shops do this at least half the time as the Twin cowls are sooo much fun to put back on.

Hope one of these fixes your issue.

[Carl Thuringer]

OK, we took the bird up today. In level flight I was not able to reproduce the near redline CHT temp that the engine shoots up to on take off. It did climb but it was far less dramatic. The left CHT seems to have a consistent 50-75 degree difference from the right, even while taxing. The shop did bench test the gauge, but I am really beginning to think that the reading is inaccurate. The baffling appears to be good. I am going to go down to Buldoc Aviation in Anoka MN and see if they cannot rule out some things. Thank you for all of the advice.