CHT confusion

During runup today the Dynon voice started nagging, "Cylinder Head Temperature". Sure enough #1 was shooting up toward 490. I pulled back power and it came back down pretty quickly. I slowly put power in again and sporadically the temp jumps back up. EGT reads nothing out of the ordinary within a few tens of degrees from all the other cylinders. I'm figuring a bad CHT probe so I taxi around a little bringing in power and pulling it back.

Temp consistently goes way up with power and comes back down to the level of the other cylinders (low 300s) when I pull power back to idle. EGT is in lock step with the other cylinders. Switching mags seems to do nothing unusual. Went back to the hangar and pulled the cowl. Nothing visually out of the ordinary with the probe or anything else.

I'm not so sure it's just a bad probe. When a probe fails wouldn't it likely read lower temps? What real engine problem might present these symptoms?

I would check the probe adn the wiring. A cylinder head can't go from 300 to 490 just from running at 1800 RPM for 30 seconds with the possible exception of severe detonation, which would be very uncommon for 1800 RPM. Just not enough heat produced in the runup scenario.

Larry

Try swapping the probe with the one on the adjacent #3 cylinder, without changing the wires (e.g. that probe will still show as Cyl. 1 on your EFIS) and run the engine again. If the problem still shows on Cyl 1 then your problem is with the probe/wiring/EFIS. If however the high temp now shows up as Cyl. 3 (e.g. it's cyl 1 that is still hot), then you likely have an engine problem.

Thank you. I will swap the probe out as a test. What I really want to know is what engine problem would present these symptoms? In other words, assuming the probe is good, what might then be wrong with my engine?

jwilbur said:

Thank you. I will swap the probe out as a test. What I really want to know is what engine problem would present these symptoms? In other words, assuming the probe is good, what might then be wrong with my engine?

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Nothing beyond uncontrolled explosions (i.e. severe detonation or pre-ignition ) can cause a 200 degree rise in cht at 1800 rpm for 30 seconds. If that were acually happening you would probably already have holes in the piston crown.

jwilbur said:

. I slowly put power in again and sporadically the temp jumps back up. EGT reads nothing out of the ordinary within a few tens of degrees from all the other cylinders.

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CHT's don't move sporadically. That is a sign of an instrumentation issue.

I had the same thing happen to me. The #1 CHT approached 500 on climb out. I pulled the throttle and circled back and landed. I went back the next day to trouble shoot it and found #1 was 20 degrees above ambient, and all other temp probes. Turned out to be a bad crimp at the factory installed spade connectors. Dropping the probe in boiling water before and after recrimping confirmed the problem and the fix.

Brian