Temporary Erratic CHT on One Cylinder

Engine is an O-235-C1 and the CHT's have always been very stable. CHT monitors all 4 cylinders (MGL unit). The sensors are under-the-spark-plug type.

Towards the end of a 90 minute flight the MGL warning light comes on and the #3 CHT reading is rapidly cycling. The cycling is so fast its hard to read the numbers.

I have the warning light limit set at 400 degrees and I think I saw a 400 number at one point. I was just about to enter the pattern at this point and throttled down. Almost immediately the #3 cht returned to a stable 320 degrees, which is normal for this engine. Other chts remained unchanged and stable while this happened. EGT's were all in the normal range when this happened as well.

Also, no loss of power or engine roughness.

The fact that the reading on the cylinder was cycling so fast you could barely read it is what surprised me. This was not the normal change rate at all.

Any ideas where I should start to investigate this?

I will check the wiring connections and pull the plugs on the cylinder.

TIA
Gary

Thermocouple or loose connection

I've experienced something similar where CHT readings oscillated randomnly between high and low, much quicker than real CHT's could actually be changing. The problem was caused by a faulty thermocouple.

I think you are on the right track with checking the connections to that thermocouple and its wiring and potentially replace that thermocouple with a new one, they are inexpensive.

That sounds like a connection issue. I don't think a failed thermocouple would behave that way. It is definitely not a reflection of actual temp.

PaulvS said:

I've experienced something similar where CHT readings oscillated randomnly between high and low, much quicker than real CHT's could actually be changing. The problem was caused by a faulty thermocouple.

I think you are on the right track with checking the connections to that thermocouple and its wiring and potentially replace that thermocouple with a new one, they are inexpensive.

Click to expand...

That is exactly what it was doing.