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I went out to the hangar this afternoon, pulled the cowling, then rigged up a metal can hanging from a step ladder to boil some water. Three of the four CHTs went to 202±1°F when the water was boiling furiously. CHT 1, the one that reads 50 - 60°F lower than the others in flight, went to 178°F. I looked at the wiring, and did find that one of the connectors at the back of the EIS 4000 was a bit loose. I tightened it and repeated the CHT checks, but there was no real change.

So, it looks like one third to one half of the low CHT indication is caused by some sort of instrumentation error. I need to sort that out before I make any more baffling changes. I removed the one change I had made, as I no longer trust the CHT indication on #1 cylinder, and don’t want to do anything that could cause it to go too high.

I had checked the CHT calibration using boiling water back in July 2006. At that time they all indicated about 3°F too low. Today the ambient temperature was just above freezing, and three of the four CHTs read about 10°F too low. The CHT indications use a reference junction inside the EIS 4000, and there is perhaps some sort of temperature sensitivity to it. In flight, I would have had the cockpit heat ON, and the cockpit temperature would hopefully have warmed up, so I suspect the error in the CHT would have been less. The atmospheric pressure was high today, so the pressure altitude was about 100 ft. Thus the boiling point of the water would have been 212°F.

I’m not sure what could be causing the low CHT #1 indication. I’ll contact Grand Rapids Technologies to get advice on what to do next.

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