Some strange happenings with CHT and EGT

When I say ground I mean all grounds. The main ground for the plane, the engine ground the battery ground. They all need a screwdriver or wrench and as Sean mentioned may need cleaning. I use a Scotchbrite pad and scrub the ground wire end. Apply dielectric grease and re=mount the ground. Since more than one item is affected (CHT & EGT) then it's more likely a main ground for these since it's more than a single item affected. Some instrument sending units work on a restrictor setup so a little difference in a poor ground can make them malfunction.

John,

It’s interesting that the over-temp readings are happening (reliably) when you reduce throttle. Assuming these high temp readings are not real, then it would seem that the only relevant thing that would change when you reduced RPM is voltage output from the alternator.  Have you noticed a significant voltage reduction when you reduce throttle?

Since voltage overcomes resistance, this may be a ground problem as others have suggested, I’m just trying to reconcile the relationship to RPM. If you don’t find a ground problem, you should also check connections at the Garmin Engine Box (GEA-24), and also discuss the problem with Garmin support.  Their support folks for the G3X are fantastic and they may have seen similar issues with the GEA-24.  

Thanks all for the responses.  All very helpful, and I enjoy the feedback.

I will certainly log this with Garmin, and I will endeavour to go over grounds etc, and see if I can find anything that will fix this annoying problem.

Cheers

John

On my last aircraft and on this yet to fly new one, I have installed a ground (-) busbar, that goes directly to battery neutral (-) for all cockpit connections. In addition, all engine compartment electrical devices are connected and engine, vie ground distribution circuits, directly back to battery neutral. I worry about the wing tank fuel level senders, that are grounded through the aircraft frame - could not find a suitable location for a return wire.