On start - Weird EGT on cylinder 4
 

The other day when I started the plane #4 EGT wasn't rising, per the engine log it was lowering to 285 for about a min. One I started taxing it started then spiked to 1305 and then over the next minute normalized in line with the other 5 cylinders.

Flew normal for the next hour and looking at the log nothing else erratic. Could this be a plug or lose probe or something entirely different?

Either the cylinder wasn't firing and the probe had an issue. It is possible that the plugs fouled from being overly rich during the start and took a minute to clear up before they could begin firing. Probe doesn't need to be tight to work properly. It is not grounded via it's body. Sticking valve could do it as well, but not likely on a new engine and uncommon on dead cold engine.

Larry

You have heard of "morning sickness"?

Thanks, wrapped up my oil change at lunch. Started right up and all temps were normal. It is possible that I pumped to my gas in for a cold start.

certainly have and most report the issue at take off and climb after a cold start and not at idle immediately after a start. I believe that the tight clearance, causing the morning sickness, causes problems at initial high power on a cold engine, as the valve heats up and expands faster than the guide, causing the sticking. As the guide heats up, the clearance opens back up a bit and the sticking goes away. Never heard anyone report morning sickness as happening at idle for one minute then going away, especially on a new engine, which I believe the OP has.

Larry

EGT Probe Replacement
 

Just went through the same thing. In this case it was the probe. I swapped the probe from number four to number two cylinder. It happened again indicating cylinder four was faulty when in fact the probe was in cylinder two. Before I got a new probe, the probe would work properly for awhile and then fault. Common fault was at start the EGT temp would go through the roof.

yes. i often have very rough running for 10-15 seconds iif i over prime, due to plug fouling. i ifeel probe issue is more likely though

I've personally experienced exactly that, firsthand. Now you've heard of it.

Thanks for sharing that.

A cause for missing on cold start, especially on new or overhauled engines, is
that with cold oil and tight clearance the oil pressure can spike as high as 150 lbs holding the lifter up until warm oil can get to it. If you would like to check this, attach a pressure gauge to the oil galley at the front of the engine. the pressure may not show on the planes instrument due to the lag in the capillary tube, you may be surprised.