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Diagnose This

stewartb

MEMBER
At typical sitting in the tiedown warmup idle my #4 EGT is 1400+, a solid 250+ above any of the other three. CHT is only about 20* warmer. Slow idle, fast idle, rich, leaned... the differential remains the same. In flight? #4 EGT comes down and the other come up so they're all pretty even. CHTs are fairly even.

Why the higher EGT at #4 with low throttle settings?
 
Didn't the guy on Mt Man TV show just have this problem on his C150/150? Intake leak was the cause on that show?

Glenn
 
Might be a leak but switching to single mag should show a slight temp rise going from Both to R and L. Works best above idle so experiment. Single mag run lets more unburned fuel mixture out exhaust valve and warms up the probe in the exhaust header.

Gary
 
Agreed, Intake leak or a clogged injector (if injected). Either way it sounds like it is lean. One way to check if it is in fact running lean, is if you lean the mixture, at some point that cylinder will stop burning and actually cool off. So when you start leaning, watch the high egt. If it falls off before the others it is surely running lean. The culprit could be a intake leak or, if injected, a clogged injector.
 
Good comments. It's odd how it becomes normal when I throttle it up to fly. The only high EGT spread is at idle and taxi power. Even then it's not a problem as much as a curiosity. Mike will remember the low power intake leak I had on the 180 a few years back. That was an expensive lesson but it apparently didn't stick! ;)
 
If there's an induction leak it'll be worse at closed throttle when intake system vacuum (or rather air pressure differential across the system components) is high, than with the throttle open when intake vacuum and differential is low. Shoot some propane gas with a long metal probe near the suspected leak and note any increase in rpm at closed throttle.

Gary
 
Agreed with Gary, but after that and before you do anything too drastic,swap your sensors (with leads and all) to be sure you're attacking the correct cylinder.
 
There's all manner of possible causes for differential temp indications. Intake leaks, panel electronics, sensors and wiring, ignition quality, valve timing, and fuel mixture in no particular order and probably some more. The interesting part is how the OP noted it normalized somewhat across cylinders at a higher throttle opening and power. That initially suggests to me a reduction in lean run or better ignition for whatever reasons. I'll leave why to others but assume a mag drop check and noted EGT rise was ok at both idle and in cruise?

But yes knowing what cylinder is being monitored should be first base.

Gary
 
Being a brand new motor and knowing that sometimes bolted parts need retorquing I’ll look for an induction leak next time I see the plane. The G3X uses Alcor EGT probes and I’ve never seen one of those be wonky but I’ll keep it in mind. The plane is an hour from home until I take posession of that house and then it’ll be a dozen steps in stocking feet out the kitchen door. Much to do in the time between so I won’t work on it anytime soon.

One of the things that confused me is that with a big Continental an induction leak is a three or 6 cylinder thing. This time it’s cylinder specific. It makes sense now but it wasn’t as clear in the pilot seat. Thanks for the comments.
 
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