EGT high on one cylinder

I've been through every thread I can find regarding high EGT on a single cylinder. Here's the facts. Just completed 8th annual condition inspection. Changed from champion plugs to tempest... same size and heat rating. That's the only thing different. On cylinder three the temps have jumped by 150 deg or so and at times 200 especially at higher power settings. At cruise or less, cylinder 3's EGT are a little more reasonably in line with the other three. An interesting note... The engine never did idle well. I always had to keep a little rpm on it for smooth running. With no throttle carb adjustments made, with the new tempest plugs it idles perfectly smooth. Also the champion plugs that were removed with the exception on one, failed the ohm test horribly.

What I've done/confirmed

1. Changed air intake gaskets
2. rechecked compression on cylinder 3, (75/80), incase of carbon or speck of trash on the exhaust valve seat
3. all plug wires checked with cable tester and all check good
4. swapped out probes with cylinder 1. Still indicates the high EGT are on cylinder 3
5. although all plugs are new, I Just swapped out plugs with another cylinder to see if that moves the problem. Haven't test flown yet.
6. mags are timed correctly at 25 BTDC, (rechecked).
7. at run up mag checks are good. About 80 rpm drop on each and EGT rising in all cylinders with each side then back down when placed on both
8. All four CHT temps are good in all flight modes, even cylinder 3.
9. Engine running smooth as silk at all times. No valve sticking symptoms


If swapping the plugs with another cylinder doesn't change anything then it seems the only thing left is to pull the plugs from cylinder 3 and put a couple of the champions back in and test fly it, (for a single test flight only CAN I DO THAT one cylinder with champion and the other three running tempest)?
The only thing that changed was the new plugs! If I can do it and it brings the EGT back in line with the others at all settings, I'm going to change back to champion and will have 8 new tempest plugs for sale.

Discussion: I've watched Mike Busch's online video regarding CHT and EGT and understand his points however..... It IS different and it puzzles and concerns me.

Any thoughts???

You have better combustion in Cylinder 3 generating a hotter EGT. The probe also maybe slightly closer than the other 3 cylinders causing slightly higher temp readout. Either way EGT is no indication of a problem and the CHT is the one to watch and you say they are fine. Stop trying to think there is a problem and fly on.

Have you compared temperature spread between full rich EGT and leaning to absolute peak EGT on ground and in flight for each cylinder?

Normally a pretty good indicator if you're dealing with an engine issue, a probe placement issue or a faulty probe (the latter which you've obviously been able to strike out).

Is it "possible" . . . ?

My wild guess, this cylinder stayed the same and all the others dropped. With the bad idle and now it is good, it could be (rare) that each other cylinder had a misfiring plug and was essentially retarded timing.

Odds are against this, but could be possible given your comment of the really bad plugs removed.

Worth what you paid . . . .. stay well Paul.

You really could do with some before and after data.

And do a mixture sweep from rich to LOP and capture that data. Do an inflight mag check when LOP.

Then you have meaningful data.

Check the plug that screws in to the head where a primer would go, check for leaks with any MAP sensors or anything connected to C3. Rubber coupling leaks.

If you have a leak when ROP that cylinder will have a higher CHT than before.

Maybe a bad mag. You said you have done 8 condition inspections. That's 8 years on the mags. Have you rebuilt the mags yet?

My guess: high manifold pressures place the most demand on the ignition system to get a spark. Cylinder 3 is only getting one plug firing at full throttle. Easy to check; do a full throttle run up mag check. If it stumbles on one mag you have confirmed the guess. Issue could be mag is getting marginal and cyl 3 has a plug with a larger gap. More likely cylinder 3 plug or its harness is marginal. Swapping plugs can also help narrow down the problem to the plug or harness.

Problem found

Hey everyone... happy to report I discovered the problem. Embarrassed because I SHOULD have thought of this sooner. In my original post I mentioned what I had done including changing the wires out between Cylinder 1 and 3. When I did that all I did was to swap them at the spade connectors just before firewall penatration. Today I was thinking there's no way doing that can rule out a still functioning but failing probe. Soooo... today I pulled the probe from cylinder 1 exhaust and covered that hole with a hose clamp. Placed the probe from cylinder 1 into cylinder 3 and zip tied the cylinder 3 EGT probe out of the way. Flew this evening and cylinder 3 EGT's are once again during all power settings, inline with the others.

I did fly earlier today BRIEFLY. Was going to get the information such as fuel flows etc as suggested above. It was horribly bumpy!!! Heck with this and back on the ground in no time. Tonights flight was much better. Thanks everyone, for your input.

IMG_2871 by Paul Gray, on Flickr

IMG_2872 by Paul Gray, on Flickr