High EGT Single Cylinder

[rob]

Yesterday, in the middle of a 3 hour flight, I was cruising at 8000' trying to take photographs of crossing traffic (A B2 Bomber). When I looked down at my engine analyzer, I noticed that the number 3 cylinder EGT had jumped about 75 degrees. Unfortunately, I did not leave the configuration alone long enough to properly troubleshoot as I was a bit freaked out about the sudden jump. From what I did observe, though: none of the other EGTs looked strange, and the CHT's were all normal also. The engine continued to make full power- there was no noticeable decrease in manifold or airspeed. After the jump, I enriched until the EGTs all dropped and then re-leaned to where I was before and everything remained normal for the next hour of flight. When I landed, I checked the mags and again, everything came back normal.

Research has pointed at this being an issue with one of the spark plugs in the offending cylinder. This brings me to my question. How long are the plugs good for? These have approximately 600 hours on them. I have not ever had any issues to speak of with fouling or anything of the sort. They have been rotated top to bottom a couple times, though not on regular intervals. Should I just replace them all? Replace half? Suggestions are appreciated.

[Oscar Avalle]

I would just clean the plugs and monitor the engine...

BTW can you share the B2 pictures?

Oscar

[rob]

I wish I could share the B2 pictures but there's nothing to share. They were about 5 miles out. It is fun to fly in Whiteman AFB's airspace though, you see all sorts of things. Last time it was two A10s.

[1964-M20E]

I had a similar thing happen to me I switched each mag off and found to offending plug. It cleared up by time I landed and did not give me any problems on my return flight. So most likely something was on one of your sparkplugs.

[rob]

I had a similar thing happen to me I switched each mag off and found to offending plug. It cleared up by time I landed and did not give me any problems on my return flight. So most likely something was on one of your sparkplugs.

I agree that it was one of the plugs. The question is what to do about it. How long should a plug remain in service? Should I just clean, rotate and gap, or should I go beyond that and replace?

[fantom]

Assuming you have a Lyc 4 banger, that #3 cyl likes to play with your probe so might want to check that also.

Also consider posting your model and year. It would help us help you.

Oh, and plugs really differ, and are replaced 'on condition' after inspection.

Good luck!

[rob]

Ack! My signature is gone; year and model used to be in there.... I am flying an 1965 M20C.

As far as the probe. That's the cylinder I generally monitor and I've never noticed a problem in the past...

[fantom]

A coupe of years ago my #3 probe went crazy with no prior notice, EGT's all over the place, over water, on the way to Key West with my wife. Snuck out to the airport early the next morning cause I didn't want to alarm her, removed cowl on the ramp, everything looked OK, swapped probes with #1, took it around the patch and sure enough, #1 was doing the EGT dance.