Abnormally high EGT

[Swingin]

So today I'm flying back from visiting relatives in my '67 M20E.  I can count the number of times on one hand that I've cruised anywhere below 10,000' MSL, but today I had to go for about 30 minutes under low overcast deck before it broke up enough to climb through VFR.  While low (~2,500' MSL) I ran the mixture to keep my EGTs between 1200-1300, which is where I keep them for climb (usually low 1200s).  

Once I found a hole, I popped up to 8,500', then leaned for cruise.  Normally I run 50-degrees LOP, set off of the #3, which is the last to peak in my plane.  50 LOP usually ends up around 1390 or 1400.  #4 sits about 10-degrees colder while LOP, all of the time.  Today though, #4 wouldn't get below 1450 unless I was well ROP, and it even peaked above 1500, which I've never seen from any of them.  Also it didn't behave normally, it would heat up a little bit even if I kept leaning, and never cool below 1400, no matter how lean I took it, until it started to stumble.  It spooked me enough to run 100-ROP for the rest of the leg, at the expense of about another 3.5GPH.

Took off again for the second leg, and managed to get #4 steady at 1420, still 30 hotter than my normally last-to-peak cylinder.  After about 30 minutes of flight though it ticked back to 1380 and stayed there, at 8.9GPH, for the next 4.0 hours, just like normal.  No further problems noted at all.

Any thoughts on this, or what could cause a cylinder to burn hot and not behave normally with respect to mixture control?

 

 

[DonMuncy]

You may have a probe going bad. If it happens again, consistently enough to diagnose, you might try swapping probes to see if it follows the probe.

Don 

[Shadrach]

It appears that one of the spark plugs on #4 is going south.  A bad probe typically gives very unstable readings. What happened to CHT?

Some other things worth noting:

1) A mag check would have told you a lot. If something like this happens and you're alone, a mag check is a good idea. If you have passengers, then it might not be, depending on their experience/comfort level.

2) Why are you 50LOP at 8500 and up? That is way too lean at that altitude to my way of thinking. You're likely leaving 10kts (or more) on the table running that lean at that alt.

3) A cylinder fired by a single plug has a much slower combustion event. The result is that the combustion event is still taking place when the exhaust valve opens. This means that the EGT probe is know being hit with burning F/A mixture, which is why EGTs increase on a sinlge plug. If you ever lose a mag in flight you will see a uniform EGT rise and typically a slight CHT drop.

 

 

 

[Cruiser]

yep, sounds like a spark plug or the plug wire on #4. EGT values in and by themselves are not critical numbers but they do offer insight to engine operation as you have seen.

50°F LOP on the leanest cylinder makes your richer cylinder even leaner, what is the spread between richest and leanest?

[Shadrach]

Quote: Cruiser

yep, sounds like a spark plug or the plug wire on #4. EGT values in and by themselves are not critical numbers but they do offer insight to engine operation as you have seen.

50°F LOP on the leanest cylinder makes your richer cylinder even leaner, what is the spread between richest and leanest?

[Swingin]

Mags checked good on the ground for both legs @ 1700RPM.  CHTs were normal for all phases of flight.

Shadrach:  What would you recommend running instead of 50LOP, and why?

Cruiser: EGTs are within 20 of each other. #3 peaks last so that's what I set LOP off of.  #4 is always about 10 degrees cooler when LOP.

[PilotDerek]

A buddy I fly with had an issue with high EGT temps on a flight. He couldn't figure it out. Then when he was calling it quits on trying to troubleshoot it he noticed he was only on the left mag not both. I learned a couple lessons from his mistake. 1. Always do a mag check (he didn't do one and has since learned) 2. Abnormally high EGT will first draw my attention to the mags. So I agree with Ross and cruiser. Check your plugs and/or wires might be a loose connection that's why it’s being intermittent.

[danb35]

I'd suggest running closer to peak, maybe 10-20 LOP, at higher altitudes, to get more power out of the engine (so long as CHTs remain acceptable, at or below 380).  Did you try a mag check in flight?  Those are much more informative than the normal ground checks as part of the runup.

[Shadrach]

Quote: Swingin

Mags checked good on the ground for both legs @ 1700RPM.  CHTs were normal for all phases of flight.

Shadrach:  What would you recommend running instead of 50LOP, and why?

Cruiser: EGTs are within 20 of each other. #3 peaks last so that's what I set LOP off of.  #4 is always about 10 degrees cooler when LOP.

[KSMooniac]

Ross and Dan are correct and giving great operational advice.  I also concur that you likely have a plug going south, and a cruise LOP mag check will tell you which one.

[testwest]

+1 for Dan and Ross. Ross's explainations are dead nuts on. Also +1 for Ross on inflight diagnosis of the bad mag. It has been said many times, but bears repeating for casual readers of this post....an all-cylinder engine analyzer is the best money you will ever spend on your plane.

When you pull the plugs on the #4 cylinder, check the center electrode resistance. The nominal new value is 1000ish ohms. More than 5000 ohms, think seriously about replacing them even if all other checks look OK. Tempest plugs have a far better design in this regard, and they are slightly less expensive for massives than brand C.

 

[danb35]

Quote: testwest

an all-cylinder engine analyzer is the best money you will ever spend on your plane.

[Shadrach]

Quote: testwest

+1 for Dan and Ross. Ross's explainations are dead nuts on. Also +1 for Ross on inflight diagnosis of the bad mag...

[testwest]

You are welcome, sir.

And thank you again for taking time to make thoughtful, factual posts.....everyone wins.

[Swingin]

I have a new exhaust system and I'm seeing lower than expected CHT values now.  While having an engine analyzer is nice, I think it makes me somewhat of a hypochondriac when it comes to my plane!  At least I learn a lot from it, especially when I ask questions here.

Here are the CHT values I was seeing today in the radar pattern at 6K' MSL, WOT 2500 RPM, 50 ROP, and cowl flaps closed:

#1 290 #2 190  #3 190  #4 320.  Does it mean anything that 2 and 3 rarely get much above 200, even in the climb?  OAT today probably 35F.

edit: I have a cowl enclosure mod, not a full 'J' cowl but it closes off the bottom opening to the spinner.

[danb35]

My money says instrumentation error--just doesn't seem likely at all that (1) there would be that much difference between cylinders in a well-running engine, or (2) that two cylinders could be running that cool at a cruise power setting.  Was anything done to the probes in the exhaust installation?  Possibility of connector damage, etc.?  What happens if you swap wires between 1 and 3, 2 and 4?

[Shadrach]

Quote: Swingin

I have a new exhaust system and I'm seeing lower than expected CHT values now.  While having an engine analyzer is nice, I think it makes me somewhat of a hypochondriac when it comes to my plane!  At least I learn a lot from it, especially when I ask questions here.

Here are the CHT values I was seeing today in the radar pattern at 6K' MSL, WOT 2500 RPM, 50 ROP, and cowl flaps closed:

#1 290 #2 190  #3 190  #4 320.  Does it mean anything that 2 and 3 rarely get much above 200, even in the climb?  OAT today probably 35F.

edit: I have a cowl enclosure mod, not a full 'J' cowl but it closes off the bottom opening to the spinner.

[Cruiser]

where are the probes mounted for these cylinders? Are all of them the same? Which cylinder is the OEM (panel) CHT connected and what is it reading?