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EUREKA!! Probable Hot #3 Cylinder Reason Found

GalinHdz

GalinHdz

Well Known Member
I have been fighting an intermittent hot #3 cylinder on my Lycoming O360 for years and nothing seemed to work. When the problem shows up I have to run the engine quite rich to bring the #3 CHT to just under 400F but at the same time the the other cylinders were at least 50 degrees cooler. Worst still on many flight the CHT's were fine and all 4 cylinders were very close to each other. I tried changing the CHT probe with no luck and my baffling was perfect, or so I thought. Pardon the pun, but this was baffling.

After reading a completely un unrelated thread here on inexpensive borescopes, I bought a Vividia VA-400 Borescope. Out of curiosity I used the borescope and looked into the firewall with the cowl on. I found that, when installed, the top cowl was folding part of the baffling down and almost completely blocking air to, you guessed it, the #3 cylinder head. :eek:

You can clearly see it in the picture:
view


With the cowl top off, the baffles are perfectly aligned with no blockage whatsoever so everything looks good. With the cowl top on, you can't see the baffling near cylinder #3 since cylinder #1 blocks the view. I could not have found this problem without a borescope. I will be "fixing" my perfect baffling so it doesn't get folded over with the cowl top on and do a test flight but I am confident it will drop the #3 CHT by a significant amount.
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#3 is hot too

Galin, I have the same problem with a hot #3 on my RV4 and have been working on it for a year, changing injectors, checking cowling pressure in flight, doing two different baffle design changes to get more airflow on the backside of #3. Nothing has worked. I hope you've found something, and please let us know.
 
mach25 said:
Galin, I have the same problem with a hot #3 on my RV4 and have been working on it for a year, changing injectors, checking cowling pressure in flight, doing two different baffle design changes to get more airflow on the backside of #3. Nothing has worked. I hope you've found something, and please let us know.
Click to expand...
I did the "repair" today but didn't get a chance to fly. I will fly on Wednesday and report the results but I am cautiously optimistic. ;)
 
mach25 said:
Galin, I have the same problem with a hot #3 on my RV4 and have been working on it for a year, changing injectors, checking cowling pressure in flight, doing two different baffle design changes to get more airflow on the backside of #3. Nothing has worked. I hope you've found something, and please let us know.
Click to expand...

I don't want to claim victory yet but todays flight went GREAT. Flew 2hrs from St. Augustine, FL to Ft. Benning, Georgia and the #3 CHT stayed within 5 degrees of the other 3 CHT's for the entire flight. Now my "hottest" cylinder is #4 but only a few degrees hotter than #3. All 4 cylinders are within 10 degrees of each other. Not bad for a carbureted engine. We fly back on Friday so let's see how it goes. Still cautiously optimistic but feeling better.

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Completed a second 2hr flight today (from Ft. Benning back to St. Augustine) and everything went perfect again. The #3 CHT continued to stay within 5 degrees of the other 3 CHT's for the entire flight. My "hottest" cylinder was #2 but again only a few degrees hotter than #3. All 4 cylinders are still within 10 degrees of each other. Since I have flown more than 4hrs with this configuration with very similar results, I am now optimistic that I finally fixed the problem.

So if you are experiencing one cylinder getting much hotter than the others and no matter what you do nothing seems to fix it, check your baffles with the cowl ON. Make sure that, when installed, your cowl isn't bending a baffle preventing air from cooling a particular cylinder. In my case, and probably most others, you can't check the rear cylinder baffling area without a borescope.

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I now have 10hrs of flying since the baffle "fix" and the CHT's have stayed within 10degrees (between 360 and 370) of each other. The #3 CHT continues to stay within 5 degrees of the other 3 CHT's for all the flights. My "hottest" cylinder is still #2 but again only a few degrees hotter than #3. I am now confident the problem is permanently fixed.

:cool:
 
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