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Sam James Cowl overheat question

my53cad

Member
I have a RV 9A with an O-320-E2D engine and an overheat question. #3 cylinder runs about 25 degrees hotter than all other cylinders. Over 400 degrees on takeoff and climbout. Compressions on cylinders are all good and 78/80PSI. I have a Sam James cowl and have checked the plenum for leaks and found none.. Is there a way to check whether the upper chamber (high pressure) vs lower cowl (lower pressure) is functioning as designed? Thanks
 
Functioning Properly?

There are ways of installing static pressure sensors, but that may not answer your question. The flow throughput past each cylinder depends on many variables. What you are looking for is functional cooling right?

Some things you might have done already:

A quick check to narrow down the assumptions might be to swap probes and verify that the difference is actually in the temperature rather than the indication.

Next thing to rule out is the possibility that that cylinder is running leaner than the others. Somehow producing more excess heat rather than getting less cooling.
 
Last edited:
Sam James Cowl overheat question.

Hi Ron, I have the Sam James Cowl and plenum on my 6A.

Couple of things come to mind. Behind #3 cylinder you can put a washer between the back of the cylinder and the baffle. This allows a little extra air past the cylinder.

Check that there is no casting flash between the cylinder fins in this area, mine were partially blocked. (Should check all the cylinders for this issue)

Oh, and double check your timing.


Terry
 
Just Adding to What the Others Said

Are your EGTs pretty much in line with each other? A high CHT along with a high EGT points pretty strongly to a plugged injector.

And Terry is absolutely correct in stating that the #3 cylinder often runs hot due to the lack of an air path through cooling fins on that back side where the baffle touches the cylinder head fins. A washer on that back screw will give just a bit of breathing room for air to pass by. If you remove the plenum and look straight down, you'll see where the casting of the cylinder goes right up to the back edge of the fin, effectively stopping airflow in that small area.

If your plenum is fitting well and you don't want to create a gap with the washer trick, an alternative is to cut out a section of the back baffle, fit in a small spacer rectangular ring (0.062" thick material will work), then rivet in a back cover to seal it up. This approach gives the air a path around that blocked area down to the lower section of the fins. This is what my EAA tech counselor recommended when I didn't want to modify my plenum. Other than the hassle of removing that back baffle, it really didn't take much time. I may not be explaining it well, but you can search on this site and see pics of what people have done. It's a relatively small area that's affected; maybe less than 2" wide.

One more thing: you said you've checked the plenum for leaks and it's good. Have you checked everywhere that the baffles fit against the cylinders and the crankcase? There are several gaps made into those baffles where they fit against the engine--some are almost 1/4" wide at the cylinder flange. The inter-cylinder baffles installed by the engine supplier are also a notorious source of leaks. If you haven't already done so, get a flashlight and drop it down between 1 & 3 and between 2 & 4. If you can see light underneath that's not a fin, it needs to be sealed with RTV. You can dab RTV on the end of a dowel to get into the hard to reach areas. Again, my tech counselor had me do this. I was skeptical, but it absolutely made a difference.
 
Adding a little bit more...

The #3 rear baffle (CB-907A) has 3 sections that need to be bent to mate with the cylinder profile.

It is not uncommon for the innermost tab (closest to the case) to remain unbent, thereby leaving a 1" x 1.25" hole for air to escape the upper plenum and do no useful work.
 
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