Q: I bought a 1972 Cessna 177B with an O-360 a few months ago. It had 900 hours on the engine. My first 20 hours I would use 23 squared and pull the mixture back to 10 gph or less, then watch the cylinder head temperatures, monitoring to keep the hottest (#2) around 385°.
When the ambient temps started rising, I noticed #2 and #4 were spiking into the 420° CHT level on climb out. When I level off and set to 23 squared, I can only keep #2 below 400° by going full rich (12.5 gph) and occasionally opening the cowl flaps.
If I use 10 gph, #2 will stay around 415°. #4 will run in the 380s, and #1 and #3 will run in the 350s.
Swapping probes did not go as planned, as the problem didn’t change. I am growing concerned and looking for advice.
Thank you for your time.
Greg McKeever, via email
A: Here’s a situation that may have several contributing factors that could result in the numbers you’re seeing.
My first inclination is, for some reason, we’ve had a change in air flow through the cowling, especially in the front where the #2 cylinder is located.
I’d recommend you take a very close look in that area and you may find there are some gaps that may have been a result of cowling removal and re-installation or a piece of baffling that has gotten out of place.
Sometimes it doesn’t take much to allow incoming air to sneak past where it’s supposed to go and change temperatures in that area.As ambient temperatures increase, it becomes even more important that the air coming into the cowl be directed properly to assure best cylinder cooling.
With the difference between the #2 and #4 cylinders when you are at 23 squared confirms we need to focus on the baffling and cowl fit at the #2 cylinder area.
If the aircraft is a 1972 year model and this is the original engine with 900 hours on it, I’d be curious to learn the overall condition of the engine baffling.
Don’t forget, the baffling lives in a very hostile environment from all of the engine heat, so make certain it’s properly located and not drooping, allowing air to escape rather than being directed for proper cooling.
If the baffling does require replacement, don’t go with the cheap stuff!
Another thing that I’d like you to check is to be absolutely certain that you don’t have an induction leak on the #2 cylinder intake pipe either at the cylinder head end (leaking gasket) or at the intake pipe connection point at the oil sump.
If there is an induction leak, it may be indicated by a high manifold pressure reading on your manifold pressure gauge at engine idle of 650 to 700 RPM.
With no leaks in the induction system I’d expect to see a reading of right around 10″. If you see a reading higher than that, say 11 or 12, this could indicate there may be an induction leak.
I appreciate the fact that you provided some actual temperatures you recorded during flights. Using those numbers as references, I think you’re still going to be okay, but let’s see what else we might want to check.
First of all, the maximum CHT for your O-360 series engine is 500°F, and for maximum service life, we’d like to keep it at 400°F or below.
There is no doubt that as ambient temperatures increase we know this will have an impact on engine CHTs. Let’s assume nothing has changed except for the increase in ambient temperatures. That gives us two choices when it comes to cooling the engine.
You can either cool it with more fuel, which you already know, or by using the cowl flaps, which you also know.
If during very hot days you can keep the #2 cylinder in that 415° area, I wouldn’t be too concerned because I don’t have a great deal of confidence in the accuracy of the CHT gauge as being dead on.
The other thing that I get concerned about is the accuracy of the tachometer. It’s not uncommon for engine tachs to read a little low, so maybe the 2,300 RPM you are seeing on the tach may actually be 2,400 RPM or a bit more.
That means the fuel flow you are putting into the engine isn’t quite enough for the additional horsepower you’re taking out of the engine. This may be the reason that when you went to 12.5 gph at 2,300 RPM you were able to keep the #2 cylinder below 400°.
I’ll wrap this up by saying you are very close to being where we want you to be, and that a little tweaking here and there should get you in the ballpark.
One last comment with regard to using more fuel to keep the engine cool: If you really stop and think about it, one of the least expensive things you can put in your engine is fuel.