Q: My #1 cylinder is running 100° hotter than the rest on my Lycoming O-360 engine. What do you think could cause this ?
John Madsen,
BC Canada
A: John, I can believe you have a difference between the number one cylinder and the remaining three cylinders.
On a normally aspirated engine, with a carburetor, the fuel distribution between all of the cylinders is not equal. I’ve covered this subject in some of my previous articles and I wouldn’t be concerned at all about what you are seeing.
If you take a close look at the length of the intake pipes you’ll notice they are not all the same length. This in itself can cause a different amount of the fuel/air mixture to be slightly different between the cylinders.
The key thing to keep in mind here is not to exceed the maximum cylinder head temperature set forth in the Lycoming Operator’s Manual. For the parallel valve engines like your O-360 series, the maximum CHT is 500° for maximum continuous operation. Ideally, you’d want to keep your CHT between 350° and 435° for best service life.
I would caution you about the accuracy of your CHT temperature gauge, so make certain it’s providing you with an accurate reading. In other words, how old is it and when was the last time it was calibrated?
Another simple check is if you have a four point CHT system, I’d suggest you swap the number one cylinder with the number two cylinder to confirm the actual reading on the number one cylinder. If the temperature moves to the number two cylinder, then you know that probe may be the problem.
If this were my aircraft, I really wouldn’t be too concerned, but I think a little troubleshooting like swapping the probes may help you sort out the issue.
Number one cylinder should be one of the coldest for CHT. I guess we’re Assuming he’s talking about CHT, if it’s EGT that’s nothing. If number one CHT is 100 degrees higher than all the others there’s a problem and it’s not an intake leak problem, that only manifest itself in low RPMs/ manifold pressure.
100F above the others is incomplete data. What are the other CHT numbers for all cylinders? Is this in WOT climb at Vx, or in leaned cruise? Are cowl flaps involved?
Is the data derived from an engine monitor, or?
According to GAMI, a redline of 500F is engine abuse because detonation margins are probably less than zero.
At WOT, intake vacuum differences are nil. Assuming the unspecified CHT instrument is accurate, Isn’t there a simple WOT/lean test to see if overheating is a lean-mixture problem vs a cooling problem?
I would consider a leak check of the intake system. Mainly at the cyl head where the intake is connected. Does this engine idle smoothly. Sometimes a water spray bottle can be used at idle rpm. If the engine gets a bit smoother with water spray you found it.
Some variation between cylinders is expected, but 100 degrees is too much. He needs to borescope that cylinder and find out what’s going on.
That and check baffling. Borescoping should be part of regular maintenance, done at least at each annual.
A borescope will show damage from detonation/pre-ignition and valve damage. What does one look for with a borescope to disclose a source of high CHT?