Ask Paul: Why are my #3 cylinder CHTs so high?

Q: Recently — 50 hours ago — I had a Lycoming O-360-A4A overhauled with new ECi Titan steel cylinders installed. The #3 cylinder has had high-ish CHTs from the beginning. The first flight was 475° during takeoff. Even now, after 50 hours, there can be a 70°-100° difference during takeoff, and 50°-75° difference during cruise.

As for conditions: My ground is almost 6,000 MSL, and during the first 20 hours I headed east and was able to keep it at 6,500 and full power.

Neither the overhaul shop nor ECi consider 475° to be a problem at takeoff, but it seems unusual to me. I updated the baffling and swapped probes and nothing seems to make any difference.

Any ideas or should I just disconnect the engine monitor so I won’t be bothered anymore?


 A: Hmmm, this doesn’t sound like such a difficult question to answer, so let’s see where it takes us. First of all, it is not unusual for engine operating temperatures to increase following the replacement of the cylinders. As a matter of fact, this is quite common following complete engine overhaul, as well. This is simply a result of closer tolerances of the engine components.

Not knowing what aircraft your engine is installed in, I can say from my experience that in most four-cylinder engines installed in a typical Piper, Cessna, or Mooney, it is usually the #3 cylinder that runs the hottest just because of its location.

Lycoming actually does not know which cylinder runs the hottest on any particular engine installation. This is always defined by the airframe manufacturer from the various tests they conduct during the certification of the aircraft. I still feel comfortable in placing my bet on the #3 cylinder being the hottest in most all installations though.

If your concern is seeing the #3 cylinder peak at 475° during takeoff, my thought is if you are confident in the accuracy of your instrumentation, I would not be alarmed.

The question I have is: Does it ever get hotter than 500° continuous? The maximum allowed cylinder head temperature for your engine is 500° F, but for maximum service life I’d like to see between 385° and 400° or a little higher for continuous operation.

This is why it is so important to know your instrumentation accuracy and justifies having the instrumentation calibrated from time to time. Not knowing the accuracy could end up costing you a lot more money than the cost of calibration.

One other thing you may want to check is the intake pipe gasket for leaks on the #3 cylinder. It may be that the gasket is not installed correctly, allowing the #3 cylinder to run a little on the lean side, in turn elevating the cylinder head temperature. While we’re in this area, you should also check the integrity of the intake pipe hose at the sump for clamp tightness. It doesn’t take much of a leak to allow a cylinder to run a little lean which, in turn, may result in a higher CHT on that cylinder.

If there are no leaks, the information you’ve provided is accurate, and you’re confident in the CHT readings you are getting, I’d have to say I agree with the ECi folks and the overhaul shop. I would expect that as you continue to put more time on the engine the temperatures you are now observing will come down as the break-in continues.

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  1. Dave Palacios says

    Let me take a shot here. with no induction leak at cruse power (maybe 75% or more) you can expect a best power fuel air mixture, with a induction leak (upstream from the fuel metering device) you probably would go thru a stochemetric mixture and get detonation and lots of heat. Granted if you get enough air (or much more than enough), you are on the other side of the curve and not enough gas for combustion and lower temps.
    Dave P.

  2. Thom Cook says

    Please explain how a cylinder can be hotter when you are decreasing the fuel to it and therefore more air in the combustion cycle so it would be logical for it to have a LOWER CHT and not a higher one. If my understanding of basic combustion is incorrect, please advise.

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