Ask Paul: Too much information about our engines?

Q: I have a factory reman Lycoming O-540 E4B5 with 300 hours and 2.5 years on it. At idle (1,000 rpm), my JPI is showing alarms on the #1 EGT for having a large differential — #1 EGT is only 600° while the rest are 1,200-1,300°. Above 1,500 rpm and higher everything seems fine with normal readings and maybe #1 CHT a little lower, but I expect that since it is first in the airstream. I have full data printouts from the JPI.

I have checked the plugs even though my mag checks are fine and replaced the probe on #1 EGT with no change. I’m scared to think I have stuck valves. My mechanic thought that Marvel Mystery Oil (MMO) may do the trick, but I would like to have a more firm diagnosis and result. Do you have ideas on the issue, troubleshooting, or recommendation on a fix. Could MMO help this?

BRIAN MANLEY, via email

A: Brian, your question doesn’t come as a shock to me. I’ll try to explain the reasons for what you are seeing on your JPI.

First and foremost, I do not believe you have a problem with the engine. I believe with the sensitivity of the JPI system, you are seeing results of what we’ve known as “normal” operation for an O-540 series Lycoming engine. Actually, this is common to most all normally aspirated engines using a carburetor. This is a result of a less than perfect fuel distribution — in other words, it’s poor at best through the induction system on any engine utilizing a carburetor. One of the factors that has an impact on this distribution is the length of the intake pipes, where some are longer than others. This fact alters how much and how the fuel/air mixture enters the combustion chamber. Since the #1 cylinder is the most forward, this little difference can be what you are seeing. Because you are seeing this at 1,000 rpm it also may be at that point the carburetor is in the transition between the idle jet mixture and the main jet. Once you increase the rpm and get a more even fuel/air mixture, the fuel supply becomes more even between the cylinders and the EGT temperatures fall in line.

I think you can dismiss the possibility of a sticking exhaust valve unless you have experienced other symptoms. Typically, if you have a sticking exhaust valve, you will feel a noticeable roughness of the engine on the first start-up of the day, which should alert you that something isn’t just right. Shutting down the engine at this point is a good idea. Have a look see under the cowl for a bent pushrod housing or an oil leak in the shroud tube area.

Regarding the use of MMO, I know it is not approved for use by Lycoming, but I’m not blind to the fact that it has been used over the years. I honestly don’t believe it would harm anything, but don’t necessarily think it would do any good either. At this point considering what you have noted, I don’t think anything like that is needed at this time.

Just as aside, I’d like to share a true story with you about having too much information available to you in the cockpit. Many years ago a gentleman named Max Conrad set many world records for long distance flights mostly, if not all, in Piper aircraft. I once asked Max what he would do if he were several hundred miles out over the North Atlantic and his oil pressure gauge began to fluctuate. He looked directly in my eye and said “I wouldn’t do anything because I wouldn’t know it.” Shocked, I asked why he wouldn’t know it. He said the answer was simple: “I disconnect the oil pressure gauge before the flight!” Could this be where the statement “what you don’t know can’t hurt you” came from? I certainly can’t say I subscribed to this type of thinking, but maybe the excellent information provided by the JPI instrument may be just a little too much information in your particular situation.

If you’ve put 300 hours on the engine in 2.5 years I commend you for that and hope you continue that type of usage. The worst thing for any engine is to suffer extended periods of inactivity and that’s certainly not the case with your aircraft.


Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:




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