Is it a sticking valve? An induction leak?

QUESTION: I have a 1993 Glasair experimental that has a standard Lycoming 180 hp IO-360A1B (fuel injected). Some months ago, my #2 EGT suddenly started to climb to about 1480° in cruise and at altitude. The other cylinders stayed the same and have never changed. I tried all different power settings and still no change. It’s repeated itself a number of times. I just had an EDM-700 unit installed with all new probes and this, too, shows a 90-100° EGT difference in cylinder #2 (and CHT is about 418 in cruise, with EGT finally settling down to 1435 in cruise, 50° rich of peak, with all other cylinders in the mid-1300s). I just had all fuel injectors cleaned thoroughly, and they were a bit dirty, but not terribly dirty. My plugs were just cleaned and regapped during a recent annual (after this EGT rise began) and still #2 EGT is elevated. One mechanic tells me not to worry, but seeing the #2 EGT rise during climb is somewhat worrisome.

Is this a sticking valve? An induction leak? A cylinder issue? If this were a magneto/timing issue, wouldn’t all cylinders be affected, not just #2? I am tempted to just change the plugs on #2 and see if this resolves it.

ANSWER: I enjoyed your question and can assure you that you’re not the first nor will you be the last to encounter a situation like this. I think some basic troubleshooting is called for here, so let’s see if we can assist you in finding the problem.

First of all even though the EGT on the number 2 cylinder has reached 1480°, it has not exceeded any limits nor has the CHT at 418° F in cruise. The maximum EGT is 1650° F and on a normally aspirated (non-turbocharged) engine I don’t think you could get there under normal circumstances. I don’t think this number is published anywhere, but is based on the maximum temperature for turbocharged engines. The maximum CHT for your engine is 475° F.

Some of the things I’m about to suggest you may have already checked, but I’ll go over several things that could contribute to what you have seen. One of the first things I’d check is the actual fuel flow from each individual nozzle. This is relativity easy and I’m certain you may be familiar with this process. Just in case you’re not, here’s how to conduct the test. Remove each nozzle from the cylinder and reattach it to its fuel line. As it hangs on the line put it into a glass jar such as a baby food jar or something similar. After you’ve done this to each nozzle, have someone turn the master switch on and put the throttle at the full throttle position and the mixture control in full rich and turn the boost pump on. Observe the flow pattern of each nozzle, which should be a nice straight pencil point stream into the bottles. Any distortion or wiggly flow may indicate an obstruction, which could cause restricted fuel flow, increasing the EGT and CHT. If this condition is noticed I’d suggest soaking the nozzles in Hoppes No. 9 gun oil for a period of time followed by blowing “”clean”” shop air through the nozzle in the direction the fuel flows. Following this step, I’d suggest you put the nozzle between your thumb and forefinger while holding it up to a light and roll it back and forth to get a full rotation. What we are looking for is a foreign object that may be floating around from the air side to the fuel side of the nozzle. If all appears OK, then I’d hook up the nozzles and again check the flow into the bottles before reinstalling them.

Another thing that may cause what you are seeing is a small induction leak, so checking the integrity of the intake pipes and hoses is recommended. An easy way to do this is to use soap and water and brush it around the area and look for bubbles while running the engine at idle rpm. If there is an intake leak you may also notice this by seeing a higher manifold pressure at idle rpm. For your engine the “”normal”” MAP at idle should be around 10″” or so. If it’s running up around 12″” to 13″” that could indicate an induction leak. You might also look for fuel stains around the intake pipe flange and gasket too.

I wouldn’t be too concerned about the 90-100° difference in the EGT reading because there are so many factors that could impact this and, again, no limits are being exceeded, including the CHT. As far as the CHT goes, I’d recheck the baffling and make certain you haven’t overlooked something in that area, but from what you’ve told us, I’d say the baffling is probably OK and the CHT is being dragged up by the change in EGT, so I’d look for something on the fuel side (restriction) or an induction leak.

No, I do not think it’s a sticking valve situation. Yes, it may be an induction leak rather than a cylinder issue.

You are correct that if it were a timing problem all cylinders would probably see the same affect. I do not think changing the spark plug will solve the issue unless the plug had been dropped prior to being installed in the engine and then I’d think you’d have noticed it in other ways.

Thanks again for your interesting question.

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


  1. Gianni Zuliani says

    On my Lycoming O-320 E2D I found recently that my Cyl 3 has bad combustion.
    I can see it from the exhaust (I’m using 4 indipendent exhaust tubes) which is black and soothed. Inspection of intake pipe revealed presence of oil in its lower part where inserted into the sump.
    Could it be oil sipping through a crack in the sump?
    Maybe oil pumped through a valve guide?
    What is the best way to determine the cause and remove it?
    Many thanks

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