What is causing this engine’s vibration problem?

QUESTION: I have a vibration problem with my IO-540-C4B5. I had a computer analysis done and it seems to be the engine instead of the prop. The test showed to add 96 grams at the 72° position and this seemed way out of tolerance. What can cause vibration instead of the prop being out of balance? A local IA seems to think it is carbon build up in the exhaust valve guide vanes. Any help or suggestions would be appreciated.

Lou Lacy
via email

ANSWER: Lou, I’ll take a shot at this one even though it’d be great if I had more information. I’m going to assume several things regarding the engine and maybe, just by blind luck, I can get a seed planted that may help you solve your problem. This is one of those questions where you may have already tried everything I’m going to mention, but just humor me and try these things again.

My first assumption is that since the engine model is an IO-540-C4B5, it’s probably installed in a Piper Aztec. My reason for making that assumption is because that was the most common installation for that specific model. Before we really get started, I’d like to know when this vibration was first noticed. Can it be tied into just a routine maintenance event, following engine overhaul, prop change, the prop making contact with the concrete runway, or anything that may have some operational impact on the entire installation? I’d suggest a very close visual inspection of the entire engine compartment looking for any hoses, lines, or anything else that may be out of place.

If you can’t pinpoint anything, then my next question is have the engine mount rubbers been replaced with new ones of the correct part number? It’s amazing how many vibration problems and other things that appeared to be engine related were corrected by changing the mount rubbers.

Is the vibration constant throughout the rpm range or just in certain ranges? While you’ve got your nose under the cowl, check the baffles to make sure one of them isn’t contacting an engine mount tube improperly. The oil cooler and lines should also be inspected closely for any improper point of contact. Keep in mind that things do move around under the cowl during normal operation so take this into consideration during your inspection because what may look fine under static conditions may change once the engine is started.

If we take the idea that your local IA suggested, that can be checked by complying with Lycoming Service Bulletin 388B and following up, if required, with Service Instruction 1425A. Other general maintenance things that should be checked are spark plugs, fuel injector nozzles, and ignition wires — just in case a little engine roughness is being confused with a vibration.

A good walk around of the aircraft is also in order since it could be something as simple as a loose avionics antenna or gear door.

Have you swapped the props from one engine to another? If the vibration remains with the prop, then we have some place to start. If it stays with the engine, then we’ll focus our attention there. If it does stay with the engine, it’s important you check closely those things we mentioned. I guess the worst case situation could be that the engine crankshaft has the incorrect crankshaft counterweight rollers installed or they’re installed in the wrong location. I’d suggest doing a lot of good commonsense troubleshooting to eliminate everything else before I’d dig that deep.

Starting out with a broad question allows me to go off in almost any direction — all of which may be wrong — but at least we can give lots of general ideas where the trouble may be found.

I can see all of those “”so-called experts”” that used to visit our booth at Oshkosh and attend our maintenance seminars rolling their eyes and saying under their breath, “”Why doesn’t he just answer the question!”” Well, it’s just not that easy, is it? If you’re going to eat an elephant, you’ve got to do it one bite at a time. With a question like this, it’s very difficult to know where to begin.

Lou, this is just one of those situations that really require you to research the past operational and maintenance history of the engine in order to find a good starting point. Hopefully, some of the things suggested here will get you started off in the right direction to solve the problem.
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Paul McBride, recognized worldwide as an expert on engines, retired after almost 40 years with Lycoming. Send your questions to: [email protected].