Troubleshooting a rough-running engine

Q: My Lycoming TIO-540 engine on my Cessna T206H is running very rough at max power setting. I recently replaced one dud cylinder due to very low compression. The ring gaps were wide and exhaust valve guide and seat were bad. Presently all compressions are good, ignition timing is good, and turbo giving boost. The prop looks OK. The engine has TT of 375 hours. The plane is AOG in a remote location. Any ideas?

COLIN WARD, via email

A: It sounds to me as though you are in a sticky situation. First, I’m surprised that you’ve already had to replace one cylinder due to low compression with the low hours on this engine. The information you supplied as to the condition of that cylinder would lead me to believe it had been run excessively lean either at takeoff power and/or over a longer period of time and possibly both. The results you mentioned are almost always related to lean operation, which causes excessive heat which, in turn, may cause excessive valve guide and seat wear.

You mentioned the wide ring gap but I’m going to assume you meant ring groove. Usually ring gap relates to the ring end gap. The wide ring groove may be caused by rapid descents at low manifold pressure where the propeller is actually driving the engine rather than the engine driving the propeller. If this is the case, the top ring groove will usually become wider and eventually could lead to an engine failure if the rings breaks and the piston material contaminates the oil system.

You didn’t mention the specific hours when that cylinder required replacement, but it’s unusual for this to occur even at a TT of 375 hours. You also didn’t mention if you have a six-point CHT, EGT system or just CHT and TIT. If you have a six-point EGT system I expect you would have noticed a much different reading on the cylinder you replaced as an indicator of some kind of a problem. You may also have noticed a higher than normal fuel flow at takeoff power, which could be an indication of a partially plugged nozzle, which results in a lean cylinder.

Where do we go from here? The first thing I would do is remove all of the fuel nozzles from the cylinders and flow check them for proper flow. Important information for this procedure is in Lycoming Service Instruction 1275C. Since we’re doing this check with the engine on the aircraft, we’ll do things a little differently, but the results we’re looking for will provide us with the information we’re looking for. The information in SI 1275C will provide important information regarding cleaning and proper torque.

Caution should be taken when reattaching the fuel line to the nozzle so as not to over-tighten the fuel line nut. Over-tightening this nut may cause the steel insert used in the two-piece nozzle to cause small hairline cracks in the top of the nozzle itself, which may cause an erratic fuel flow, resulting in poor operation and possible engine damage.

Following removal from the cylinder, reconnect the nozzle to its respective fuel line and put each nozzle in a clean bottle or small jar (like a baby food jar). It’s important that we can view the flow coming from the nozzle during this test. The next step is to turn the master switch on and move the throttle and mixture controls full open and then turn the boost pump on for about 30 to 45 seconds. Now we want to observe each nozzle and its flow pattern. It should be a nice straight pencil-point stream from each nozzle. If you observe any nozzle that has a wiggly, air bubbly spray pattern, this nozzle should be inspected for cracks at the top using a magnifying glass just in case this nozzle experienced an over-torque situation. The reason for the magnifying glass is simply because even the slightest of cracks will open up when the fuel line nut is attached and tightened, but when the top of the nozzle is in a relaxed state the cracks may be very difficult to see with the naked eye.

If no cracks are observed, then the next step is to clean all nozzles as described in SI 1275C. I highly recommend using Hoppes No. 9 Gun Cleaning Solvent for this. The nozzles should be soaked for at least 20 minutes. Following the soaking process, clean the nozzles with clean Stoddard solvent and blow dry with clean shop air, always blowing through the nozzle in the same direction the fuel flows.

You may want to reflow the suspect nozzle after the cleaning process and if it still demonstrates the same poor pattern, then this nozzle should be replaced before further operation. The nozzles may now be reinstalled using new packing rings. Be sure to inspect the rubber hose that connects the tube assembly of the nozzle to the vent line.

Colin, this is a good start to troubleshooting the problem and, hopefully, these corrective actions will rid you of your problem.

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


  1. Steve Ells says

    Another important part of the “baby bottle” flow check is determining if the nozzles are matched; do they all deliver the same AMOUNT of fuel. 

Leave a Reply

Your email address will not be published. Required fields are marked *