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Help Troubleshoot Lycoming/ McCauly Combo

Brockster

Well Known Member
Have been trying to help a friend troubleshoot a Lycoming O-360 / McCauly constant speed prop combination on his Cessna 177 Cardinal. Engine rpm varies approximately 100 rpm or so while in cruise flight. Creates a definite hesitation while flying. There does not seem to be any pattern to it as it comes and goes sometimes 30 seconds or several minutes between occurances. The occurance only lasts a few seconds. Occurs also at reduced power settings but tends to be less severe. Manifold pressure and oil pressure are solid before, during and after occurrances. Mag check on the ground and in flight are both good. In flight with either mag off does not stop problem. McCauly prop currently installed was brand new and has only a couple hundred hours on it. Previous McCauly prop which could not be overhauled also had the same problem. Original McCauly govenor overhauled and did not help. Switched to newly overhauled Woodward govenor and again did not help. Accomplished Lycoming Service instruction #1462A, Propellor oil control leak test procedure and no findings. Mags have been torn down and checked per overhaul manual and nothing found. Spark plugs and leads tested with no findings. Really starting to scratch my head as to what the problem could be. Any suggestions would be greatly appreciated. Problem has been there i'm told for some time but never gets worse or better. Engine has 1750 hours with New ECI cylinders 325 hours ago. Will be trying all new plugs but after that I am out of ideas.
 
RPM's go down. Compressions were all good this last recent annual except number 3 which needed the exhaust guide replaced and seat ground. Also found top plasma coated compression ring was peeling coating. This is supposedly common for ECI cylinders with this first generation ring type. Newer generations have a modification to the ring to prevent this. Was told other cylinders could also have this happening to them but should not be a problem.
 
ECI cylinders

Serial numbers should be checked against the AD. If those cyl were off some bodies shelf they may fall into the ad for cracking
 
worth a try

Try a test flight with the P leds disconnected from the mags. Years ago I spent thousands changing out components to solve a semilar problem. An old mechanic finally unhooked the P leds and the problem went away. The old noise filters were trying to ground the P leds.
 
Try a test flight with the P leds disconnected from the mags. Years ago I spent thousands changing out components to solve a semilar problem. An old mechanic finally unhooked the P leds and the problem went away. The old noise filters were trying to ground the P leds.

Never would have thought of that one! Makes perfect sense.
 
EGT does not change but all cylinders are not monitored. Aware of the AD on the cylinders and visual inspection and compression check shows no cracks. I will try disconnecting p leads as that is very easy to do and doesn't cost anything. Will be a few days before it can fly as the cowl is being painted after repairs to fiberglass nose piece. Thank you very much for the recommendations and will follow up with the results.
 
If EGT were rising in any cylinder, I'd probably look first at ignition. If EGT isn't rising, then I might be looking for something going on in the valve train - issue with lifter assembly, possibly broken spring, etc.

Dan
 
Was there ever a resolution on this issue? I’m chasing one that sounds identical. 1956 Cessna 172, Bolen TW conversion, Avcon STC, Hartzell prop and governor.
 
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