Question for our Engines Expert Paul McBride: I have an issue where my 800 hour Since Major Over Haul (SMOH) Lycoming O-540-C4B5 engine won’t hold its RPMs. We’ve done the low-hanging fruit of things like having the governor overhauled.
While the governor was off, we put a Lycoming test plate on the governor attach and then ran the engine with the prop now acting as a fixed-pitch. Even then, the RPMs are always hunting.
Fuel injectors cleaned in acetone. Still hunting.
12 new iridium plugs. Still hunting.
Compressions are all in the 70s. Oil pressure is solid at 80 PSI.
Do you think I should be looking at the throttle body or distribution spider at this point? Anything else?
Mike Lupo
Answer: Here are a few suggestions that may assist you in figuring out your situation with the “hunting prop” RPMs.
Before I go any further, I hope that you’ve checked the oil filter by cutting it open and inspecting for any metal contamination?
I’d also check the oil suction screen in the sump for the same there.
If I may assume that this engine is installed on a Piper Aztec, have you considered swapping governors from the other engine? I wouldn’t do this until you can confirm there is no metal contamination in the engine experiencing the problem.
Assuming you find nothing in the filter, and even though you’ve put the blank plate on the governor pad, I’d like to refer you to Lycoming Service Instruction 1462A. This check will give you the true results of what is happening at the front main bearing area of the engine, which may be the most suspect area in your situation.
You didn’t mention anything about the engine fuel flow. I’d be curious to know what it is at full throttle?
Even though you’ve cleaned the fuel nozzles, there made be a partial blockage, which is tricking the engine. If at full throttle the fuel flow goes to the top of the gauge, then we may have a partial restriction in one of the nozzles.
If your engine has the old one-piece nozzle, make certain that during installation someone hasn’t pushed the shield down over the air vent screen, which could cause issues.
If you have the two-piece nozzles, make certain there are no very small hairline cracks where the B-nut from the fuel line attaches to the nozzle.
I’d suggest you inspect each nozzle with a magnifying glass, looking for any indication of small cracks. Keep in mind that when the nut is tightened, it may open any small cracks, which may disrupt the fuel flow to that particular cylinder.
At this point Mike, I don’t think the throttle body or the flow divider is suspect.
Your troubleshooting seems to be on track so far, and now with a few more things to check, I think you’ll find the cause of your problem.
Just one of my pet peeves. Saying “RPMs” is incorrect. RPM stands for revolutions per minute. It is already plural. It is incorrect to add an “s” to RPM.
It is also incorrect to say that you had a pitch link “brake” on take off.
Sorry, but I am of the generation that was taught correct grammar and spelling by the nuns.
As Paul mentioned it would be prudent to check the oil transfer passages, etc. with SI 1462A but I’ve seen several instances where the engine passes those checks but we’ve found the steel plug inside the front of the crankshaft (it creates a sealed passage for the oil to get to the prop) loose in varying degrees. It appeared tight visually, but when a strong magnet was placed on it (and the sludge cleaned out) it was evident it was completely tight and allowed leakage. On one engine it was just flopping around in the passage. Both these engines ran normal on the ground, but shortly into the flight and after the engine warmed up, either complete rpm control was lost, or the rpm wandered around. Easy fix to put in a new plug. If your issue is related to fuel, you should be able to see indications on the fuel flow/pressure gauges and most likely the manifold pressure. If these are stable when the prop is hunting, then it’s got to in the prop system somewhere.
While checking, you might as well grab your prop blades and try increasing / decreasing pitch. If there is any easy “wiggle” you could have worn out pitch links inside the hub. Just a thought…
I had one brake once on take off… that was quite a ride around the patch. Like your efforts, took for ever to figure out what was wrong, until I happened to grab the right (bad) prop.
Good luck.
Plus sucking air upstream of the oil pump would cause modulation and engine bearing damage though not necessarily detectable on the oil pressure gauge since pressure is adjustable.
My two cents: Why are you replacing anything without proving that it will actually repair the fault? Would you do that with your automobile? Firing a parts cannon at the plane will work, but it is a foolish way to “repair” anything.
It sounds like a fuel issue to me. I’d be looking at that.
Hydromatic Propellers use a piston to control blade pitch. If the seals go bad oil will be on both side of the piston creating a pressure problem inside the hub. I’ve sent a number of low time props back to the overhaul shop for warranty fix because of RPM wondering. Usually the problem shows up before 500 hours after overhaul but, on-the-other-hand, most all constant speed propellers have a recommended 72 month seal replacement. Commercial operators are required to overhaul propellers each 72 months.