Hang around an aircraft maintenance shop long enough and you’ll hear about every problem imaginable—and unimaginable. A recent one comes from the squawk list of a flight school Cherokee Warrior with a Lycoming O-320 that runs-on for about two revolutions upon engine shutdown. This is somewhat common and worth talking about. We chuckled at the student’s writeup: “Mixture control must have got disconnected.” Well, not exactly.
If you’re faced with this issue, one way to stop the engine is to first close the throttle and then retard the mixture to idle cutoff with the ignition switch in the Both magneto position until the engine stops. What’s happening here with this gasoline engine is called dieseling, a popular term in maintenance circles for engine run-on. Real diesel engines, otherwise called compression ignition engines, will run without any source for ignition due to the spontaneous combustion of air and fuel under a heavy compression ratio, commonly 22:1. Remove the air or fuel and the engine quits turning.
This happens because fuel and air are not sufficiently reduced in volume to eliminate the combustion process. Poor mixture cutoff due to a worn mixture control valve barrel or pulling of fuel into the induction system through the manual primer circuit are the two main causes for this condition. A high carburetor float level, leaking float needle and seat assembly or an overly rich idle mixture can also contribute to the problem.
A solid suggestion is to set idle speed and mixture to maintenance manual specs and plug off the fuel primer circuit to eliminate the obvious things. If the problem continues, it’s time to remove the carburetor for further evaluation. As some have noted, Lycoming’s SB 366C for Lycoming engines equipped with Marvel-Schebler, Facet, Precision, or Volare updraft carburetor models MA-3A, MA-3PA, MA-3SPA, MA-4SPA, MA-4-5, MA-4-5AA, and MA-6AA is important. It goes back to 2016 with compliance within the next 50 hours of engine operation and every 50 hours thereafter during routine engine maintenance. Instances were reported of leakage through the gasket between the bowl assembly and throttle body of the carburetor, evident by fuel stains in the area of the leak. Moreover, the bulletin notes that leakage of this type can be accompanied by loose screws that attach the bowl and throttle body.
This is a common symptom of a carb that has loose carb bowl screws Lycomming SB 366 C. If you are getting the diesel’ing, I would ground the plane until it can be properly inspected. This could be a symptom of a much riskier issue.
Since the ignition is on, there will be a spark, so this is different from a car engine doing the same thing.
If the dieseling happens when the temps are very warm and/or when using mogas, you might have a situation known as pneumatic lock. This happens when fuel boils in the carb bowl or fuel line to the carb, building pressure in the line. This can lead to fuel getting past the cutoff and allowing the engine to stumble on. Check for a loose carb bowl first, but know this is a possibility.