Piston engine troubleshooting

Piston Engine Troubleshooting By Thomas Ehresman March 1998 You work in a shop that specializes in turbine equipment. The few cabin class piston twins for which the FBO is responsible get their maintenance done elsewhere. From time to...

There are innumerable problems that can and do occur in piston engines. There is, however, a group of problems that are generally the most common. What follows is an aid to help quickly troubleshoot six frequently encountered problems. We'll call them the top six.

1. No start/hard starting:
There are three requirements to get an engine to start — fuel (in the right proportion), oxygen (air), and fire (timed spark). If the engine won't run, one of these things is missing.

Air is almost a given. Even with a significant intake obstruction there will be at least enough air getting into the engine to get it to fire at an idle speed. One exception here is a totally sealed off induction. This would most likely occur after the engine has been removed from storage and someone forgot to remove the intake plug. This source of the no start problem is, however, rarely encountered.

Fuel is a much more common culprit. Over priming an engine will prevent it from firing even once. The presence of a strong fuel smell in the exhaust pipe indicates a flooded engine. Moving the mixture to idle cutoff and throttle to wide open while cranking will solve this one whether you have a carbureted or injected engine. Letting the engine sit for awhile also helps get rid of excessive fuel. Use wide open throttle and mixture idle cutoff while cranking.

Lack of fuel can be just as puzzling. A slight fuel smell in the exhaust pipe can usually tip you off that you need more prime. A note on the Teledyne Continental TSIO-360. These engines take a seemingly excessive amount of prime to start. Fuel may be running out the induction drains but the engine often still does not have enough fuel to start. Best method: one, three second shot of prime and small bursts of the same while cranking.

The last starting requirement can be a little more troublesome. The magneto on a piston engine is designed with a start feature that retards the timing to prevent "kick back" and provides additional energy to the spark event. This additional energy is required because of the slow cranking of the engine during starting. These requirements are commonly achieved in two ways — the impulse coupling and the Bendix "Shower of Sparks."

The engine with an impulse coupling will have a snapping sound while starting. The engine employing the Shower of Sparks system will have a buzzing sound in the cabin whenever the starter switch is activated. Know which system the aircraft has before attempting to start the engine.

The absence of the clicking in an impulse coupling engine denotes a problem with the impulse coupling, probably a broken main or flyweight return spring. This problem requires removal of the magneto(s) with the impulse coupling for repair. If the engine is very cold, stiff oil can also prevent the flyweights from returning to "snap" the coupling. A thorough preheat solves this one.

The absence of buzzing in the shower of sparks system, or the malfunction of the retard points in the magneto, is accompanied many times by the occasional firing of the engine when you let up on the start switch but no firing when the start switch is engaged. This is because of the grounding of the right and left main breaker points by the start switch when in the start position. When the start switch is returned to the normal position, the mags are back in the "both" position. (This is also true of Shower of Spark systems with separate mag switches. The R & L main breaker points ground with the start switch engaged.) Since the engine is still turning at this instant, a cylinder at or slightly after normal firing position may fire and even allow the engine to start. If the Shower of Sparks unit is not buzzing, the unit needs repair or replacement. If there's "buzz" but no spark, check the continuity and points in the retard system.


Hot starts can be troublesome if you're not experienced with the procedures for the particular engine fuel system. Space doesn't allow for the discussion of this problem here. Consult the Pilots Operating Handbook or engine operators manual for specifics.

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