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...


Another common problem encountered is magneto timing. If the engine has just shown up from the overhaul shop or maintenance has just been performed on the magneto, look for these symptoms and their source problems.

Kickback is caused by the start firing event happening too soon, before the piston reaches top center. The usual causes here are: improper retard point timing (Shower of Sparks system) or improper magneto to engine timing (Shower of Sparks and impulse couplings).

If no firing by the engine is observed at any time, but you know the magneto is firing (don't use your tongue), the probable cause is the magneto-to-engine timing 180 off. When the "flower pot" method is used it is not too difficult to set timing to before top center on the exhaust stroke instead of the compression stroke.

Piston Engine Troubleshooting

By Thomas Ehresman

March 1998

2. Miss/rough running:
The two most common sources of this symptom are ignition misfire and clogged fuel injector. The misfire most usually shows up on the preflight mag check. To find the offending plug, run the engine on the magneto with the bad drop. If a four or six point EGT/CHT system is installed, note the cold cylinder. Trace the ignition lead back to the magneto with the miss. That plug circuit (plug, high tension lead, or tower in the distributor block) has a short or open. The most common cause here is a dirty/bad spark plug or a chaffed ignition lead.

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A clogged injector will act very similar to the misfire. The miss is always apparent if the injector is fully clogged (or nearly so). If the injector is partially clogged, the miss will show up while leaning the engine for cruise. (On the multiple point EGT check for a very early peak and a higher head temperature on one cylinder.) In either case, run the engine in the configuration with the noted miss (rich or leaned out) and note the cold cylinder the same as you would for a misfire. Hint: If the aircraft is not equipped with a four or six point EGT/CHT or some of the probes aren't working, run the engine in the configuration with the noted miss. While running with the noted miss or immediately after engine shutdown, use a squirt gun or bottle and squirt a small, fine stream of water on each exhaust collector approximately one inch from the cylinder exhaust port. The cylinder with the miss will show up with a much colder exhaust collector at this point.

Another common malfunction that is difficult to troubleshoot on the ground is the altitude miss. This is where a plug circuit will misfire only when the aircraft is at or above a certain altitude. If the aircraft has the four or six point EGT/CHT, you can find this one in the air. If not, the "shotgun" method should be used on the ignition system.

The most common culprit is large plug gaps. Clean, gap, and test all the plugs. Also, test the ignition high tension lead harness with a high tension lead tester. Check the distributor block for cleanliness, tracking, and cracks.

If none of these are the problem, other but less common sources are things such as improper E-gap (magneto internal timing) and weak rotating magnets or coils. For these internal mag problems, the magneto(s) must be removed and troubleshot on a test bench. Note of caution: Do not attempt to do in-flight mag checks for high altitude miss diagnosis. Very often high altitude misses turn into crossfires. Crossfire is when the electrical energy from a miss-fire finds a path of least resistance in one of the electrodes next to it. A mag check while attempting to diagnose which magneto is miss/crossfiring can literally blow the exhaust system into pieces. This is especially true of turbo-charged engines. Mag checks at even low cruise power settings will over temp most cylinder and exhaust components.

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