Nuts & Bolts

Craft & Techni

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what is not. your airplane's engine, You and your meyou or an a i r f r a m e and chanic meet at the airpowerplant (A&P) meplane on Monday chanic, you can't fix it evening. You describe the problem as best you can u n t i l you troubleshoot Troubleshooting piston engine problems the problem and discover while you remove the cowling. From your its probable cause. Let's MICHAEL L. LEASURE, EAA 218380 phone conversation, your say you're ready to fire up and take off for a fly-in mechanic knows this is a breakfast. Everything is perfect—unrough-running engine problem, and she has brought some tools. Intil you start the engine. cluded in the tattered gray bag are a Instead of purring, the engine compression tester, a f l a s h l i g h t , shakes and misfires. Hoping for the spark plug wrenches, an ignition best you cycle the magnetos, try to lead tester, a crayon, screwdrivers, a run the engine a bit faster, and, fismall set of wrenches, sockets, and a nally, lean the mixture. Nothing sandwich. helps. When you reduce power to A crayon? Your mechanic exidle the engine dies. It is obvious plains t h a t if the crayon doesn't that something's wrong with the enmelt when she touches it to an exgine, and it will need attention. And haust stack, she's found a cold cylinyou're going to eat breakfast at the der, which will help her isolate the local fast food place. problem. The crayon is safer than By observing the engine's probburnt fingertips and more profeslems and the conditions under which sional than spitting on each stack. they occurred, you've begun the With the airplane tied down outtroubleshooting process. It was mornside the hangar, you climb in and ing, the temperature moderate, the humidity low, and the engine cold. country, and you experienced no run through the checklist exactly as you do each time you start the enThe problem was evident the mo- problems at all. These are crucial bits of informa- gine. You shout "clear" and silently ment the engine's different control settings didn't change the way it ran. tion that will help you and your me- hope the problem is still there. An The t a n k s were f u l l , and you c h a n i c diagnose the engine's engine problem that comes and goes found no water or debris when you problem in the shortest time possi- is much worse than one that stays. As you crank the engine, your drained all of the fuel system sumps ble. Some information may be irreled u r i n g your preflight inspection. vant, but at this point, it is difficult mechanic stands outside and Your last flight was a long cross- to sort out what is important and slightly behind the propeller arc. NO MATTER WHO MAINTAINS

Why Won't It Run Right? By observing the engine's problems and the conditions under which they occurred, you've begun the troubleshooting process.

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Craft & Technique She's looking for leaks and the color of the exhaust and listening for strange sounds—anything out of the ordinary. The engine starts and dies. You restart, and it catches and runs really rough. Your mechanic waves her arm, signaling, "I've seen enough, shut the darn noise off." She touches the crayon to each exhaust stack, and Cylinder 1 does not wear a melted button of color. It's cold, and this narrows things considerably. The problem is probably an ignition lead, a spark plug, an injector line, or an injector, and it will be connected to Cylinder 1. Looking at the engine and finding nothing visually amiss, she removes the injector line A crayon is useful for determining which cylinders are firing (hot) without the danger of burns or damage to the exhaust.

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and injector. After looking through the injector, she clears its blockage with a blast of compressed air and reconnects everything. . Back in the cockpit you run your checklist, and the engine performs like it has hundreds of times previously. Are you done? If this were your car, the answer would probably be yes. With airplanes you not only want to fix the problem—you want to find out what caused it to keep it from happening again. A piece of debris from a deteriorating rubber line, a bad filter, or another failing component could have caused the blockage. Only a complete fuel system service will reveal the source of the foreign material.

Compression lest

If the problem had not been so readily apparent, several other tests could have isolated the problem. A compression test determines the condition of valves and cylinders from the compression rings up into the head. Using

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A differential compression tester is used to supply compressed air into a cylinder and to measure the amount of air escaping. An 80-psi input is the industry standard.

with a plastic h a m m e r with the piston at bottom dead center. This may dislodge any fouling on the seat or valve. A Continental Engines Service B u l letin covers staking, and it's something you should do carefully.

Ignition & Spark Plug Tests

An i g n i t i o n lead tester An ignition lead tester attached to an Ignition helps to diagnose engine harness. This lead checks okay, as there is a misfiring. If your engine is spark in the tester window when the button rough and the problem on the tester is pressed. follows one magneto during a r u n - u p check, its a tester that screws into the spark cause is probably a fouled spark plug hole, you pressurize the cylin- plug or a failed ignition lead. A lead der with compressed air after mak- tester simulates the magneto by firing sure the piston is at top dead ing a spark through the ignition cacenter on the compression stroke. ble and shows if it reaches the lead's The industry standard starting terminal end or if the spark jumps pressure is 80 psi, and this is called a to the ground before reaching the differential pressure test because the terminal. Electricity takes the path of least tester measures the air that leaks out of the cylinder. A typical good cylin- resistance, and if the insulation on der compression reading is 80/78 the lead is chafed and passes near psi. A bad cylinder will read below engine parts or the engine mount, the electricity takes this shortcut in25 percent of ideal, or 60 psi. What's really handy about this stead of flowing to the terminal and test is that it tells you from where f i r i n g the spark plug. A snapping the air is leaking. If you hear a hiss- noise during the test will also indiing airflow noise from the crankcase cate the spark is probably not reachbreather, piston rings are the prob- ing the terminal end of the lead. Spark plugs are best tested and lem. Air noise coming from the exhaust pipe tells of a bad e x h a u s t serviced with a machine designed valve or a cracked exhaust port. Air for this purpose. Most FBOs have noise from the carburetor tells about one of these cleaners/testers and will a bad intake valve or port. A poor compression test reading may simply mean all of your ring end gaps are lined up. To eliminate this possibility, run the engine on the ground and retest the compression. Debris on the valves or their seats can cause exhaust or intake air leaks, and ground running the engine and retesting will eliminate this possibility, too. If you feel that debris still Spark plugs are tested and serviced with exists on the valves, you can equipment such as this. Combustion pres"stake" it, or hit the top of sures are simulated during testing to see if the appropriate rocker arm the plug will fire properly under pressure.

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Craft & Technique no doubt let you use it to test your spark plugs. Visually, a spark plug may look fine, and it may even spark properly in free air. But the tester simulates combustion pressures up to roughly 110 psi and will give a much better measurement of plug performance in actual operation. If you drop a spark plug w h i l e servicing your engine, throw it away. The ceramic insulator is brittle, and it will crack deep inside the plug. The spark will then j u m p to the grounded shell of the plug rather than progressing to the firing end. In short, it's junk (pun intended).

Occasionally an engine problem defies explanation. What this often means is that you have more than one problem.

higher than normal manifold pressure readings. Most of the engine problems I diagA stuck valve will cause rough ennose are caused by induction leaks, gine operation at all speeds, with stuck valves, or failed ignition com- possible quitting at idle. ponents. Induction leaks show up A failed ignition component will at low rpms and are indicated by follow a magneto check, and you hard starting, poor if any idle, and can isolate it with the cold-cylinder

Engine Symptoms

crayon check. Run the engine on the bad magneto and then use the crayon to find the cold cylinder— and the bad lead or plug. Occasionally an engine problem defies explanation. What this often means is that you have more than one problem. These are the hardest to isolate, but the same procedures apply. The last situation you may face involves someone working on an engine and doing some procedure that is not normally done. It doesn't work out, so they are reluctant to give you the information you need to find the problem. "You mean I didn't tell you I took off the accessory case, removed the gears, and reinstalled everything at random?" This situation requires that you assume nothing is as it should be—and you start from the very beginning. Good luck and happy hunting.

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