Troubleshooting and Failure Flashcards

A technician is diagnosing an engine that starts easily when cold but stalls after 5-10 minutes of operation. Once it stalls, it cannot be restarted until it has cooled down for a significant period. What is the MOST likely cause of this issue?

Answer: Faulty ignition coil

Ignition coils can develop internal shorts that only manifest when the component heats up and its internal windings expand. This creates an open circuit, killing the spark. As the coil cools, the connection is restored, allowing the engine to start again. A clogged fuel filter would likely cause issues under load, not consistently after a specific runtime. Worn rings and incorrect valve lash would cause consistent problems like low power or hard starting, regardless of temperature.

Upon starting a four-stroke engine for a customer complaint of high oil consumption, a technician observes a consistent stream of blue-tinted smoke from the muffler. This symptom is a classic indicator of which internal engine problem?

Answer: The engine is burning oil

Blue or grayish-blue smoke is the result of engine oil entering the combustion chamber and being burned with the fuel/air mixture. This is typically caused by worn piston rings, valve guides, or seals. Black smoke indicates a rich fuel mixture, while heavy white smoke often points to a coolant leak (on liquid-cooled engines) or a blown head gasket.

A two-stroke engine on a string trimmer starts and idles properly, but bogs down and loses all power as soon as the throttle is applied. The carburetor has been cleaned and fuel flow is confirmed to be good. Which of the following components should be inspected next for a potential restriction?

Answer: The spark arrestor screen

A clogged spark arrestor screen in the muffler is a very common cause of this symptom, especially in two-stroke engines. The carbon buildup prevents exhaust gases from escaping efficiently, creating excessive backpressure that chokes the engine under load. While a dirty air filter can cause low power, it usually presents as a rich running condition (black smoke). A bad coil or rings would likely affect idle as well.

A lawnmower engine abruptly stopped after the blade struck a large tree root. Now, the engine has a weak or non-existent spark, and the starter rope kicks back violently. What is the MOST likely cause of these symptoms?

Answer: The flywheel key has sheared

The soft metal flywheel key is designed to shear during a sudden crankshaft stoppage to protect the crankshaft from damage. When it shears, the flywheel spins on the crankshaft, misaligning the flywheel's magnet with the ignition coil. This severely alters the ignition timing, causing a weak/absent spark or a spark at the wrong time, which leads to violent kickback. A fouled plug is possible but less direct. A stuck float affects fuel, not spark.

A technician performs a compression test on a single-cylinder engine and gets a reading of 45 PSI, which is well below the manufacturer's specification. To quickly help determine if the low compression is caused by worn piston rings or a poor valve seal, what is the immediate next diagnostic step?

Answer: Add a small amount of oil to the cylinder and re-test

This procedure is known as a 'wet' compression test. Adding about a teaspoon of engine oil into the cylinder through the spark plug hole temporarily helps seal the piston rings. If the compression reading increases significantly on the re-test, it indicates the rings are worn. If the reading stays about the same, the leak is likely at the valves or head gasket. This is a faster initial diagnostic step than a leak-down test.

A V-twin, air-cooled engine is running rough, overheating, and pushing oily smoke from the crankcase breather tube. A compression test reveals one cylinder has significantly lower compression than the other. What is the MOST likely failure?

Answer: A blown head gasket

A blown head gasket can create a leak between the combustion chamber and the crankcase (often via the pushrod gallery). This allows hot combustion gases to enter the crankcase, pressurizing it and forcing out oily smoke through the breather. It also directly causes the loss of compression in that cylinder, leading to a rough run condition and overheating. A bad coil would cause a misfire but not pressure in the crankcase. A clogged injector would cause a lean condition. A sheared key affects ignition timing.