Engine Performance & Diagnostics Flashcards

A cylinder leakage test shows 30% leakage with air escaping from the radiator filler neck. What is the most likely cause?

Answer: Leaking head gasket or cracked head

Air bubbling into the cooling system indicates combustion chamber leakage into a coolant passage, typically a failed head gasket or cracked head.

During a running compression test, one cylinder shows normal cranking compression but very low snap-throttle compression. What does this suggest?

Answer: Restricted exhaust or worn camshaft lobe

Low snap-throttle readings point to a breathing problem such as a worn cam lobe or restriction rather than a sealing problem.

A vacuum gauge reading that drops steadily while the engine is held at 2,500 RPM indicates what condition?

Answer: Restricted exhaust system

A restricted exhaust causes backpressure to build at sustained RPM, which steadily lowers manifold vacuum.

Long-term fuel trim reads +22% at idle but drops to +3% at 2,500 RPM. What is the most likely cause?

Answer: Vacuum leak

A vacuum leak has the greatest effect at idle, when unmetered air is a larger share of total airflow.

A technician sees a P0300 random misfire code along with positive fuel trims on both banks. Which is the best next step?

Answer: Check fuel pressure and volume

Random misfires with lean trims on both banks point to a common fuel delivery problem such as low pressure or volume.

With a scan tool, a technician performs a cylinder power balance test and sees little RPM drop on cylinder 4. Which test should come next to isolate the fault?

Answer: Swap the coil and recheck

Swapping the coil to another cylinder shows whether the misfire follows the coil or stays with the cylinder.

An engine has a rough idle that smooths out when the PCV valve hose is pinched off. What is the likely problem?

Answer: PCV valve stuck open or hose leaking

A PCV valve stuck open lets excess unmetered air into the intake, causing a lean, rough idle.