Diagnostic & Troubleshooting Methods Flashcards

An engine emits white smoke from the exhaust that persists after full warm-up. A chemical combustion gas test of the coolant is positive. What is the MOST appropriate next step?

Answer: Perform a cylinder leakdown test to identify the affected cylinder(s)

After confirming combustion gas in the coolant, a cylinder leakdown test pinpoints which cylinder(s) are leaking into the cooling system through a failed head gasket or crack.

Which piston ring installation error would MOST likely cause immediate high oil consumption but normal compression on a newly rebuilt engine?

Answer: Oil control ring expander ends overlapped

Overlapping the oil control ring expander prevents the ring from exerting proper radial pressure, severely compromising oil scraping and causing immediate high oil consumption.

A technician uses a dial bore gauge and finds that a cylinder bore is within wear limits but has 0.003" taper. What is the BEST course of action?

Answer: Bore or hone the cylinder to remove taper before fitting new rings

Cylinder taper prevents uniform ring contact throughout the stroke; the cylinder must be bored or honed to correct the taper regardless of whether absolute dimensions are within limits.

When inspecting a failed engine, the technician finds the rod bearing on cylinder #3 is severely spun and wiped. The other bearings show normal wear. What is the FIRST question to investigate?

Answer: Was the correct bearing size installed on that rod?

An isolated bearing failure at one cylinder most often indicates a local cause such as wrong bearing size, lack of crush, or assembly error—check bearing selection and installation first.

What does a vacuum gauge reading that slowly drops from a normal value to a low value and then recovers, in a repeating cycle at idle, indicate?

Answer: A sticking valve—likely a valve that is intermittently not seating

A cyclically dropping and recovering vacuum reading at idle is the classic signature of a sticking valve that temporarily holds open and then releases.

An engine rebuilder performs a magnetic particle (Magnaflux) inspection on a crankshaft and finds indications running transversely across a journal fillet radius. What is the CORRECT action?

Answer: Reject the crankshaft—transverse cracks at fillet radii are stress risers that cause fatigue failure

Transverse cracks at fillet radii are critical stress concentration points where fatigue fracture initiates; the crankshaft must be rejected and replaced, not repaired.

During diagnosis of an engine with low power and excessive fuel consumption, a technician measures exhaust backpressure using a pressure gauge at the O2 sensor port and finds 8 psi at wide-open throttle. What does this confirm?

Answer: A restricted catalytic converter or exhaust system is reducing engine output

Exhaust backpressure above 1.5–2 psi at WOT indicates a restriction in the exhaust system (commonly a clogged catalytic converter), which reduces power and causes rich-running symptoms.