AERA Diagnostic & Troubleshooting Methods 5 — Questions and Answers
Question 1: 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?
- Replace the radiator
- Perform a cylinder leakdown test to identify the affected cylinder(s) (Correct answer)
- Replace all spark plugs
- Flush the cooling system and retest
Correct 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.
Question 2: Which piston ring installation error would MOST likely cause immediate high oil consumption but normal compression on a newly rebuilt engine?
- Oil control ring expander ends overlapped (Correct answer)
- Top compression ring installed upside down
- Second ring gap aligned with the top ring gap
- Rings installed in wrong grooves (top ring in second groove, second in top)
Correct 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.
Question 3: 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?
- Install new rings without honing since the bore is within limits
- Deglaze only to allow ring seating
- Bore or hone the cylinder to remove taper before fitting new rings (Correct answer)
- Install oversize rings to compensate for the taper
Correct 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.
Question 4: 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?
- Was the correct bearing size installed on that rod? (Correct answer)
- Was there a manufacturing defect in the crankshaft?
- Was the oil changed regularly?
- Was the cooling system functioning properly?
Correct 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.
Question 5: 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?
- A sticking or partially stuck choke
- A sticking valve—likely a valve that is intermittently not seating (Correct answer)
- An exhaust restriction building up backpressure
- A fuel delivery problem causing intermittent lean condition
Correct 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.
Question 6: 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?
- Polish the fillet to remove surface discoloration and reuse the crankshaft
- Regrind the journal undersize to remove the indication
- Reject the crankshaft—transverse cracks at fillet radii are stress risers that cause fatigue failure (Correct answer)
- Apply a surface treatment and retest before deciding
Correct 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.
Question 7: 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?
- The engine has a leaking exhaust manifold gasket
- A restricted catalytic converter or exhaust system is reducing engine output (Correct answer)
- The engine has excessive cylinder leakdown
- The fuel injectors are over-delivering fuel
Correct 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.
AERA Diagnostic & Troubleshooting Methods 5
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?