AERA Diagnostic & Troubleshooting Methods 3 — Questions and Answers
Question 1: A vacuum gauge connected to the intake manifold shows a steady low reading of approximately 12–15 in-Hg at idle. What does this indicate?
- Leaking head gasket
- Late ignition or valve timing (Correct answer)
- Sticking exhaust valve
- Normal engine operation
Correct answer: Late ignition or valve timing
A steady but low vacuum reading at idle consistently points to retarded ignition timing or retarded valve timing affecting volumetric efficiency.
Question 2: When a cylinder leakdown test shows air escaping into the crankcase, what specific component failure is indicated?
- Leaking intake valve
- Blown head gasket between cylinders
- Worn piston rings or cylinder walls (Correct answer)
- Leaking exhaust valve
Correct answer: Worn piston rings or cylinder walls
Air escaping into the crankcase (detectable at the oil fill cap or dipstick tube) during a leakdown test confirms worn piston rings or cylinder wall damage.
Question 3: What does a fluctuating oil pressure gauge reading during steady engine operation MOST often indicate?
- A failing oil pump
- Air in the oil due to a low oil level or foaming (Correct answer)
- Normal pressure cycling behavior
- A clogged oil filter
Correct answer: Air in the oil due to a low oil level or foaming
A fluctuating oil pressure gauge typically indicates air entrainment in the oil caused by low oil level, aerated oil, or a cracked pickup tube drawing air.
Question 4: An engine produces a deep, rhythmic knock that is most pronounced at light throttle and disappears under heavy load. This is MOST characteristic of:
- Worn rod bearings
- Piston slap (Correct answer)
- Loose torque converter bolts
- Excessive main bearing clearance
Correct answer: Piston slap
Piston slap is typically loudest at light load when combustion pressure is insufficient to hold the piston against the cylinder wall, and often diminishes under heavy load.
Question 5: After a valve job, an engine runs rough and has low power. A leakdown test shows 5% leakage on all cylinders. What is the MOST likely cause of the symptoms?
- Worn piston rings
- Incorrect valve timing due to improper cam timing
- Excessive valve lash set on all cylinders (Correct answer)
- Defective spark plugs
Correct answer: Excessive valve lash set on all cylinders
With acceptable ring seal (low leakdown), excessive valve lash on all cylinders after a valve job would cause rough running and power loss by reducing effective lift and duration.
Question 6: A technician suspects a cracked engine block. Which test is MOST definitive for detecting an internal crack that communicates between a water jacket and an oil gallery?
- Pressure testing the cooling system
- Performing an oil analysis for coolant contamination
- Magnaflux (magnetic particle) inspection
- Pressure testing both systems simultaneously and cross-checking for leaks (Correct answer)
Correct answer: Pressure testing both systems simultaneously and cross-checking for leaks
Simultaneously pressurizing both the oil and coolant systems then checking for cross-contamination or pressure loss in each is the most definitive method for locating a communicating crack.
Question 7: An engine emits blue-gray smoke under deceleration but not during acceleration. Which condition MOST likely explains this pattern?
- Worn piston rings
- Leaking valve stem seals (Correct answer)
- Cracked piston crown
- Excessive fuel enrichment
Correct answer: Leaking valve stem seals
Leaking valve stem seals allow oil to be drawn past the valve stems during the high intake vacuum generated on deceleration, producing blue smoke specifically on decel.
AERA Diagnostic & Troubleshooting Methods 3
A vacuum gauge connected to the intake manifold shows a steady low reading of approximately 12–15 in-Hg at idle.
What does this indicate?