AERA Diagnostic & Troubleshooting Methods 4 — Questions and Answers
Question 1: Which measurement BEST determines whether a crankshaft journal has developed an hourglass (barrel) shape during engine operation?
- Outside micrometer reading at one location
- Multiple micrometer readings along the journal's length (Correct answer)
- Dial indicator runout check
- Plastigage measurement
Correct answer: Multiple micrometer readings along the journal's length
Taking multiple micrometer measurements at several points along the journal's length reveals taper or hourglass wear that a single reading would miss.
Question 2: A technician performing a valve spring pressure test finds that several springs are 15% below the manufacturer's specified seat pressure. What is the MOST likely result if these springs are reused?
- Valve float at high RPM and loss of power (Correct answer)
- Increased valve guide wear
- Bent pushrods under load
- Premature camshaft lobe wear
Correct answer: Valve float at high RPM and loss of power
Weak valve springs with insufficient seat pressure allow valves to float (bounce) at high RPM, causing power loss, misfires, and potential valve-to-piston contact.
Question 3: When diagnosing an overheating engine that loses coolant with no visible external leaks, which test should be performed FIRST?
- Block combustion gas test (chemical test for exhaust gases in coolant) (Correct answer)
- Cylinder leakdown test
- Cooling system pressure test at cold start
- Radiator flow restriction test
Correct answer: Block combustion gas test (chemical test for exhaust gases in coolant)
The combustion gas (block) test quickly confirms whether exhaust gases are entering the coolant system (indicating a head gasket failure) before committing to more invasive tests.
Question 4: What does a higher-than-specified connecting rod side clearance primarily affect?
- Oil pressure at the main bearings
- Oil throw-off to the cylinder walls and piston pins (Correct answer)
- Crankshaft end play
- Piston ring seal
Correct answer: Oil throw-off to the cylinder walls and piston pins
Excessive rod side clearance increases oil throw-off from the rod journal, which can over-oil cylinder walls and affect piston pin lubrication.
Question 5: A newly rebuilt engine develops a coolant leak only under high load conditions. During a cold pressure test the system holds pressure perfectly. What is the MOST probable cause?
- Warped cylinder head sealing surface (Correct answer)
- Faulty radiator cap
- Cracked exhaust manifold
- Defective water pump seal
Correct answer: Warped cylinder head sealing surface
A warped head sealing surface may seal adequately when cold but leak under the thermal expansion and cylinder pressure of high-load operation.
Question 6: A technician uses Plastigage to check main bearing clearances after reassembly and finds one journal shows much wider Plastigage crush than the others. What does this indicate?
- That journal has less clearance than the others
- That journal has more clearance than the others (Correct answer)
- The bearing cap was over-torqued
- The crankshaft journal is undersized for the installed bearing
Correct answer: That journal has more clearance than the others
Plastigage spreads wider when clearance is greater; a significantly wider crush reading on one journal indicates excessive bearing clearance at that location.
Question 7: During a post-rebuild road test, a knocking noise appears only when the engine reaches full operating temperature. Which condition MOST likely causes heat-dependent knocking?
- Excessive piston-to-wall clearance that worsens as the piston expands with heat
- A loose vibration damper bolt
- Worn rod bearings with excessive clearance (Correct answer)
- Detonation due to incorrect fuel octane
Correct answer: Worn rod bearings with excessive clearance
Worn rod bearings with borderline clearance often allow more knock as oil viscosity drops with heat, making the knock apparent only after full warm-up.
AERA Diagnostic & Troubleshooting Methods 4
Which measurement BEST determines whether a crankshaft journal has developed an hourglass (barrel) shape during engine operation?