AERA Diagnostic & Troubleshooting Methods 2 — Questions and Answers
Question 1: When performing a cylinder balance test on a fuel-injected engine, what does a cylinder that causes the largest RPM drop when disabled indicate?
- That cylinder is contributing the most power (Correct answer)
- That cylinder has a misfiring injector
- That cylinder has excessive compression
- That cylinder is burning oil
Correct answer: That cylinder is contributing the most power
The cylinder causing the largest RPM drop when disabled is contributing the most power, while a cylinder with little or no RPM drop indicates a weak or non-contributing cylinder.
Question 2: A rebuilt engine exhibits excessive blowby immediately after startup. Which condition is the MOST likely cause?
- Worn valve guides
- Improperly installed piston rings (Correct answer)
- A leaking head gasket
- Excessive main bearing clearance
Correct answer: Improperly installed piston rings
Improperly installed piston rings—such as rings installed upside down, gaps aligned, or rings not seated—cause immediate excessive blowby after a rebuild.
Question 3: What is the purpose of performing a running compression test compared to a static compression test?
- It measures compression at cranking speed only
- It evaluates camshaft timing and valve operation under running conditions (Correct answer)
- It checks for head gasket leaks specifically
- It tests the accuracy of the compression gauge
Correct answer: It evaluates camshaft timing and valve operation under running conditions
A running compression test reveals issues with camshaft timing, valve timing, and restricted intake/exhaust that a static cranking test cannot detect.
Question 4: An engine rebuilder finds that oil pressure is within spec at idle but drops dangerously low at high RPM. Which worn component is MOST likely responsible?
- Oil pump relief valve stuck open
- Worn main and rod bearing clearances (Correct answer)
- Clogged oil pickup screen
- Faulty oil pressure sending unit
Correct answer: Worn main and rod bearing clearances
Excessive main and rod bearing clearances allow oil to escape faster than the pump can supply it at high RPM, causing pressure to drop under load.
Question 5: During engine diagnosis, a technician detects a high-pitched metallic tapping that increases with engine speed but is unaffected by load. What is the MOST likely source?
- Rod bearing knock
- Collapsed valve lifter
- Piston slap
- Excessive valve lash (Correct answer)
Correct answer: Excessive valve lash
Excessive valve lash produces a high-pitched metallic tapping that increases with speed but does not change with engine load, unlike bearing or piston noises.
Question 6: A wet compression test shows a significant increase in compression pressure over the dry test in one cylinder. What does this indicate?
- A leaking intake valve
- Worn or broken piston rings (Correct answer)
- A blown head gasket
- A leaking exhaust valve
Correct answer: Worn or broken piston rings
When oil added during a wet test temporarily seals the rings and significantly raises compression, worn or broken piston rings are the source of the low compression.
Question 7: Which diagnostic tool is BEST suited to detect an intermittent crankshaft position sensor fault that causes a no-start condition?
- Compression tester
- Oscilloscope or graphing scan tool (Correct answer)
- Vacuum gauge
- Cylinder leakdown tester
Correct answer: Oscilloscope or graphing scan tool
An oscilloscope or graphing scan tool captures the sensor's waveform pattern over time, making it the best tool to catch intermittent dropout events.
AERA Diagnostic & Troubleshooting Methods 2
When performing a cylinder balance test on a fuel-injected engine, what does a cylinder that causes the largest RPM drop when disabled indicate?