ASE4 General Engine Diagnosis 1 — Questions and Answers
Question 1: A customer complains of a rough idle that disappears at higher RPMs. Which tool is BEST used to identify a misfiring cylinder?
- Vacuum gauge
- Compression tester
- Scan tool with misfire counters (Correct answer)
- Fuel pressure gauge
Correct answer: Scan tool with misfire counters
A scan tool displaying misfire counters can identify which specific cylinder is misfiring under live conditions.
Question 2: A cylinder power balance test shows that shorting cylinder #3 causes no RPM drop. This indicates:
- That cylinder is producing too much power
- That cylinder has a lean condition
- That cylinder is not contributing to engine power (Correct answer)
- The ignition timing is advanced on #3
Correct answer: That cylinder is not contributing to engine power
No RPM drop when a cylinder is disabled means that cylinder was already not contributing power, indicating a fault.
Question 3: Low compression on two adjacent cylinders most likely indicates:
- Worn piston rings
- A blown head gasket between those cylinders (Correct answer)
- A faulty intake manifold gasket
- Excessive valve clearance
Correct answer: A blown head gasket between those cylinders
A blown head gasket between two adjacent cylinders allows compression to leak between them, causing both to read low.
Question 4: When performing a cylinder leakage test, air is heard escaping from the throttle body. This indicates:
- A leaking exhaust valve
- A leaking intake valve (Correct answer)
- Worn piston rings
- A cracked piston
Correct answer: A leaking intake valve
Air escaping back through the throttle body during a leak-down test indicates a leaking intake valve on that cylinder.
Question 5: An engine has good compression but produces blue smoke at startup that clears after a few minutes. The MOST likely cause is:
- Worn piston rings
- Leaking valve stem seals (Correct answer)
- A cracked cylinder head
- A rich fuel mixture
Correct answer: Leaking valve stem seals
Valve stem seals allow oil to seep into the combustion chamber when the engine sits, burning off as blue smoke at startup.
Question 6: A technician connects a vacuum gauge and observes a steady low reading of 12 in-Hg at idle. This MOST likely indicates:
- Late ignition timing (Correct answer)
- A leaking head gasket
- A restricted exhaust
- Worn piston rings
Correct answer: Late ignition timing
A steady but low vacuum reading at idle typically points to late ignition timing reducing combustion efficiency.
ASE4 General Engine Diagnosis 1
A customer complains of a rough idle that disappears at higher RPMs.
Which tool is BEST used to identify a misfiring cylinder?