ASE4 General Engine Diagnosis 2 — Questions and Answers
Question 1: An engine has a wet compression test reading significantly higher than the dry test. This indicates:
- A blown head gasket
- Worn or damaged piston rings (Correct answer)
- Leaking intake valves
- A cracked cylinder head
Correct answer: Worn or damaged piston rings
Adding oil temporarily seals worn rings, causing a wet reading much higher than the dry reading.
Question 2: A vacuum gauge needle fluctuates between 14 and 19 in-Hg at idle. This suggests:
- Late ignition timing
- A restricted exhaust
- An intermittent misfire or sticking valve (Correct answer)
- Low engine compression
Correct answer: An intermittent misfire or sticking valve
A fluctuating vacuum needle at idle indicates an intermittent problem such as a sticking valve or random misfire.
Question 3: During a cylinder balance test, disabling cylinder #5 drops RPM by 50 while all others drop by 200. This indicates:
- #5 has a leaking injector
- #5 is the strongest cylinder
- #5 is weak and contributing less power (Correct answer)
- #5 has advanced timing
Correct answer: #5 is weak and contributing less power
A smaller RPM drop on one cylinder compared to others means that cylinder is contributing less power than the rest.
Question 4: A technician hears a rhythmic tapping noise that increases with engine speed. Oil pressure is normal. The MOST likely cause is:
- A worn main bearing
- Excessive valve clearance (Correct answer)
- A broken piston ring
- A clogged oil passage
Correct answer: Excessive valve clearance
Excessive valve clearance creates a rhythmic tapping that speeds up with engine RPM and is unaffected by oil pressure.
Question 5: A customer reports an engine knock that worsens under load but disappears at idle. This MOST likely indicates:
- Valve train noise
- Rod bearing knock (Correct answer)
- Piston slap
- Timing chain rattle
Correct answer: Rod bearing knock
Rod bearing knock intensifies under load because increased combustion pressure amplifies the bearing clearance impact.
Question 6: When testing cranking vacuum, the gauge reads 0 in-Hg on all cylinders. The MOST likely cause is:
- A jumped timing chain or belt (Correct answer)
- Worn piston rings on all cylinders
- A faulty vacuum gauge
- Excessive oil viscosity
Correct answer: A jumped timing chain or belt
Zero cranking vacuum across all cylinders indicates the valve timing is completely off, typically due to a jumped timing chain or belt.
ASE4 General Engine Diagnosis 2
An engine has a wet compression test reading significantly higher than the dry test.
This indicates: