ASE Engine Diagnostics 2 — Questions and Answers
Question 1: A dry compression test shows two adjacent cylinders at 40 psi while all others read 160 psi. What is the most likely cause?
- Worn piston rings in both cylinders
- A blown head gasket between the two cylinders (Correct answer)
- A jumped timing belt
- A restricted exhaust system
Correct answer: A blown head gasket between the two cylinders
Low compression in two adjacent cylinders usually means a failed head gasket between them.
Question 2: During a wet compression test, a low cylinder's reading rises significantly after adding oil. What does this indicate?
- Burned exhaust valve
- Bent intake valve
- Worn or damaged piston rings (Correct answer)
- Cracked cylinder head
Correct answer: Worn or damaged piston rings
Oil temporarily seals the rings, so a big increase points to ring or cylinder wall wear.
Question 3: During a cylinder leakage test, air is heard escaping from the oil filler opening. Which component is leaking?
- Intake valve
- Exhaust valve
- Head gasket to coolant jacket
- Piston rings (Correct answer)
Correct answer: Piston rings
Air escaping into the crankcase means it is getting past the piston rings.
Question 4: During a cylinder leakage test, air is heard hissing from the throttle body. What is the likely fault?
- Leaking intake valve (Correct answer)
- Leaking exhaust valve
- Worn piston rings
- Cracked cylinder wall
Correct answer: Leaking intake valve
Air exiting through the intake system indicates the intake valve is not sealing.
Question 5: A vacuum gauge on a warm engine at idle shows a steady reading of 14 in. Hg instead of the normal 17–21 in. Hg. What is a likely cause?
- A burned valve
- Retarded ignition or valve timing (Correct answer)
- A sticking valve
- Weak valve springs
Correct answer: Retarded ignition or valve timing
A steady but low vacuum reading typically indicates late ignition or valve timing.
Question 6: At idle, a vacuum gauge needle drops regularly by 3–4 in. Hg at a steady rhythm. What does this most likely indicate?
- Late ignition timing
- Restricted exhaust
- A burned or leaking valve (Correct answer)
- Intake manifold vacuum leak
Correct answer: A burned or leaking valve
A regular, rhythmic drop occurs each time the faulty cylinder's leaking valve should be sealing.
Question 7: With the engine held at 2,500 rpm, vacuum slowly drops toward a very low reading. What is the most likely problem?
- Worn piston rings
- Leaking intake gasket
- Weak valve springs
- Restricted exhaust system (Correct answer)
Correct answer: Restricted exhaust system
A gradual vacuum drop at steady elevated rpm indicates exhaust back pressure building from a restriction.
ASE Engine Diagnostics 2
A dry compression test shows two adjacent cylinders at 40 psi while all others read 160 psi.
What is the most likely cause?