Engine Diagnostics Flashcards

A dry compression test shows two adjacent cylinders at 40 psi while all others read 160 psi. What is the most likely cause?

Answer: A blown head gasket between the two cylinders

Low compression in two adjacent cylinders usually means a failed head gasket between them.

During a wet compression test, a low cylinder's reading rises significantly after adding oil. What does this indicate?

Answer: Worn or damaged piston rings

Oil temporarily seals the rings, so a big increase points to ring or cylinder wall wear.

During a cylinder leakage test, air is heard escaping from the oil filler opening. Which component is leaking?

Answer: Piston rings

Air escaping into the crankcase means it is getting past the piston rings.

During a cylinder leakage test, air is heard hissing from the throttle body. What is the likely fault?

Answer: Leaking intake valve

Air exiting through the intake system indicates the intake valve is not sealing.

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?

Answer: Retarded ignition or valve timing

A steady but low vacuum reading typically indicates late ignition or valve timing.

At idle, a vacuum gauge needle drops regularly by 3–4 in. Hg at a steady rhythm. What does this most likely indicate?

Answer: A burned or leaking valve

A regular, rhythmic drop occurs each time the faulty cylinder's leaking valve should be sealing.

With the engine held at 2,500 rpm, vacuum slowly drops toward a very low reading. What is the most likely problem?

Answer: Restricted exhaust system

A gradual vacuum drop at steady elevated rpm indicates exhaust back pressure building from a restriction.