General Engine Diagnosis
What the candidate must know
General diagnosis is the umbrella skill: separating mechanical faults from fuel, ignition, air, and electronic faults before tearing anything apart. Know the three essentials for combustion (adequate compression, correct air/fuel mixture, properly timed ignition), the difference between a mechanical complaint (low compression, noise, blowby, vibration) and a performance complaint (misfire, low power, hard start, poor economy), and how to use a road test, a cylinder balance or power contribution test, a compression test (wet and dry), a cylinder leakage test, and a vacuum gauge reading to localize a fault.
Traps in questions
- A wet compression test that raises compression points to rings; one that does not raise it points to valves, head gasket, or cam timing. Questions often swap these conclusions.
- Blowby (crankcase pressure, oil filler cap dance, oil in the intake from the breather) is a ring/cylinder problem, not a PCV problem, unless the PCV system is proven faulty.
- A steady low vacuum reading can be late valve or ignition timing; a fluctuating needle can be a valve or head gasket leak. Do not confuse a low but steady needle with a wavering one.
- A misfire that disappears when the engine warms can be a lean condition or a failing ignition component, not necessarily a mechanical fault. Always confirm with a cylinder contribution test before condemning hardware.
- Noise diagnosis: rod knock is a deep, rhythmic knock that changes with load; lifter tick is high-pitched and often load-independent; piston slap fades when warm. Match the noise signature to the cause.
Worked example
A truck comes in with low power and oil consumption. Dry compression on #4 is 260 psi; wet is 340 psi; other cylinders are 380 psi dry. The wet test raising compression by 80 psi says rings/piston on #4, so the head does not come off first. A leak-down test confirming air escaping past the rings (into the crankcase) supports the same conclusion.
Rule of thumb
Compression should be within about 10–15% across cylinders. A cylinder more than 15% low, or one that changes significantly wet vs. dry, is the suspect.