Engine and Engine Support Systems
Cycle, sealing, and support
You must be able to explain the four-stroke cycle and the valve events that make it work: intake, compression, power, exhaust, with valve overlap and lobe separation affecting cylinder pressure, idle quality, and emissions. Camshaft phasers and variable valve timing change these events at runtime, so a stretched chain, worn tensioner, or mis-indexed phaser produces cam/crank correlation codes, rough running, and hard starting. Timing marks must align exactly to the OEM procedure - never assume a mark from memory.
Diagnose sealing with pressure, not opinion. A compression test is run with the throttle wide open, all plugs removed, and a comparable number of compression strokes per hole; cylinders should sit within roughly 10–20 % of the highest. A wet test that raises a low cylinder points to rings; no change points to valves or head gasket. A cylinder leakdown test localizes the leak: air into the intake indicates an intake valve, into the exhaust an exhaust valve, into the crankcase rings, and bubbles in the cooling system a head gasket or crack.
Support systems matter: cooling (thermostat opening, pressure cap raising boiling point roughly 2.5–3 °F per psi, coolant concentration, water pump flow, electrolysis), lubrication (pressure versus viscosity, relief valve, gallery restrictions), mounts, PCV, intake leaks, and turbocharger boost control. Torque-to-yield head bolts are single-use, oiled or sealed per spec, and tightened in sequence by angle.
Trap: condemning rings on a single low cylinder before a wet and leakdown test.
Rule of thumb: a 10 % spread in compression is acceptable; a 20 % spread warrants diagnosis.