Engine Mechanical and Cooling System Servicing
Domain 3 covers the mechanical health of the engine and the cooling circuit that keeps it alive: compression and leakage testing, valve clearance and timing, head gasket diagnosis, and cooling system pressure testing.
Core rules
- Compression test: engine at operating temperature, all plugs or glow plugs removed, throttle wide open, 4–6 strokes per cylinder; cylinders should be within about 10% of each other. A wet test (oil added) that raises pressure points to rings; no change points to valves or gasket.
- Cylinder leakage test: air escaping at the intake means intake valve; at the exhaust, exhaust valve; at the dipstick, rings; bubbles in the radiator, head gasket or cracked head.
- Set valve clearance cold unless the maker states hot, with the cylinder at TDC on compression; use the specified sequence and recheck after locking the adjuster.
- Timing belt or chain: align all marks before removal, replace tensioner and water pump as a set when recommended, rotate the engine two turns by hand and re-verify marks.
- Cooling: pressure-test at cap rating (commonly around 1.1 bar) and hold two minutes; test the cap separately; the thermostat should start opening at its stamped temperature and be fully open about 10–15 °C higher.
- Use a 50/50 coolant mix and bleed air from the highest point; never open a hot pressurised cap.
Worked example
An engine overheats; the top hose is hot and hard, the bottom hose stays cold, the heater blows cold and no coolant is lost. Reasoning: a cold bottom hose and cold heater core mean coolant is not circulating through the radiator, so the thermostat is stuck closed. Coolant loss and bubbles would instead point to the head gasket. Confirmation: suspend the thermostat in heated water with a thermometer - if it is still shut at 95 °C when stamped 82 °C, replace it, refill with the correct mix, bleed and re-test.
Traps
- Testing compression on a cold engine or with the throttle closed.
- Reusing TTY head bolts or tightening them out of sequence.
- Filling with water only, or mixing OAT and silicate coolants.
- Ignoring a weeping water-pump tell-tale hole or a collapsed upper hose.