Lubrication and Cooling Systems
Diesel lubrication does more than reduce friction: the oil also cools pistons, actuates injectors on some systems (HEUI), and carries soot. Know the path: pan → pickup → pump → filter/cooler → galleries → bearings → drain back. Understand oil pressure vs. oil flow, relief valves, and bypass valves that open on a clogged filter to protect the engine.
Cooling on a diesel must handle high heat rejection and often includes an EGR cooler and sometimes an oil cooler. Know thermostat operation, coolant flow direction, water pump drive, and the role of the degas bottle/pressure cap in raising boiling point. Air pockets cause hotspots; bleeding is required.
Traps: confusing a low oil pressure gauge with low oil level; a faulty sender is common. A clogged filter bypass keeps pressure but stops filtration. Coolant loss with no external leak can be an EGR cooler, head gasket, or oil cooler. White sweet-smelling exhaust suggests coolant burning. Do not overlook cavitation on wet-liner engines, which needs correct coolant chemistry (SCA/DCA additives).
Worked example: Engine overheats only under load. Check thermostat first, then radiator airflow/fan clutch, then water pump impeller, then EGR cooler restriction. A restricted EGR cooler raises coolant temp under load without an external leak.
Rule of thumb: Overheat at idle → airflow/fan; overheat under load → coolant flow, radiator, or EGR cooler.