Plant machinery overheating — the full diagnostic checklist
Overheating has more possible causes than most fitters check on the first pass. Working through them in the right order — cheapest and most common first — saves time and avoids replacing parts that were never the problem.
1. Coolant level and leaks
Always start cold — never open a hot cap. Check the level against MIN/MAX, and check the cap itself for oily contamination, which can be an early sign of a head gasket issue. Then inspect hoses, the radiator, water pump body and block for staining or active drips before assuming it's just low.
2. Radiator blockage
Plant machines block quickly with mud, chaff and site debris. Check the core face and any hydraulic or charge air cooler sitting in front of it, and clean with compressed air from the engine side outward.
3. Belt condition and tension
A slipping, glazed or loose belt turns the water pump and fan too slowly, especially under load. Check for cracking or glazing, and tension — typically no more than around 10mm of deflection under firm thumb pressure at the midpoint of the longest span.
4. Water pump pulley, bearing and weep hole
Rock the pulley by hand to check for bearing play, and inspect the weep hole underneath the pump body. Dry is normal — wet or dripping means the shaft seal has failed.
5–7. Temperature readings, thermostat and circulation
Using an IR thermometer rather than hand-feel gives objective readings at the top and bottom radiator hose. Once warmed up, they should be close to each other — a big gap points to a thermostat stuck closed. Even with a good thermostat, check for actual circulation: coolant should visibly move in the expansion tank, and the top hose should surge under pressure, not just feel firm.
8. Pressure test
This requires a cooling system pressure tester kit. It finds leaks — both external and internal — that aren't visible at normal atmospheric pressure. If you don't have the kit, this is worth referring out rather than guessing.
9. Combustion gas test, if necessary
Only needed if earlier steps point toward a possible head gasket — oily coolant contamination, or pressure loss with no visible external leak. This needs a chemical block-check kit or an exhaust gas analyser, and confirms whether combustion gases are entering the coolant.
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