7.3 Overheating, Overcooling and Thermostats

7.2 Cooling System Diagnosis, Thermostats, Coolant Service & Water Pumps

Roughly one-third of the total thermal energy released by burning gasoline or LPG in a commercial truck engine must be absorbed by the liquid cooling system and rejected to the atmosphere. When diagnosing cooling complaints under ASE T1 Area D Tasks D.6 through D.9, your first diagnostic step is always to isolate the exact operating regime—low-speed/stationary PTO operation versus high-speed highway load, or chronic overcooling.

Diagnosing Engine Overheating vs. Overcooling (Task D.6)

1. Low-Speed, Curb-Idle, or Stationary PTO Overheating

If a commercial truck overheats while idling in traffic or running a stationary hydraulic PTO (such as an aerial bucket lift, refuse packer, or rollback winch at 1,200–1,600 RPM), yet cools right back down to normal (195°F–205°F) once driven at 55 MPH on the open highway, the root cause is an airflow deficiency. At 55 MPH, natural ram air forces thousands of cubic feet per minute (CFM) through the radiator core without assistance from the cooling fan. At stationary idle or PTO speeds, ram air is zero and the fan assembly must move 100% of the cooling air. Primary culprits include:

• Failed or slipping thermostatic/electro-viscous fan clutch or inoperative electric cooling fan.
• Cracked, unbolted, or missing radiator fan shroud or missing radiator-to-core-support rubber air baffles (allowing the fan to recirculate hot engine-bay air in a loop around its blade tips).
• External debris plugging the heat-exchanger stack: Insects, mud, chaff, leaves, or grease packed into the narrow air gap between the front A/C condenser, transmission/power-steering cooler, and radiator fins.
• Slipping accessory drive belt or glazed water pump/fan pulley.

2. High-Speed or Heavy-Grade Towing Overheating

If the engine holds a normal 195°F temperature at curb idle and light city driving, but climbs past 230°F–245°F during sustained highway cruising or steep mountain grades under heavy GCWR load, the cooling system cannot transfer heat fast enough to match peak fuel burn. Primary culprits include:

• Internally scaled, calcified, or silicate-gelled radiator tubes reducing effective liquid-to-metal heat transfer area.
• Thermostat stuck partially closed, restricting maximum gallons-per-minute (GPM) flow at high RPM.
• Water pump impeller cavitation erosion or loose composite impeller slipping on the pump shaft at high RPM.
• Collapsed lower (suction) radiator hose: High water-pump suction at 2,800–3,500 RPM pulls a softened lower hose flat if its internal reinforcing spring or structural wall has deteriorated.
• Excessive combustion heat or exhaust restriction: Retarded ignition timing, lean air-fuel mixtures (common on misadjusted LPG vaporizers), a melted/plugged catalytic converter, or a leaking head gasket / cracked cylinder head pumping superheated combustion gases into the upper cooling jacket and displacing liquid coolant from around the exhaust ports.

3. Engine Overcooling (DTC P0128)

When an engine fails to climb to its calibrated closed-loop regulating temperature (185°F–195°F) within the PCM's warm-up airflow/run-time model, the PCM stores DTC P0128 (Coolant Thermostat — Coolant Temperature Below Thermostat Regulating Temperature). Drivers typically complain of poor cab heater output and increased fuel consumption. Overcooling is caused by a thermostat stuck open, a torn or missing thermostat flange rubber seal allowing cold coolant to bypass around the closed poppet valve, an improper low-temperature thermostat (160°F), or a seized fan clutch in sub-zero weather. Never ignore overcooling: operating with oil temperatures below 180°F prevents water vapor and unburned fuel from boiling out of the crankcase, rapidly forming acidic black crankcase sludge, accelerating cylinder bore wear, and diluting the engine oil with fuel.

Thermostat, Coolant Bypass & Housing Testing (Task D.7)

A wax-pellet thermostat relies on a sealed brass cup filled with copper-impregnated paraffin wax. As coolant heats up, the solid wax melts and expands (~12% volumetric increase), pushing against a synthetic rubber diaphragm and stainless steel center pin to open the poppet valve against return spring tension.

To bench-test a thermostat, suspend both the thermostat and a calibrated immersion thermometer on wires in a container of water (never rest either component against the hot bottom of the pan) and heat gradually while stirring using a controlled method rated for the required temperature; plain water at atmospheric pressure cannot be heated beyond its local boiling point:

1. Start-to-Open Temperature: The main valve must begin cracking open within ±3°F (±1.7°C) of the temperature stamped on the thermostat flange (for example, 189°F–195°F for a 192°F rating). You can suspend the thermostat by trapping a thin strip of cellophane or a 0.002 in. feeler ribbon in the closed valve; the exact temperature at which the thermostat drops off the ribbon is its start-to-open point.
2. Full-Open Temperature and Lift: The example acceptance criterion requires the thermostat to reach full open travel (minimum ~0.300–0.350 in. / 7.6–8.9 mm valve lift) at approximately 20°F (11°C) above its start-to-open rating (~212°F–215°F), and must close completely without sticking as the water cools.

Inspect the thermostat housing (water outlet neck) for warpage with a straightedge (max 0.002 in.) and check for galvanic pitting around the O-ring land. Always install the thermostat with the copper wax-pellet bulb pointed INWARD toward the heated engine block/cylinder head and orient the jiggle pin (air bleed valve) or bleed notch at the 12 o'clock (highest) position to purge trapped air during refilling.