Overheating Only at Highway Speed Under Load
A 2.5 L turbodiesel pickup with 280 000 km on the clock is booked in with "uses water and the needle climbs". The driver reports it is normal around town, but after about 30 minutes at 100–120 km/h with a 700 kg load the gauge rises above normal, and drops back as soon as he slows down. Coolant loss is roughly 0.5 L per 200 km, with no puddle under the vehicle and no white smoke from the exhaust. Workshop findings: coolant is 30% glycol by refractometer; the pressure cap releases at 0.7 bar against a 0.9–1.1 bar specification; a cold pressure test at 1.0 bar holds for 10 minutes with no drop; the thermostat starts opening at 82 °C in a pot and is fully open by 95 °C; the viscous fan turns easily by hand when the radiator is hot; about 20% of the radiator core face is blocked with mud and insects; the bottom hose feels soft and flattens when the engine is revved.
1. Explain why this symptom pattern points away from the fan and towards coolant flow and heat rejection. Then judge each workshop finding as a fault or as normal.
Read the worked answer
At 100 km/h the ram airflow through the core is far greater than anything the fan can move, so a failed fan coupling shows up as overheating at idle and in slow traffic, then clears once the vehicle is moving. Here the opposite happens: the vehicle is fine in town and overheats only when engine load and road speed are both high. That means heat production is rising with fuelling and the system cannot get rid of it - either coolant cannot circulate fast enough or the radiator cannot transfer the heat. Slowing down cuts fuel and heat output and reduces load, so the gauge falls, which fits this explanation.
Findings:
- 30% glycol - fault. Too weak to give the designed boil-over protection or corrosion inhibition.
- Cap releasing at 0.7 bar against 0.9–1.1 bar - fault. It vents early, so coolant boils at a lower temperature and is pushed out as vapour.
- Cold pressure test holding with no drop - normal, and it usefully rules out a cold external leak, but it does not reproduce hot, flowing, pressurised conditions.
- Thermostat opening at 82 °C, fully open by 95 °C - normal and probably not the cause, so replacing it would be guesswork.
- Fan free when hot - fault. The viscous coupling has failed, but this explains low-speed overheating, not the highway symptom.
- 20% blocked core face - fault. It reduces heat rejection exactly when the highest heat load is being produced.
- Soft bottom hose collapsing on revving - fault, and a direct cause of the highway symptom: at high rpm pump suction collapses the hose and throttles circulation.
2. Explain what the cap and the 30% glycol mixture do to the boiling point, and why the combination of a weak cap and coolant loss with no visible leak is consistent.
Read the worked answer
Water boils at 100 °C at atmospheric pressure. Two things raise that:
- Glycol concentration. A 30% glycol mix raises the boiling point by only about 3–5 °C (roughly 104 °C); a 50/50 mix would give about 10–12 °C. So the weak mix is giving away around 6–8 °C of protection.
- System pressure. Each 0.1 bar of cap pressure raises the boiling point by roughly 2.5–3 °C. A correct 0.9–1.1 bar cap therefore adds about 23–28 °C; this 0.7 bar cap adds only about 18 °C.
Together, this system boils its bulk coolant somewhere around 120–123 °C instead of about 128–130 °C. Under sustained highway load the metal surfaces and local hot spots run well above bulk temperature, so boiling begins at the liner walls even before the gauge shows a problem. Steam bubbles displace liquid, the pump loses its prime, and the expanding vapour pushes coolant out through the cap and overflow.
The loss therefore leaves as vapour, not liquid, which is why there is no puddle and no white smoke. It also explains why the cold pressure test held: it tested the system, not the cap's release point under hot running conditions, and the cap - not a hose or core plug - is the escape route. A cap that vents early is both a cause of overheating and the visible symptom of coolant loss.
3. Give the repair sequence and the tests you would use afterwards. Explain why simply fitting a new thermostat would be the wrong first move.
Read the worked answer
Repair sequence - cheapest and most certain faults first, before spending on diagnosis that the evidence has already covered:
- Replace the bottom hose and its clamps, and check whether the original had an internal stiffening spring; fit the correct part, not a straight hose bent to fit. Pressure-test again.
- Fit the correct 0.9–1.1 bar cap, checked against the part number, not just a cap that screws on.
- Clean the core externally from the engine side with low-pressure air or water, straight through the fins, without bending them; check for corrosion and fin separation that cleaning cannot fix.
- Replace the viscous fan coupling, and check the fan shroud and seal are intact.
- Flush the system, refill with the manufacturer's specified glycol concentration (usually 50%), and bleed the highest point and heater circuit properly.
Verification: hot pressure test; infrared temperature readings at the upper and lower hoses to confirm a real drop across the radiator; observe the fan roaring when the engine is hot and idling; confirm the heater works and that the gauge is stable during a loaded road test of at least the duration the customer described; recheck coolant level after the vehicle cools.
Why not the thermostat first: the pot test showed it opens at 82 °C and is fully open by 95 °C, so it is functioning. Fitting one would leave the weak cap, collapsing hose, blocked core and failed coupling untouched, the symptom would continue, and the customer would pay for a part that was not needed. Fix what the evidence identifies, then re-test before condemning anything else - such as the head gasket or water pump - that a combustion gas test can rule in or out.