Cooling in hot weather: diagnosing overheating on an old car with an engineer's method

When outside temperatures rise, the same phrases keep coming up: 'old cars run hot, it's normal', 'you have to remove the thermostat', 'you need a bigger radiator', 'use pure water', 'fit a more powerful fan'. These reflexes are understandable, but they often mix up symptom, cause and solution.

An old car in good condition should not overheat simply because it is hot. It may have less thermal margin than a modern car, particularly at low speed, in traffic jams or in the mountains, but the cooling system was designed to dissipate a given thermal output under real-world operating conditions.

In automotive engineering, a cooling circuit is not validated “by eye”. We speak of cooling capacity, stabilisation temperature, margin before boiling, water flow, air flow, circuit pressure and pressure losses. The SAE J819 standard, for example, defines an “air-to-boil” type approach, i.e. the ambient temperature limit at which the system reaches boiling under given conditions. So it is not a matter of folklore: a cooling system is measured and diagnosed.

1. The urban myth: 'old cars run hot, it's normal'

An old car that runs hot is not 'normal' as a matter of principle. What is true is that it is often more exposed to accumulated faults:

• partially clogged radiator;
• oxidised or scaled-up core;
• incorrect thermostat ;
• tired fan;
• imprecise thermal switch;
• pressure cap not holding pressure ;
• worn water pump;
• missing air guides ;
• coolant too old or an inconsistent mixture;
• incomplete bleeding ;
• faulty sensor or gauge;
• poorly adjusted injection, carburation or ignition.

A cooling system comes down to two transfers: engine → coolant, then coolant → air. In simplified terms:

Q = thermal power transported, ṁ = mass flow rate, Cp = specific heat, ΔT = inlet/outlet temperature difference. Cooling depends not only on the radiator, but also on the coolant flow rate, the nature of the coolant and the usable temperature difference.

2. Two cases that must absolutely be distinguished

3. First case: the gauge indicates overheating, but without vapour

3.1. Check the real temperature with an infrared thermometer

The infrared thermometer measures a surfacetemperature, not the internal coolant temperature, and depends heavily on emissivity: a shiny surface (bare aluminium, clean brass) returns a false value.

Useful measurement points: cylinder head near the water outlet, thermostat housing, top hose, radiator inlet and outlet, top and bottom of the core, several areas of the radiator, pump body, heater core.

3.2. What the temperature differences tell you

Hot cylinder head, cold radiator. The hot coolant does not circulate correctly towards the radiator: thermostat stuck closed, fitted backwards or unsuitable, air pocket, ineffective pump, slack belt, loose impeller, crushed hose, blocked circuit.

Radiator hot everywhere, but engine too hot. The heat reaches the core; the problem is on the air side: obstructed core, crushed fins, fan that doesn't switch on / too slow / reversed, missing shroud or guides, air leak, hot-air recirculation. A CFD study by Tata Technologies/Tata Motors shows that eliminating recirculation and leaks increased the flow through the radiator by 34 %. A new radiator that is poorly guided in terms of airflow cools badly.

Radiator hot at the top, cold at the bottom. A large inlet/outlet ΔT = radiator that exchanges well but insufficient water flow. Conversely, a low ΔT + high temperature everywhere = radiator that cannot reject heat to the air (airflow, clogged core, fan, degraded radiator). The ΔT is always read in context.

Localised cold spots. Internal clogging: sludge, oxidation, scale, old leak-stop products, particles, incompatible mixtures, mineral-rich tap water. MAHLE points out that these contaminants sometimes require a flush and reduce heat exchange.

4. Check the reading: sensor, earth and gauge

You compare the physical reality with the display. If the cylinder head stays consistent, the radiator shows a logical gradient, the fan regulates and there is neither steam nor blow-back, you should suspect the measurement chain: tired sensor, wrong reference, sensor incompatible with the gauge, degraded engine earth, connector oxidation, parasitic resistance, faulty voltage stabiliser, imprecise gauge.

5. Check the fan: activation, regulation and flow direction

On the road, the dynamic airflow is enough and the fan is secondary. When stationary, in traffic jams, on a slow climb or after a heavy load, it becomes essential.

5.1. Does the fan switch on?

To check: thermoswitch, relay, fuse, power supply, earth, first-speed resistor, fan motor, connectors, brushes. On two-speed vehicles, the first speed can be missing without you noticing: the car waits too high before the fan kicks in.

5.2. Does the fan regulate?

Normal behaviour: the temperature rises, the fan switches on, it stabilises then drops a few degrees, the fan cuts off, the cycle starts again. If it switches on but the temperature keeps rising, the air is not really passing through the radiator, or the radiator no longer exchanges heat.

5.3. Is the fan turning in the right direction?

After replacement or polarity reversal, it can spin the wrong way: it stirs the air without creating the right flow. It must be puller (behind the radiator) or pusher (in front).

6. The thermostat: why you should not remove it randomly

'In summer, remove the thermostat': sometimes true in an extreme case of a stuck thermostat, but a bad general rule. It is not a simple tap, but a thermal-regulation component and hydraulic. MAHLE explains that it opens the radiator circuit at a defined temperature and sometimes closes the bypass at the same time. Gates: stuck open = engine too cold; stuck closed = overheating.

6.1. The flawed reasoning about the “water speed”

Limits do exist: pressure losses, cavitation, poor distribution, pump outside its efficient range, bubbles, stabilisation time, engine architecture.

6.2. How to test the thermostat

On the vehicle: with the engine cold at idle, follow the temperature near the housing and the top hose; opening expected around the nominal temperature, with a rapid rise of the hose at the moment of opening (Gates recommends the infrared thermometer). Off the vehicle: immerse the thermostat in water, heat it, check the opening temperature, verify full opening and closing on cooling.

7. Check the water pump and its drive

The pump sets the flow rate: without flow, no heat transfer. Gates points out that it is the heart of the circuit; belt, roller and tensioner are inspected along with it. To check: belt present and tensioned, no slipping, pulley driven, no bearing noise or play, no leak from the weep hole, condition of the coolant, visible circulation.

7.1. The broken or slack belt

Simple case: pump not driven, temperature rising fast. Depending on the engine, driven by the timing belt or by the accessory belt.

7.2. The worn, corroded or loose impeller

Gates reports that fin corrosion makes the pump ineffective, that deposits hinder its operation and that cavitation damages the components.

8. The radiator: clean or replace?

It is a heat exchanger: its performance depends on the exchange surface, the active tubes, the condition of the fins, the materials, the coolant and air flow rates, the flow distribution and the internal/external cleanliness.

8.1. External cleaning

8.2. Internal cleaning

More uncertain: a flush removes light sludge but does not recover an oxidised, scaled-up or blocked core. MAHLE recommends checking the old coolant (oil, rust, scale, particles) because these contaminants indicate faults.

8.3. Replacement or re-core

Cold spots, small active surface, obstructed tubes: replacement becomes serious. But don't think only in terms of size: a thicker radiator can work less well if the air does not pass through the whole thickness. Fin density, core quality, pressure losses and air guides matter enormously.

9. The coolant: why the water/glycol proportion matters

Coolant is not 'coloured water': it is a compromise between heat capacity, antifreeze, anticorrosion, boiling point, pump lubrication, seal/metal compatibility, conductivity, viscosity and chemical stability.

9.1. Specific heat capacity

If Q is identical and Cp decreases, ΔT increases.

9.2. Thermal conductivity and viscosity

Glycol has lower conductivity and higher viscosity: increased pressure losses and reduced real flow in small passages, narrow radiators and heater circuits. It is not about running on pure water, but about avoiding inconsistent mixtures or ones too heavily loaded with glycol, especially in summer.

9.3. Why you should not run on pure water for long

Avoid: hard tap water, unknown mixtures, incompatible coolants, old brown coolant, repeated top-ups without checking, excessive antifreeze for summer use.

10. Air guides: aerodynamics matter as much as the radiator

On many old cars, the air guides, foams, closing panels or shrouds have disappeared. These are functional : the air takes the path of least resistance and bypasses the core if it can. The CFD by Tata Technologies/Tata Motors shows that panels, shroud, grille, floor and leaks directly influence the flow: reducing recirculation and leaks improved the flow through the radiator core by up to 34%.

To check: original deflectors, sealing foams, closing panels, fan shroud, fan/radiator gap, stray air passages, hot-air recirculation, unobstructed grille, plate/accessory blocking the air intake, a different radiator fitted without adapting the guides.

11. Injection, carburation or ignition settings: when the engine produces too much heat

The radiator, pump, thermostat and fan can be fine but overwhelmed because the engine abnormally produces too much heat. Classic case: too lean a setting, especially under load. On a petrol engine, the air/fuel ratio directly influences the combustion temperature, the exhaust-gas temperature and the heat to be dissipated.

An engine that is too lean can show: hotter exhaust, hotter cylinder head, stressed valves, risk of knock/pre-ignition, hotter oil, rising coolant (higher thermal load), overheating visible mainly under load, on climbs, on the motorway.

11.1. The possible causes of a too-lean operation

• air leak at the intake; cracked vacuum hose; faulty manifold gasket;
• blocked injectors; low fuel pressure; tired pump; clogged filter; faulty regulator;
• jets too small or blocked; incorrect float-bowl level;
• faulty flow meter/load sensor; temperature sensor distorting the ECU info; incorrect lambda or richness correction;
• excessive advance; too lean a map; wrong CO setting; poorly synchronised carburettors.

11.2. The typical symptom: temperature rises under load

Fitting a bigger radiator can mask the problem but does not remove the cause. On an old car — especially after engine modification, a change of injection/carburettors/exhaust or long storage — checking the mixture is part of the thermal diagnosis.

12. The cabin heater: a logical backup solution

13. Second case: steam, boiling or coolant blowing back

13.1. Check the cap: an often underestimated part

The cap is a pressure valve (and often a vacuum valve) that keeps the circuit pressurised to raise the boiling point. Behr Hella states that it increases the pressure by about 0.3 to 1.4 bar, which raises the boiling point. At atmospheric pressure water boils around 100 °C; under pressure, higher. A tired cap causes overflow, coolant loss, early boiling, hoses crushed on cooling, air ingress, corrosion, instability.

13.2. Local boiling: the danger of vapour pockets

When the coolant boils locally (cylinder head, chambers, exhaust seats), heat exchange degrades sharply: vapour carries heat poorly, the hot zone gets hotter — a vicious circle. Consequences: damaged head gasket, warping, hot spots, knock, cavitation, corrosion, cracks. The circuit pressure is essential to keep the coolant in the liquid phase.

14. In case of steam: go back through the whole diagnosis, priority on pressure/flow

14.1. Level and bleeding

A lack of coolant reduces the mass that absorbs the heat; an air pocket reduces circulation and creates hot spots (MAHLE). Air symptoms: cold/irregular heater, gurgling, unstable temperature, partly cold radiator, varying level, rapid rise after a drain. On some old cars, bleeding requires a precise procedure (bleed screw, heater open, tank raised, slow filling, stabilised revs, check after cooling).

14.2. Water pump

A loose impeller gives a misleading diagnosis: everything turns, but the real flow is missing. Signs: hot cylinder head, radiator not hot enough, weak heater, little coolant movement at the tank, overheating increasing with load, noisy or leaking pump.

14.3. Thermostat stuck closed

The engine heats up fast, the radiator stays cold or warms up late. The infrared thermometer before/after the housing settles it (Gates method).

14.4. Blocked radiator

Works at low load then becomes insufficient in summer. Thermal mapping identifies it: cold spots, irregular ΔT, hot top/cold bottom, heater that helps a lot, dirty coolant, a history of leak-stop products.

14.5. Combustion gases in the circuit

15. Practical 'engineer's' diagnosis method

1. Qualify the symptom — gauge only? with steam? stationary, on the road, on a climb, after stopping? slow drift or sudden rise? only under load?

2. Measure — cylinder head, thermostat housing, radiator inlet/outlet, top/bottom, multiple zones. Always on a matte surface or black tape.

3. Read the gradients — hot cylinder head + cold radiator = circulation; radiator hot everywhere + hot engine = air exchange/capacity; cold spots = blocked radiator; huge ΔT = low flow; low ΔT + hot = lack of air; hot mainly under load = mixture/advance.

4. Airflow — does the fan switch on? right speed, right direction? shroud and guides? clean core? clear grille?

5. Water flow — pump driven? belt, thermostat, hoses, bleeding, circulation, radiator not blocked?

6. Pressure — cap at the right rating? seal, vacuum valve, tank, return, leaks?

7. Combustion — mixture richness, fuel pressure, injectors, air leaks, carburettors, ignition advance, spark plugs, AFR/lambda under load, EGT ?

8. Only upgrade after diagnosis — sensor, earths, correct thermostat, bleeding, cap, radiator, air guides, fan, pump, water/glycol mix, cleaning, engine tuning.

16. Solutions ranked by real effectiveness

Backup solutions: cabin heater fully on, reduce load, temporary stop, bonnet open when stationary, resume after the temperature drops.

Conclusion

An old car that runs hot in hot weather should not be written off automatically. Cooling is not a matter of luck, but of thermal balance: the engine produces heat, the coolant carries it, the radiator transfers it to the air, the fan and the guides set the airflow, the thermostat organises circulation, the pump sets the flow rate, the cap maintains the pressure, the coolant determines the heat capacity, and the injection/carburation/ignition determine the heat to be dissipated.

Technical sources

• MAHLE Aftermarket — Vehicle cooling: circuit, thermostat, bypass, radiator, circulation faults.
• Gates — Cooling system troubleshooting: thermostat, pump, hoses, radiator, cap, infrared checking.
• Fluke — Emissivity in infrared thermometry.
• Dynalene — Ethylene Glycol Engineering Guide: Cp, conductivity, viscosity.
• Behr Hella Service — Radiator filler cap: system pressure, valves.
• Chacko, Shome, Kumar, Agarwal, Katkar — Numerical Simulation for Improving Radiator Efficiency by Air Flow Optimization (CFD).
• NASA Technical Report — fuel-air ratio, EGT and CHT.
• MIT OpenCourseWare — Engine Heat Transfer.
• Colorado State University — Intake and Exhaust System Heat Transfer.
• ICCT — Fuel enrichment under high load.