Recent record temperatures have taxed car cooling systems to new limits. Most modern cars can cope, and with their air-con on you’re hardly aware of the huge exchanges of heat as you sit coolly in the cabin, but things can get marginal in a classic car for engine and occupants alike. Sometimes the radiator (we’ll assume water-cooling) just can’t get rid of heat as quickly as it goes in, even with the cooling fan’s help. Your engine gets ever hotter, most likely when sitting in traffic with no natural airflow through the engine bay, and a boil-up threatens. Turn on the heater for a while to use it as an extra radiator, blower on full speed. You’ll melt even more, but it should bring the engine temperature down. If your car’s cooling capacity isn’t quite enough on a hot day even when cruising at speed, again you can resort to help from the heater for as long as you can bear it.
But overheating might not just be an inherent characteristic of your car-type; your radiator could be clogged up with silt or scale. A flushing solution might help, but a new core or complete radiator is better. Another reason for overheating is external blockage of the radiator fins, perhaps with mud or oil residue. Don’t clear them with a power washer because you’ll very likely destroy the fins, as this writer knows from experience. Or some fins might have corroded away in which case, again, re-coring or replacement is the answer. Some radiators have ducting ahead of them to ensure air goes through the radiator rather than round it; check it’s all in place and complete.
Retarded ignition can cause hot running by allowing still-burning mixture to enter the exhaust ports and heat up the adjacent water channels. And is the thermostat opening fully, and at the right temperature? A lower-temp one can help in summer. If your car overheats in traffic and has an engine-driven fan, a replacement electric one will help because it runs faster than the fan of an idling engine and sucks air more efficiently. Then there’s waterless coolant, which divides opinions; it has a higher boiling point but the engine will still get hot. The ultimate solution? A more efficient radiator, using the latest type of core – and maybe some racy louvres in the bonnet to let the hot air out.