We're into the third heatwave in as many months this week, and I already know what next week looks like in the workshop. It looks like flat batteries, shredded tyres and steaming radiators. Breakdown callouts jump by around 20% when the UK gets a proper hot spell, and after 40 years on the tools I can tell you the frustrating part: most of those breakdowns were sitting there waiting to happen. The heat didn't cause them. The heat found them.
Here's what hot weather actually does to a car, and the five-minute check that catches almost all of it before it catches you.
Why does hot weather make cars break down?
There's a myth I hear every summer: "modern cars don't overheat, that's a 1970s problem." I understand why people believe it, because cooling systems really are far better than they used to be. But the three things that strand people in a heatwave haven't changed in my working lifetime. They're tyres, batteries and cooling systems, in that order, and heat attacks all three at once.
The important bit is that heat rarely breaks a healthy component. What it does is finish off a weak one. A battery on its way out will start the car fine on a mild Tuesday and leave you stranded in a hot Friday traffic jam. A tyre with a bulge or low tread will roll along happily at 15 degrees and let go at 30. That's why the "nothing I can do, it's just the heat" attitude annoys me. There's plenty you can do, and none of it needs tools.
What does heat do to your tyres?
Tyre pressure rises with temperature, roughly one to two PSI for every 10 degrees. That doesn't sound like much until you remember most of the tyres I see are underinflated to start with. An underinflated tyre flexes more, flexing builds heat, and heat is what destroys tyres. Add scorching tarmac and a car loaded for the school holidays and you've got the recipe for the summer blowouts you see on the hard shoulder every July.
The check takes two minutes at any petrol station. Inflate to the loaded figure in your door jamb or handbook, not the everyday one, if you're carrying passengers and luggage. And while you're down there, look at the tyre. Cracks in the sidewall, bulges, or tread worn near the 1.6mm legal limit are exactly the weaknesses heat goes looking for.
Can a heatwave kill your car battery?
Yes, and this surprises people, because everyone associates flat batteries with January. Winter exposes weak batteries, but summer is what weakens them. Under-bonnet temperatures in a heatwave accelerate the chemical reactions inside the battery and evaporate the electrolyte faster. The damage is cumulative and silent, and the battery that dies on a cold morning in November was often wounded in July.
If your battery is more than four or five years old, or the engine has started turning over lazily, get it tested before the hot week rather than after it. It's a five-minute job with a proper tester, and it's exactly the sort of thing we do at your driveway with our mobile battery service.
Why do engines overheat in traffic?
Your cooling system works hardest when the car is working slowest. At motorway speed there's a gale blowing through the radiator. Crawling through Merry Hill in 32 degree heat with the air-con flat out, there's no airflow at all, so the electric fan is doing everything. A slightly low coolant level, a tired fan, or a sticking thermostat will cope fine at speed and then boil in a queue.
So check the coolant. Engine stone cold, look at the expansion tank, and the level should sit between the MIN and MAX marks. If it's below MIN, top it up, but pay attention, because coolant doesn't get used up. If it's low, it's going somewhere, and you want to know where before it becomes a head gasket conversation.
If the gauge does start climbing in traffic, do the counterintuitive thing: turn the air-con off and the heater on full. You'll be miserable, but the heater is a second radiator and it can pull enough heat out of the engine to get you somewhere safe. Never, ever open the coolant cap on a hot engine. I've seen the scalds, and forty years on I still wince.
And if the air-con itself is the casualty of the week, blowing warm just when you need it most, I covered what's actually wrong and when a regas is worth paying for in Elliot's air-con post from May.
The 5-minute heatwave check
Do this before the hot spell, not during it. On a cold engine:
1. Coolant. Between MIN and MAX on the expansion tank. Low means a leak, so book it in.
2. Oil. Pull the dipstick, wipe, dip again. Hot engines punish low oil.
3. Tyres. Pressures to the loaded figure, and a proper look at tread and sidewalls on all four, spare included if you have one.
4. Battery. Over four years old or cranking slowly? Get it tested this week.
5. Water. Top up the washer fluid, and put a couple of bottles of drinking water in the boot. If you do end up waiting for recovery in 30 degree heat, you'll be glad of them.
That's it. No tools, no ramps, no expertise. Five minutes on your drive against an afternoon on the hard shoulder.
My honest take
Most of the heatwave breakdowns I've attended in four decades were preventable, and I mean the large majority, not half. The car was already carrying a weak battery, a tired tyre or a slow coolant leak, and the owner either didn't know or had been putting it off. A hot week is just the debt collector.
If you'd rather have a professional eye over it, our Summer Ready guide covers everything we check, and a full service deals with all of the above and plenty more. And if the worst does happen this week, our emergency breakdown service covers the Black Country seven days a week. But honestly? Do the five-minute check first. It's free, and it's the best odds you'll get all summer.
Stay cool out there.