Drivers 'should use 15-min rule' and not start engine during 'peak' times this week

As the mercury soars into the 30s across the UK this week, motorists are being warned of a hidden danger that could leave them stranded with a hefty repair bill – and it's all about timing. With the fourth heatwave of the summer triggering amber health alerts, drivers need to be aware that firing up their car during peak afternoon heat - typically seen after the hottest hours of the day, which in the UK is generally 11am to 3pm - could seriously damage their engine, warns an expert.

The lethal mix of blistering temperatures and a day's worth of heat accumulating under the bonnet creates the perfect storm for mechanical catastrophe. Kazimieras Urbonas, a car expert and Supplier Excellence Manager at Ovoko, one of Europe's biggest online marketplaces for used car parts, has witnessed firsthand the havoc that extreme heat can wreak on vehicles.

"Most drivers don't realise that their car's engine bay can reach temperatures far higher than the outside air temperature during a heatwave," Urbonas explains. "When you try to start a superheated engine, you're asking it to work even harder when it's already under incredible stress."

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Why peak heat hours are your engine's worst enemy

Urbonas warns that the real issue begins well before you even start your car. "Throughout a hot day, heat builds up steadily under the bonnet, turning your engine bay into an oven. By late afternoon, when temperatures peak, the metal components are already superheated and struggling to cope."

On a typical summer's day, the temperature inside the engine bay can rise 20-30 degrees higher than the outside air. When you start a car that's been sitting in 34C heat for hours, you're essentially asking an already overheated system to generate even more heat through combustion.

"The cooling system becomes overwhelmed," Urbonas explains. "Your radiator and cooling fans are working overtime just to maintain normal temperatures, but they're fighting a losing battle against the ambient heat."

The hidden damage you can't see

Starting a superheated engine not only puts strain on the cooling system, but it can also cause costly internal damage that won't become apparent until it's too late. Metal components expand differently under extreme heat, and sudden temperature changes from starting the engine can lead to warping.

"We see a lot of damaged parts coming through our marketplace during heatwave periods," Urbonas points out. "Cylinder heads, gaskets, and cooling system components that have failed due to heat stress. These aren't cheap fixes either - we're talking hundreds or even thousands of pounds in repairs."

The most susceptible components include rubber seals and gaskets, which can deteriorate under scorching temperatures, resulting in coolant leaks and engine overheating.

Four telltale signs your engine is heat-stressed

Urbonas highlights these crucial warning indicators that your engine is battling with the heat:

Temperature gauge rising above normal levels

Even without reaching the danger zone, a higher-than-typical reading during ignition serves as a clear alert.

Coolant odour or vapour

"If you smell sweet coolant or see steam when you open the bonnet, your cooling system is already under severe stress," cautions Urbonas.

Strange engine sounds

Pinging, knocking, or abnormal rattling noises may signal that engine parts are expanding beyond their standard limits.

Lethargic performance

Should your vehicle feel less responsive or struggle with normal acceleration, the heat might be impacting engine function.

The 15-minute rule that could save you thousands

The remedy is remarkably straightforward, according to Urbonas: "Wait 15-20 minutes after peak heat hours before starting your car, or take steps to cool it down first. It's such a small change that can prevent massive headaches later.

"If you must drive during peak heat, start the car and let it idle for a minute or two before driving off," recommends Urbonas. "Give the cooling system a chance to circulate before you put the engine under load."

Urbonas advised: "Prevention is always better than repair when it comes to protecting your engine during extreme heat. Simple steps like parking in shade whenever possible, using reflective windscreen covers, and checking your coolant levels before heatwaves can save you significant money and stress.

"Regular maintenance becomes even more important during hot weather. Make sure your cooling system is in top condition: check for leaks, ensure your radiator is clean, and consider having your coolant changed if it's been a while. A well-maintained cooling system is your first line of defence against heat damage.

"If you do notice any warning signs, don't ignore them. The cost of a simple coolant top-up or radiator flush is nothing compared to replacing a warped cylinder head or blown gasket. At Ovoko, we see too many expensive failures that could have been prevented with basic precautions and early intervention."