Should I let my car warm up before driving?

 

Warming up your car up before driving it: With a modern car is this a good idea or bad idea?

 
 
 
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Short answer: No. You don’t need to warm up the car. If you’ve got a reasonably modern car - say less than about 15 years old - just turn it on and drive it.

The whole concept of warming a car up is a waste - a throwback from the 1970s and ‘80s. There’s a catch, and we’ll get to that.

But first, why does this myth persists today?

Basically, in the olden days, cars had carburettors. Remember those?

With a carbie, managing the air-fuel ratio on cold starts was a bitch. It was accomplished with a manual choke. The term ‘compromise’ springs to mind. Engines ran quite badly until they warmed up.

There was a cable that literally choked the air inlet, and manually throttled the engine up. The air-fuel ratio went through the roof. You could get high from the fumes - not recommended, but it did smell pretty good while it damaged your DNA.

Chokes got better, eventually. They became automatic. They used diaphragms and other mechanical voodoo to remove from the driver the burden of pushing the choke back in. Still imperfect, but better.

Still a good idea to warm the car up until the auto choke turned itself off.

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Then we got fuel injection, in the 1990s, and the management of the mixture improved across the whole range of operating conditions. And there’s been a continuum of improvement ever since.

Modern throttles are ‘by wire’ and there’s a lot of measurement of the inlet air, the metering of the fuel, and measurement of the exhaust, leading to better control. There are mass airflow sensors - MAF sensors - telling the computer the weight of air going into the engine.

There are oxygen sensors in the exhaust - telling the computer how complete the combustion is in the engine, essentially, in real time, knock sensors for microprocessor ignition advance control, and a bunch of other feedback and control mechanisms that were unthinkable in the 1970s and 1980s.

So: your modern car today - just start it up and drive. For most people that means getting in, starting, putting a seatbelt on, checking the kids are secure, backing out of a driveway or backing and filling out of a parking spot, or crawling out of an underground car park - whatever.

It’s not very demanding driving (in the context of the load on the engine) in the seconds following a cold cold start, for most of us.

People also say you need to give the oil time to circulate. Trust me: It is circulating - within seconds. Modern oil is very thin. The whole system is pressurised within a few seconds at the most, even in cold conditions.

Engines are also designed to warm up quickly, so basically, by the time you’re at the end of your driveway, your engine is warm.

The only catch is: Don’t thrash the engine until it’s at its full, normal operating temperature. Metals expand a little with temperature. It’s about 22 microns per metre per degree for aluminium and about 12 for steel. (Two of the most common metals in engines.)

That doesn’t sound like much, and it’s not, but thermal expansion is designed-in, in R&D. The precision parts are designed to be exactly the right size at their normal operating temperatures.

That means some of the clearances are off, just a little, in a cold engine. So, for this reason alone, I wouldn’t be pumping in the big stresses until normal operating temperature is achieved.

Obviously, if you’re in a critical driving situation and you need to floor it to get out of the path of some truck aimed right at you - whatever - don’t die trying to preserve your cold engine. Priorities are important.

What I’m saying is that thrashing a cold engine is, over time, quite a bad idea if you want that engine to last. Aside from that, I guess the only other case for warming up the car is so the heater and the demister have sufficient time to take any condensation away so you can see enough to drive safely.

There’s a good case for taking a couple of minutes to get that right, because contrary to old adage, what you can’t see will hurt you.

But if you are warming up your modern car just because Grandpappy implied non-specifically that not doing so would usher in the four horsemen of the automotive apocalypse, you’re on the wrong track.

It’s a complete waste of time.

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