Friday, December 28, 2012

Should you warm up your engine before you drive?



Should you warm up your car's engine?

Warmup is part of the ritual of driving any antique, and wise owners follow it faithfully.  Few question the wisdom of bringing an old engine up to temperature before putting it under load.  Yet many of those same people hop into modern cars and zoom out of the driveway without a moment’s preparation.  Is that wise?

Most carmakers say warmup is unnecessary, but they are under pressure from government agencies to keep fuel economy up, and warmup burns gas.  And they are in the business of selling parts . . . cars that don’t break don’t make them any money.  So I don’t know if I’d follow their advice blindly, in this area and some others . . .

When I think about the problems people have with high-end cars, oil leaks and head gasket failures are high on the list for any brand.  When we disassemble engines for oil leakage we often find gaskets cracked, which means they were not strong enough to hold against the applied fluid pressure.

It’s easy to assume they didn’t hold because they just weren’t good enough, but that’s not always the only explanation.  In fact, when an engine is cold the metal parts have contracted so the fit between two pieces of engine is fractionally looser than five minutes after start, when everything is warm

As the engine parts heat and expand, the bolts tend to expand less, so they become more tightly clamped.  Why?  Because the engines in modern cars are aluminum or some other lightweight alloy, and the bolts are almost always steel.  Steels expands less.

At the same time, when an engine is cold, the oil is thick.  Thicker oil = higher oil pressure as the pump works harder to force it through the passages of the motor.  An engine that has 15 pounds pressure at 800 rpm hot might have 50 pounds when cold, and pressure could soar to 100psi or more if the motor is raced.

Looser clearances between big engine parts + higher oil pressures in warm-up = much greater chance of blowout failures in the engine’s oil system.

At the same time, stepping on the gas with a cold motor means high combustion chamber pressures.  Mix that with those loose clamping forces on the cold head gasket and you have a formula for head gasket blowout.

Manufacturers can say what they will . . . the logic and engineering sense of the points above will stand.  They may have engineered in enough strength to protect against the failures I describe, but then again, maybe they didn’t.  After all, if they did, we’d never see those failures in the shop!

The simple takeaway – five minutes of warm-up will keep your motor alive longer, with fewer leaks and less risk of failure.  And when you do drive . . . go light on the throttle until everything is up to temperature!

Happy New Year
John Elder Robison

3 comments:

Elaine said...

Thank you John for the post. I grew up in a gear head family and learned to warm up my car when I learned to drive. Question: is it just a critical when the weather is warm to mild?

John Elder Robison said...

Yes . . . the desirability of warmup is the same spring or summer. When the motor is hot the metal is about 200 degrees. WHen it's sitting cold its the temp of the air. Whether that is 30 degrees or 70 degrees it still has a long way to go to warm up.

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