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At the car wash I see guys spraying down their cars' engines and steam pouring out. Can this damage the engine or other parts? Is cleaning under the hood something I should be doing to prolong the life of my car – or is it strictly cosmetic? What's the best way to go about it?– Marc, Ottawa

If your engine came with a wash label, like a wool sweater, it would probably say "hand wash with care – but only if you know what you're doing."

"I don't recall ever seeing anything in a service manual or an owner's manual telling you to clean the engine compartment," says Mubasher Faruki, with the British Columbia Institute of Technology (BCIT) in Burnaby, B.C. "A lot of people like to do it, but it's not something I'd recommend doing with pressurized water at a car wash."

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A jet of soapy water, the foam brush and a quick rinse all get the engine clean – but you could end up with a glistening engine that won't start, or worse.

"The engine compartment's not designed for a lot of water – you could be getting water into areas where it shouldn't be and it can cause corrosion," says Faruki, chief instructor of motive power at BCIT's school of transportation. "Sure, you'll have people saying they do it all the time and have no problems, but you really have to be careful."

Faruki lists a few pitfalls of pressure washing: Spraying high-pressured warm or cool water on a hot engine could cool things too fast, stressing the metal and, potentially, causing cracks. Water could get into the alternator or the engine sensors, causing damage. It can also push dirt and guck onto belts – causing squeaks.

"If you're really unfortunate, water could get into your engine through the induction system, from around the air cleaner," Faruki says. "It's unlikely, but you could force water into the cylinders and cause huge damage."

While a modern electrical system should be able to keep out moisture – jets of water could cause trouble.

Should you be cleaning the engine at all? An engine that's clean enough to eat off will impress anyone who looks under your hood – but, unless you have a mess from a leak, that's probably the only benefit, Faruki says.

"If you have an oil leak, that attracts dirt so, yeah, you'll want to clean it," says Faruki. "But, in my opinion, if everything is working well – no oil or coolant leaks – cleaning the engine is strictly for cosmetic purposes."

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If you do want to clean under the hood, Faruki recommends skipping the car wash and using a rag and a toothbrush. A water-soluble engine degreaser is fine, if you follow the directions on the label. Don't use solvent based cleaners – they'll just attract more dirt to the engine, he says.

If you're setting out to clean the engine properly – to get it ready for a sale or for maintenance – it can be a big job. Popular Mechanics has a 28-step guide online, and it doesn't involve a hose.

If you really want to use a hose, use it under low pressure, and not when the engine's hot. Faruki also says to avoid getting water into the alternator, the engine computer and the air filter – if you can find them.

"Covering the alternator with a plastic bag or plastic wrap is a great idea," Faruki says. "It might be tough to know what to avoid getting wet or what to cover unless you're familiar with the engine."

Anything that gets wet should be dried, ideally with compressed air, and then by running the engine until everything dries out, Faruki says.

And how often? Since a wash is not something your engine normally needs, that's up to you, Faruki says.

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"I really want to stress that people should look through the owner's manual to see what maintenance is required," Faruki says. "If you do what the manufacturer recommends, you're likely going to avoid a lot of problems."

If you're not sure what parts to avoid getting wet, getting your engine professionally cleaned might be a better bet, but even then, water can cause trouble.

"In a lot at dealerships they'll get the detailing department to clean the engine of a car, and that's where I've seen these kinds of problems," he says. "You have a kid with good intentions, a hose and a can of degreaser. And then cars won't start."

Send your automotive maintenance and repair questions to globedrive@globeandmail.com

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