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I often go to a chain for a quick oil change. They frequently try to sell me an expensive complete engine flush, although I don't know exactly what that is. I thought that they drain the oil, change the filter, and apply new oil. What is this flush, and does it have some merit to it?

– Henry

An engine flush washes the gunk out of your car's engine – and most cars will never need one, says an engine expert.

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"If you do your oil changes regularly you shouldn't have a need for an engine flush," says Dennis Mott, automotive professor at Centennial College in Toronto. "Ninety-nine per cent of cars on the road will never ever need one."

In an engine flush, a technician takes out some of the oil and adds a flush additive – a chemical designed to break up carbon deposits inside the engine. The car gets driven so the chemical can work its way through – and then the deposits get loosened and trapped in the filter. Then you get an oil change and a new filter and all the gunk is taken away.

It's like some of those less pleasant cleanses sold in the health food store – but it's for your vehicle.

"You have to change the oil after an engine flush, otherwise it would plug up your system," says Mott. "That's why they sell them at the same time as oil changes."

How does gunk build up? By going too long between oil changes, doing a lot of stop and go driving, or letting your car sit unused for months at a time.

"If your car sits over the winter and oil's not circulating in the vehicle, condensation builds up," Mott says. "I have a couple of antique cars that I rarely drive – I've gone over to a high performance oil to prevent (build-up) from happening."

A flush might not be a bad idea if you're buying a used car with 100,000 km or more on it and don't know if the oil was changed regularly, Mott says.

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Even if your car had some build up, the oil change place probably wouldn't be able to tell unless they knew of some gaps in your maintenance history. And it's definitely not something you need regularly.

"No manufacturer in the world is going to recommend one," Mott says.

"How an oil change place can know if you need one is beyond me."

Send your automotive questions to globedrive@globeandmail.com

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