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I would appreciate your help with something that's been debated for a long time: decarbonizing your engine. There are chemicals and procedures, like TerraClean, that promise to blast carbon deposits in your engine. My car has direct fuel injection and I'm told, that with enough buildup, the engine could misfire. The dealership says the best way to clean the carbon isn't by chemicals but taking apart the engine and manually scrubbing. It wants almost $1,000 for this service. Is there any proof behind decarbonizing your engine through chemicals and is it worth the money to pay the dealer to do it? – Zac

You are correct – this is an area of debate among technicians and engineers, with the opinions of backyard mechanics thrown in.

One school of thought is that, with modern engines running as hot as they do, cleaner fuels and more sophisticated lubricants, there is no need for "decarbonizing."

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The most likely spot for carbon buildup is on the intake valves, which attract carbon atoms that find their way there from the oil pan through the PVC (Positive Crankcase Ventilation) system.

Until the advent of direct injection, fuel entered the system before the intake valve and cleaning agents contained within the fuel helped dissolve any carbon encountered on the way to the combustion chamber. But with direct injection placing the fuel "directly" into that combustion chamber, the additives have no chance to work on any carbon buildup on the intake valve. The cure is to get the engine hotter, to run it harder or change the timing of the valve action to allow more heat to reach the intake valve. Changes to PCV valve design have also come into play.

Cleaning fuel injectors

When I took my car in for its annual tune-up, I told my mechanic that the fuel mileage was down considerably. He told me I need to have my fuel injectors cleaned, that they have become clogged and are causing a misfire. He says this is a $1,000 job but will result in a more efficient engine and increased mileage. – Sebastien

I suspect your decreased fuel mileage is due to winter driving – cold weather means more fuel consumption. Or perhaps changed driving habits or a different route.

Having said that, there is no question fuel injectors can become partially clogged or "dirty" and need to be cleaned.

The orifice at the end of the injector through which the fuel is sprayed is so small it does not take much to cause a blockage. This is usually associated with poor-quality fuel and a buildup of varnish in the injector, restricting or altering output.

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There are various flushing systems available to technicians that use solvents under pressure to flush or clean injectors. Because these are pressure-related, they can be dangerous to use so should be left to a professional. There are a variety of aftermarket products that claim to clean injectors, but they are minimally effective.

If the injector is indeed in bad shape, it might have to be removed and cleaned or replaced. New injectors cost in the vicinity of $200 each. If all else fails, off-car cleaning may be required. This can involve reverse flow under pressure to push debris out and allows the technician to observe the flow pattern and look for problems. This can be expensive because of the time, labour and equipment involved.

The best way to minimize or eliminate the need for injector cleaning is to use gas that contains sufficient detergent to prevent varnish buildup. Most brand-name gas has enough detergent to do this and, as a rule, premium grades contain extra cleaning agents that will usually cure this problem after a tank or two.

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

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