Skip to main content

A visitor looks at an engine on media day at the Paris Mondial de l'Automobile.


I was told I need to decarbonize my engine. Is it really necessary? – Becky

That's like asking a favourite colour – there are a variety of answers.

There are two factors at play here. Carbon deposits are unburned fuel collecting at various points in an engine. The location and degree of buildup determines the solution. If on the valves, tops of pistons or in the combustion chamber, it may be possible to use an additive – either poured into the carburetor of an older engine or introduced via the fuel in an injected engine.

Story continues below advertisement

There is an emerging situation with the new direct-injection engines that introduce the fuel/air mixture directly into the combustion chamber, bypassing the intake valves. This means that a solution involving a cleaner mixed with the fuel does not work as the fuel does not wash over the intake valves.

In cases of severe buildup, it might be necessary to disassemble the engine and manually clean the affected areas. In milder cases, deposits might be removed via a cleaning process such as that recommended by your dealer. In most of these cases, you can do it yourself by using an additive poured into the fuel tank.

But, if it were my car, the first thing I would do is start using a clean fuel, especially one with a strong cleaning agent in it. I have seen demonstrations of Shell's high-test gasoline and the way it cleans internal engine components and have no hesitation in recommending that as an initial measure.

The things to look for in determining if your engine truly has a problem with carbon buildup include: engine pinging or spark knock under load, hesitation when pressing on the accelerator, a loss of performance or power or stalling in cold weather.

You can remove a spark plug and have a look – see if there are any hard black deposits on the end of the plug. If none of these conditions exist, it likely is not necessary.

Please send your automotive maintenance and repair questions to

Report an error Editorial code of conduct
As of December 20, 2017, we have temporarily removed commenting from our articles. We hope to have this resolved by the end of January 2018. Thank you for your patience. If you are looking to give feedback on our new site, please send it along to If you want to write a letter to the editor, please forward to