Virginia Auto Service

Ask the Car Guy: How Can Carbon Build-up Impact My Engine?

Matt Allen, Owner of Virginia Auto Service, cohost of Bumper to Bumper Radio
Matt Allen, Owner of Virginia Auto Service, cohost of Bumper to Bumper Radio

Dear Car Guy,

I keep hearing about carbon build-up and that it can hurt my car. Can you please explain what carbon build-up is and how it can impact my engine?

Thank you,

Bill Q., Phoenix, AZ

Hi, Bill,

Many car manufacturers are now using direct-injection (DI) technology in their new designs because it increases fuel efficiency (meaning MPG).

This technology, which was initially created to make diesel engines quieter and more fuel efficient, changes the way fuel enters the engine.  This change allows carbon, a black sooty dust, to build-up on the engine’s intake valve and other surfaces inside the engine.

Auto Repair Phoenix - carbon build up

In port-injection engines, the fuel enters the engine before the intake valve.  This means that the fuel is flowing over the valve as part of the engine’s normal operation.  Most gasoline sold today contains additives whose sole job is to help remove carbon from the intake valve and other parts of the engine and these additives do their work as the fuel flows into the intake valve.  But DI bypasses that process, injecting the fuel directly into the cylinder.  This means the additives in the fuel never come in contact with the surfaces most likely to experience carbon build-up.

When carbon builds-up in the car’s engine it can cause a couple different problems.  First, the engine will lose power.  One of the main problems carbon build-up does is to change the ratio of fuel to air in the engine.  This can happen if sensors stop functioning properly because of carbon build-up or if build-up keeps the intake valve from closing properly.  This can make the engine sluggish and cause it to stall.  Carbon build-up can also significantly degrade fuel efficiency, causing the car to get fewer miles to the gallon than it should be getting.

There are some things that will cause carbon to build-up faster that those with cars that are prone to carbon build-up problems should avoid.  Filling the tank with low grade gasoline which is less purified and usually has a higher concentration of contaminants can increase carbon build-up in the engine.  If the ratio of fuel to air is off, as mentioned above, carbon will accumulate more quickly.  Using the car primarily for short trips can also increase problems with carbon build-up as the engine doesn’t run long enough or hard enough to get hot enough to burn any of the carbon off.

The cars that are at the highest risk for carbon build-up problems are those that use DI, although not all cars that these types of fuel injection system have experienced this issue.  Audis, Mini Coopers, and some VW and BMW models are the cars that seem most prone to carbon build-up.  For owners of these cars, the carbon build-up problem means they must invest in additional maintenance on a regular basis to have the carbon cleaned from their engine.

When cars experience issues with carbon build-up, the build-up must be cleaned out of the engine.  Most auto repair shops can perform this kind of service but it can be costly, depending on the type of car and the work required to remove the build-up.  If your car is having any of these problems, have your repair shop check it over and recommend what can be done to solve the immediate issue and over the long term to protect your engine and keep your car on the road.

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