The Problem
and Mini Cooper (along with other vehicles) with Gasoline Direct Injection (GDI) engines are both cars that can experience carbon build up over time causing drivability and performance issues. GDI is a met
hod that automakers have used to inject gasoline at a high pressure directly into an engine’s combustion chamber rather than into the intake tract or cylinder port.  Fuel is better measured than older fuel injection or carbureted systems – making the vehicle more fuel efficient, as well as yielding more power with an engine of identical displacement. The AudiBMW engines and the Mini Cooper N12 and N14 (along with later engines), this means in particular that the fuel is injected directly into the combustion chamber of each cylinder and not in the intake tract. As a consequence thereof, the fuel injectors are no more placed before the intake valve, but directly in the cylinder itself.


The disadvantage of this is that the regular bathing and therefor cleaning of the intake valves by way of the fuel that they are exposed to does not happen any more. The intake valves are only in contact with air or rather blowby gases from the crankcase breathing circuit, in which substantial quantities of fuel and oil can be found. Over time, these lead to deposits of carbonised fuel and oil in the intake tract as well as on the intake valves themselves; this is also referred to as carbon build up, or carbonizing. This is not immediately bad for the engine, but over time can have a detrimental effect on its efficiency. If the valves are heavily carbonised, they may not close properly anymore, and symptoms such as a bumpy idle, vibrations and diminished throttle response can be observed; it may also contribute to increased oil consumption.

The check engine light may come on and the car can hesitate or stall.  If carbon has built up to a point where the vehicle’s performance has been affected, we have had success with a service called a walnut shell blast or “Decarb”.  Walnut shells are gentle enough to clean and sweep away the carbon without damaging the valves.  Once walnut shell blasting has been done, fuel treatments can be added to routine maintenance services to try and keep carbon build up at bay.

In theory there exist two methods to clean the intake valves and intake tracts and valves of the engine. A very elaborate and consequently expensive method is to completely take off the cylinder head and to have the valves lie in a very aggressive cleaning solution for at least 48 hours, after which they have to be cleaned manually as well in order to remove any remaining carbonising.

A modified (and much less labour intensive) variant of this method is to do the cleaning by way of a cleaning product without taking off the cylinder head. However, this has the disadvantage that you can typically only clean one intake tract at the same time, as in order to avoid for the cleaning fluid to enter the combustion chamber the valve has to be tightly shut, and usually only one valve of the six that we have is completely shut at the same time. As depending on the amount of carbonising the valve and intake tract have to be left to simmer in the cleaning solution for 24-48 hours, this can take quite a long time during which you can’t drive the car. In addition, as even the most aggressive cleaning solution is usually not sufficient to remove the carbonising completely, you have to do some manual cleaning with a brush as well. Not much fun!

Method – Safe And Reliable 
The method that we professionally utilize here at Atlantic Motorcar (and the one recommended by most car manufacturers) is the cleaning of the intake tract and valve of each cylinder with a blasting tool. This blasting tool is using pressurised air (usually 6-8 bar) to inject fine walnut shell granules into the intake tract. These granules hit the carbonising at high speed and thus remove it entirely, while at the same time this material is soft enough not to damage the metal of the intake tract and the valves. That is also the reason why no other material should be used for this method. The advantages of this method are obvious: On the one hand, a labour extensive removal of the cylinder head is unnecessary, on the other hand the method is very fast (only a few seconds per intake tract are necessary) and effective.

How Long Does This Take
In total the whole procedure including removal of the manifold took about 3 -4 hours, and for most cars, costs between $400 and $600 dollars. We’ve found that this treatment, done correctly, usually needs not be repeated in the future, or at least for 3-5 years.

Will I Really Notice The Difference?
To make it short: Yes! You feel a difference. The throttle response is noticeably much better (without hesitation), and the idle smooths out as it was when the car was new.  The difference in engine power is usually quite noticeable as well, and the end of that pesky Check Engine Light is a blessing. We can recommend this treatment without any reservation for any car with a mileage of more than 50,000 miles; in my opinion this should be a normal, regular maintenance to be done on a GDI car, Be it Audi, BMW, Mercedes, or Mini Cooper.

Not a simple repair, yes, but one, if done correctly, will last another 50,000 to 100,000 miles. That’s our goal, fix it right the first time, and prevent problems from happening in the first place. 30 years of service experience have well taught us that “an ounce of prevention is worth a pound of cure.” Be cured, once and for all, and give us a call, we’re happy to answer any service questions you might have on your BMW, Audi, Mercedes, Lexus, Volvo or Volkswagen.

Earning your trust, every time you turn the car…that’s what we do…every day…for the last 30 years.
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If you have questions, or if we can be of further assistance, just call us at (207) 882-969, we’d love to meet you, and your car!

Bruce and the AMC Service Team