Walnut shell blasting refers to the act of cleaning the intake valves and tracts of a vehicle's engine with fine walnut shell particles and a blasting tool.
This type of cleaning for vehicles became popular after engines began evolving from conventional fuel injection to direct fuel injection. Cleaning of the intake valves and tracts wasn't needed with conventional injection because the fuel acted as a natural cleaner. The fuel was injected into the intake before it reached the engine's cylinders and, as it traveled to the cylinders for combustion, kept the intake's lining clean.
Now, with direct injection, fuel is injected directly into the cylinders and never comes in contact with the tracts and valves of the intake. Because of this modern injection style and lack of clean fuel in the intake, blow-by gasses that travel through the intake create deposits. Although blow-by gases consist primarily of air, small amounts of oil and exhaust particles do exist and contribute to dirty intake tracts and valves. This buildup is referred to as carbonization.
Walnut shell blasting is a safe way to clean the lining of the intake because walnut shells, when broken down into fine granules and blasted at high speeds, effectively remove deposits and maintain the integrity of the intake's lining. The high speed blasting doesn't create unwanted grooves in the intake even though shell granules make contact with a backed pressure of around 90 psi. Unlike sand and other abrasives, walnut shell granules don't wear metal - they only remove loose deposits that aren't part of the intakes original design. In essence, walnut shell blasting only removes what is necessary, which is why it has become the most viable solution for cleaning intakes.
When carbonization occurs in the valves and tracts of the intake, engine efficiency and performance is affected. Although a small amount of carbonization in the intake isn't severe, over time, as the carbonization effect accelerates, the valves will close improperly. Symptoms of this include rough idle, delayed throttle response and an increase in oil consumption.
The consensus of many vehicle maintenance and repair experts is that valves of engines with direct fuel injection should be cleaned using walnut shell blasting every 40,000 to 50,000 miles. This special cleaning may be done through an automotive shop or at home with the proper tools and knowledge. Mechanical skills are necessary because, in order to access the intake, you have to remove the engine cover, intake manifold and disconnect various tubes. You also have to carefully cover exposed areas of the engine so that the tract you are cleaning is isolated. If shell particles enter open intake valves and reach the combustion chambers, serious problems may occur.
There are three other forms of intake cleaning besides walnut shell blasting. The first involves removing the cylinder head completely and soaking the intake valve in a special cleaning solution for about two days. The second method doesn't require you to take off the cylinder head, but requires manual cleaning with an aggressive solution and brush. Considering the amount of brushing, this way is the most labor intensive. You also have to make sure not to get solution in the open valves you are not cleaning. The third method consists of adding a solution directly into the charge pipe, turning the engine on and letting the solution mix with the intake air. This method is popular, but ineffective if you don't do it every 3,000 miles or so.
After a proper walnut shell blasting, your throttle response will improve and your idle will become smoother. Engine power output and efficiency will also benefit from the cleaning.