The process of walnut blasting, which entails cleaning the intake manifold and valves of a car’s engine with a high pressure air blast of finely crushed walnut shells (a biodegradable abrasive), is meant to help clear out carbon buildup on older gasoline direct injection (GDI) engines, thus helping the car run better.

GDI engines are designed to improve fuel economy by injecting fuel directly into the combustion chamber. But this configuration can cause the intake valves to get dirtier with built-up carbon deposits formed when trace amounts of engine oil seep past valve seals as a part of normal engine operation and then bake onto the valves.

Over time, these deposits can cause hesitation, poor drivability, or a check-engine light, says John Ibbotson, CR’s chief mechanic.

"The tough thing is that there is no long-term fix," says Chuck Lynch, the director of technical services at the Automotive Engine Rebuilders Association. "You can't just change an intake manifold to one with port injectors without changing the ECM [engine control module]."


That's what many manufacturers ended up doing. In addition to injecting fuel directly into the engine's combustion chamber, they added another injector in the intake manifold that would operate intermittently to keep intake valves clean, Lynch says.

Owners of the older GDI vehicles are stuck with the problem, though, and will likely need to have their engines cleaned periodically. The walnut method of intake valve cleaning may be best known to owners of BMWs. That manufacturer offered it as a solution for certain models with GDI engines between 2006 and 2016, though GDI engines from other brands made around the same time period, such as Audi and Volkswagen, may benefit as well. The process can be pricey because the intake manifold must be removed. One shop we spoke to said it would cost about $1,000.

Ask your shop whether a chemical intake cleaning, which can cost less, could work instead.

"Not many people are going to do walnut hulls if there's a chemical solution available," says Lynch, who notes that cleaning chemicals can be sprayed into the intake manifold without removing it. 

Editor’s Note: This article expands on the version that appeared in the September 2020 issue of Consumer Reports magazine.