Intake & inlet valve deposits
Indirect port-injection engines benefit from the air/petrol mixture being sprayed on the reverse side of the inlet valves. The fuel mix, therefore, keeps the area clean of deposits that originate mainly from the positive crankcase breather valve (PCV), which diverts oil vapour into the inlet to be burned, instead of released into the atmosphere. As GDI exhaust gas contains a relatively high proportion of soot, the particulates attach themselves to the viscosity improver element of the oil and become baked onto the surface of the cylinderhead intake. Once this happens the build rate of the deposits accelerates and reduces airflow.
Swirl flap movement within the inlet manifold can also be restricted, affecting the ECU’S ability to provide correct air/ fuel mixing and optimum combustion, when working in either homogenous (conventional) and stratified (lean burn) modes. Abrasive carbon can break loose, enter the combustion chamber and score the cylinder walls.
To an extent, sophisticated engine management can compensate for partial blockages. However, because deposit build-up is gradual, most drivers do not become aware of the situation until the engine goes into ‘limp-home’ mode and/or an engine management lamp is illuminated, when it reaches the limits of its adjustment tolerances. After the deposits have been removed, the engine management emissions data must be reset, or else the engine will not run properly until the system adjusts itself, which can take weeks. Some car manufacturers have issued instructions to dealerships on how to deal with the problem. More recently, certain engines have been introduced with both GDI and port-injection to try to get around the problem, although they are rare in the UK.
For the DIYER, solving the problem can be very difficult. Removing the inlet manifold and cleaning the deposits manually is the most common cure. Crushed walnut shells being fired into the intake with the inlet valve closed is the most popular method, but this task requires special tools and is best left to a professional. In terms of prevention, it is difficult to introduce cleaning solutions that are carried with the airflow, but some companies are marketing GDI intake cleaners to be used at every service interval; we hope to investigate these products in a future issue.
The best advice is to keep the air filter clean and use good-quality branded fuel, preferably premium high-octane blends with better clean/burn characteristics, to reduce particulate emissions.