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Intake Valve Deposits in GDI Engines

Gasoline Direct Injection (GDI) is used on a variety of late model engines and now Subaru. GDI sprays fuel directly into the combustion chamber under high pressure, rather than spraying fuel under low pressure into the intake ports in the cylinder head. GDI increases fuel economy and power 15 to 25 percent, but there is a downside that is now becoming apparent as these engines accumulate miles.

The problem is carbon deposits are building up on the inlet side (top) of the intake valves. The deposits create turbulence and can restrict airflow into the cylinders causing performance and drivability problems, such as: hesitation, stumbling, misfiring, even hard starting. The thicker the carbon deposit buildup on the valves, the worse the drivability problems.

GDI sprays fuel directly into the combustion chamber so the fuel completely bypasses the intake valves. Consequently, detergents and cleaners that are added to gasoline to prevent intake valve deposits from forming in port fuel injection engines never have a chance to do their job in a GDI engine. The inlet sides of the intake valves are never in direct contact with the fuel so the detergents cannot wash away the deposits. Because of this, fuel detergent additives that are either in gasoline from the refinery or are added to the fuel tank have almost no effect on preventing or removing intake valve deposits in GDI engines. The additives work in regular fuel injected engines, but not GDI engines.

What Causes Intake Valve Deposits

Intake valve deposits form as a result of oil slowly seeping past the intake valve guide seals and down the valve guides. This is the big problem with most current GDI engines. Due to modern unburned hydrocarbon regulations, vapors from the crankcase are usually vented into the intake stream in order to prevent oil droplets from escaping through the exhaust. In a port injection engine, these droplets are ‘washed off’ the neck of the intake valve by a relatively constant stream of gasoline droplets. In a GDI engine, the gasoline doesn’t touch intake side of the valve. As a result, the droplets have a tendency to bake onto the valve and significantly reduce performance. To add to this effect, many advanced GDI engines also include exhaust gas recirculation in order to lean out the combustion mixture and reduce in-cylinder temperatures, reducing NOx emissions. Since GDI combustion has the ability to produce far more soot than port injection combustion, the problem is magnified.

A tiny amount of oil is necessary to lubricate the guides, but when oil reaches the hot surface of the valve, it can stick and burn forming heavy black carbon deposits that gradually build up over time. The higher the mileage on the engine and the greater the wear in the valve guides and seals, the faster the accumulation of black carbon deposits on the intake valves. Low viscosity motor oils (such as 5W-20 and 0W-20) may make the problem worse because they are thinner, to reduce friction and flow more easily down the valve guides. Conventional motor oils also have a lower flash point than synthetic oils, which can also increase the formation of deposits over time.

Another contributing factor to the formation of intake valve deposits is unburned fuel vapors and oil vapors being siphoned back into the intake manifold through the PCV system. This is done to control crankcase emissions and to remove moisture from the oil. The fuel vapors, which are carbon particles and oil droplets, that the PCV system routes back into the intake manifold are reburned in the engine to reduce pollution. But these vapors can also form carbon and varnish deposits on the intake valves.

The more blowby an engine has due to cylinder and piston ring wear, the greater the volume of crankcase vapors that are pulled back into the engine by the PCV system. High mileage engines typically have more blowby than low mileage engines, so the build up of intake valve deposits is usually faster.
Even more alarming is that these deposits can dislodge and damage other downstream components, such as:  turbochargers, catalytic converters, etc.. Manufacturers have added systems to capture these oil droplets and particulates, but no system is 100% effective.

Diagnosing Intake Valve Deposits:

An engine that is experiencing drivability and performance problems as a result of intake valve deposits may or may not turn on the Check Engine light. If the engine is misfiring bad enough, it may set a P0300 random misfire code or individual cylinder misfire codes. However, many other factors can also set misfire codes, so a misfire code alone is not necessarily an indication the engine has dirty intake valves.

You cannot see intake valve deposits directly because the valves are inside the cylinder head. The only way to see deposits on the intake valves is to remove the intake manifold and peer into the intake ports in the cylinder head - unless you have a fancy tool that can be inserted into the combustion chamber through the spark plug hole to inspect the valves.

How to reduce carbon deposits on GDI Intake Valves

How fast the intake valves get dirty does not seem to be a function of fuel quality or how much ethanol alcohol is in the gasoline. Rather, it appears to be influenced most by how often the engine oil is changed. Oil and combustion vapors that are drawn back into the intake manifold through the PCV system seem to contribute most to carbon deposits on the intake valves.

The best advise is to change your oil often, if you only do short trip stop-and-go city driving, or change your oil about every 5000 miles if you do mostly highway driving. If you want to minimize carbon buildup on the intake valves, don't push your oil change intervals too high or longer unless you are using a high quality full synthetic oil.

Changing your oil regularly will help minimize the carbon buildup on the valves, but eventually they may still get dirty. If that happens, it may be necessary to clean the valves every 25,000 to 30,000 miles with an aerosol cleaner that is sprayed into the intake manifold.

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