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 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
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
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