As a vehicle approaches higher
mileage, you can generally expect intake valve deposits (IVDs) and
injector deposits. Their onset can vary widely depending on driving
conditions. Engine operating temperature, intake manifold speed
(RPM) and hot soak cycles are some of the critical operating
parameters affecting these deposits. To lubricate an intake valve,
tiny amounts of oil have to run down the valve stem. Over time, this
oil is deposited and heated on the intake side of the valve, forming
a carbon cone. This has multiple effects on the intake event. The
carbon changes the aerodynamics of the intake event - namely causing
higher gas speed and a change in the direction of intake swirl,
which affects the combustion process. The IVDs also act like a
sponge, creating a delay in fuel control. This delay not only
creates a temporary enleanment on acceleration but also causes a
temporary enrichment on deceleration. When the throttle is closed,
the intake manifold vacuum goes high, pulling the fuel out of the
carbon sponge. This affects fuel control, potentially affecting
catalytic converter diagnostics under OBDII. Because of IVDs and the
effect of phosphorus (from oil ash) on converters,
"normal" oil consumption will have to be drastically
reduced to achieve ULEV (Ultra Low Emission Vehicle) and SULEV
(Super Low Emissions Vehicle) levels.
