Air management

Air management for FSI V6 and V8 engines

In 2006, Audi introduced new 3.2L V6 and 4.2L V8 engines. Both use Audi’s Fuel Stratified Injection (FSI) system which was first introduced on the 2.0 FSI four-cylinder engine in 2004.

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The creators of these direct injection engines started from a blank sheet of paper. The V6 and V8 lost the five-valve layout to make way for a fuel injector. Moving the injector towards the combustion chamber allows better control of the air/fuel ratio and the size of the fuel droplets in the cylinder. But Audi has gone one step further by completely rethinking how air enters the engine and how crankcase fumes are regulated.

The 3.2L V6 and 4.2L FSI V8 have a lot in common with the 2.0L FSI four-cylinder engine, including carbon deposit issues. Chances are your first exposure to an Audi intake manifold won’t be a driving complaint, but rather a carbon cleaning service where the intake valves need to be cleaned.

Air management

The intake manifold of V6 and V8 FSI engines manages airflow past the throttle body with two sets of valves in the plenum and at the intake ports. If the valves are not working properly, the engine will not produce the same power and the codes will set.

Baffles in the center of the plenum change the length of the sliders to improve torque and responsiveness. Audi refers to these valves as “change-over”, “intake manifold adjuster” or “intake runner control” in service information and DTC descriptions. Deflectors direct air to longer runners for low-end torque and transitions to shorter runners as engine speed increases.

A vacuum actuator and solenoid control the deflectors. A sensor monitors the position of the valves. Audi has used this type of manifold on its V6 and V8 engines since the 1990s. When the system fails and no vacuum is present, the default position usually sets the system in the closed configuration, or the position with the longest intake channel. This will cause the engine power to decrease as the rpm increases. RS or Audi V6 and V8 high performance engines do not have this system.

Tumbling flaps

On Audi FSI engines there are “tumble flaps” before the intake valves which are actuated by a vacuum pot and an electric solenoid valve. These are small flaps that can close and restrict flow to the intake valves. This restriction creates a “Venturi effect” to smooth the suction of airflow into the engine at low rpm and light load. When the valves are open, the entire diameter of the intake runner is used.

Tumbling valves are typically closed at light loads between 1000 and 4000 rpm. This helps improve idle quality and low-end acceleration. As engine loads increase, the valves open.

The actuator may fail on some high mileage vehicles. The most common areas of failure are the linkage and the shaft. The plastic links can crack, preventing the valves from opening fully or opening too far.

A Hall effect sensor at the end of the shafts on both banks measures the position of the valves. If the valves are slow to respond or exceed the open or close setpoints, a code will be set. The shaft can wear and develop vacuum leaks at the ends.

The customer may report a loss of power when the rocker valve or intake runner actuators fail. The first diagnostic step should be to check for vacuum leaks in lines and actuators before swapping parts. Typically, leaks will cause lean codes and increase fuel trim numbers in the long run. A smoke machine can help locate leaky ports, pipes, and cracked plastic parts.

CrankCase Vapor Management

Audi takes crankcase fumes on FSI V6 and V8 engines very seriously. The system uses many valves and chambers to manage the vapors so that most of the oil can be removed and the remaining gases can be burned in the combustion chamber.

In a naturally aspirated FSI engine, crankcase pressure rises and falls with a change in engine load and speed. Combustion chamber gases can leak past the piston rings as the cylinder rises during the compression and exhaust cycles. Oil and combustion gases in the crankcase can also be drawn through the piston rings and into the combustion chamber. FSI engines manage pressure, so carbon deposits don’t form on the valves and the oil stays clean.

On Audi V6 and V8 engines, the positive crankcase ventilation (PCV) valve is more like an oil separator system that can cost upwards of $300. The oil separator is located in the engine valley on the V6 and near the rear of the intake on the V8.

These units are equipped with a control piston, a bypass valve, a two-stage pressure limiting valve and a drain valve. The valves work together to ensure that conditions are optimal for removing oil and controlling pressures inside the combustion chamber. The oil separator is warmed by engine coolant which prevents the crankcase breather from freezing due to condensation and potentially cracking the plastic.

Inside the oil separator, crankcase gases pass through two or more cone-shaped chambers called cyclones. The chambers are connected in parallel. The cyclone swirls the steam to remove the oil from the gases.

A spring-loaded piston that changes position depending on the flow controls the system. A two-stage pressure control valve adjusts the internal crankcase pressure. The bypass valve, together with the control piston, ensures that the cyclones operate at optimum levels. If there is too much or too little flow, the cyclones will not work properly. If there is too much airflow, the bypass valve will open and allow untreated gas to flow to the engine.

The separated oil is collected in an oil tank under the cyclones. Oil cannot drain from the tank until the oil drain valve is open. The oil drain valve is closed as long as the pressure in the crankcase is lower than the valves. The valve opens automatically by gravity only at very low engine speeds, or when the engine is off, because the pressure conditions above and below the valve are in balance.

If the oil separator fails due to clogging, leaks, or stuck valves, engine life will be affected in two ways. First, the oil could be contaminated and potentially cause sludge and bearing issues. Second, increased oil consumption can cause carbon deposits on the intake valves and damage the catalytic converter.

FSI V6 and V8 engines power many of Audi’s A-series cars and Q-series SUVs. For an FSI engine to work properly, all components must work properly. Look at it this way – the intake manifold is the lungs, while the injectors, intake valves and cylinders are the heart. If there is a problem with the intake manifold, the problem will be amplified by the intake valves and the combustion event in the cylinder. This is why a holistic approach must be taken when troubleshooting carbon deposits, lean codes and other drivability issues on any FSI engine.