Air Management For The FSI V6 And V8 Engines

Jun 18, 2023

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

The creators of these direct-fuel injection engines started with a clean sheet of paper. The V6 and V8 lost the five-valve arrangement to make room for a fuel injector. Moving the injector to the combustion chamber allows for better control of the air/fuel ratio and fuel droplet size in the cylinder. But, Audi took it one step further by completely redesigning how air enters the engine and how crankcase vapors are regulated.

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

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

The baffles in the center of the plenum change the length of the runners to improve torque and responsiveness. Audi calls these "change-over," "intake manifold tuning" or "intake runner control" valves in the service information and DTC descriptions. The baffles direct air into longer runners for low-end torque and transitions to shorter runners as engine speed increases.

A vacuum actuator and solenoid control the baffles. 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 typically sets the system in the closed configuration, or the position with the longest intake runner. This will cause the engine to have less power as rpm increases. RS or high-performance Audi V6 and V8 engines do not have this system installed.

On Audi FSI engines, there are "tumble flaps" before the intake valves that are actuated by a vacuum pot and electric solenoid valve. These are small flaps that can close down and restrict flow to the intake valves. This restriction creates a "Venturi effect" to smooth the airflow draw into the engine at low engine speeds and loads. When the valves are open, the full diameter of the intake runner is used.

The tumble valves are typically closed during low loads between 1,000 and 4,000 rpm. This helps to improve idle quality and acceleration at low engine speeds. When the engine loads increase, the valves open.

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

A Hall-effect sensor on the end of the shafts on both banks measures the position of the valves. If the valves are slow to react or go past set points for open or closed, a code will be set. The shaft can wear and develop vacuum leaks at the ends.

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

Audi takes crankcase vapors very seriously on FSI V6 and V8 engines. 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, the pressure in the crankcase increases and decreases with a change in engine load and speed. Gases from the combustion chamber can leak past the piston rings as the cylinder goes up during the compression and exhaust cycles. Oil and combustion gases in the crankcase can also be sucked past the piston rings and into the combustion chamber. FSI engines manage the pressure, so carbon deposits do not form on the valves and the oil stays clean.

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

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

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

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

The separated oil is collected in an oil reservoir beneath the cyclones. The oil cannot drain out of the reservoir until the oil drain valve is opened. The oil drain valve is closed as long as the pressure in the crankcase is below the valves. The valve opens automatically due to gravity only at very low engine rpm, or when the engine off, because the pressure conditions above and below the valve are in equilibrium.

If the oil separator is not functioning due to clogging, leaks or stuck valves, the life of the engine will be impacted two ways. First, the oil could become contaminated and potentially cause sludging and bearing problems. Second, increased oil consumption can cause carbon deposits on the intake valves and damage the catalytic converter.

The FSI V6 and V8 engines are in many of Audi's A-series cars and Q-series SUVs. For any FSI engine to work properly, all of the components must be functioning 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 an issue with the intake manifold, the problem will be magnified by the intake valves and combustion event in the cylinder. This is why a holistic approach must be taken when resolving carbon deposits, lean codes and other driveability problems on any FSI engine.