2.8L six-cylinder engine
High engine torque even at low revs is provided by a long intake manifold with small diameter pipes. To ensure high engine power at high revs, on the contrary, a short intake manifold with large diameter pipes is required.
To combine these two approaches, AUDI has developed an intake manifold with a variable configuration of the internal cavity. In the pipes of this intake manifold there are six flaps that close and open depending on the engine load. Thus, each cylinder has an intake pipe of different length and diameter, which can be designated as an intake pipe for maintaining high torque and an intake pipe for developing the engine's power characteristics. The flaps in the intake manifold pipes are actuated by low-pressure air depending on the engine speed. Up to 4,000 rpm, they are in a closed position, providing the engine with high torque. At engine speeds exceeding 4,000 rpm, the flaps are fully open.
If the engine's effective power drops, it is necessary to check the operation of the flaps in the intake manifold pipes.
1. First check the ease of movement of the damper rod.
2. Check the functionality of the flaps. This check is performed with the engine warmed up.
3. Start the engine and increase its speed to over 4000 rpm.
4. Watch the diaphragm valve of the intake manifold flaps. The throttle linkage should move back. If this does not happen, check the condition of the low-pressure air hoses and make sure that they are tight. If so, then:
5. Check the diaphragm valve. To do this, attach a spare low-pressure air hose to the valve connection pipe and suck air from the hose with your mouth. If the intake manifold flaps drive rod does not move or there is always air in the hose despite attempts to pump it out with your mouth, then the diaphragm valve is faulty and must be replaced. If the diaphragm valve is normal, then check the sequential intake manifold sequential valve. This valve is located next to the air mass flow meter at the rear of the engine on the right side. This valve determines the need to supply low air pressure to the diaphragm valve (see illustration).
6. Disconnect the two low pressure air hoses from the intake manifold sequential valve (see illustration). Attach a spare low pressure air hose to the valve outlet that leads to the intake manifold.
7. Draw air from this hose with your mouth. No air should pass through the hose.
8. Start the engine and increase the speed to 4000 rpm or higher.
9. Suck air from the hose with your mouth. Air should now pass through the hose. If the defect cannot be identified, which is unlikely, then contact the workshop.
Removal
10. Remove the protective cover over the engine.
11. Remove the spark plug caps and release the high tension wires from the holders.
12. Disconnect the fuel distribution line together with the injectors.
13. Disconnect the air intake hose.
14. Remove the idle speed control valve or disconnect its connector.
15. Disconnect all low-pressure hoses from the intake manifold. Also remove the hose from the throttle body and the crankcase ventilation hose.
16. Disconnect the gas cable and the throttle position potentiometer plug.
17. Unscrew the intake manifold mounting bolts. The sequence of unscrewing the bolts is the opposite of their tightening (see illustration).
18. Remove the intake manifold together with the throttle body pipe.
Installation
19. Replace the old gaskets with new ones.
20. Tighten the intake manifold mounting bolts in the sequence shown in Illustration 8.17. Tighten the bolts in four passes to the following torques:
1st stage: 5 Nm;
2nd stage: 10 Nm;
stage 3: 20 Nm;
stage 4: 20 Nm.
Attention! Dismantling and installation of the intake manifold of a six-cylinder engine with a volume of 2.6 liters are performed identically. The moments and tightening order of the manifold mounting bolts are the same as for a 2.8 liter engine.