The need for the DV stems from the fact that the turbocharger, driven by exhaust gases, keeps building boost even after the engine's throttle is closed. However, with the throttle plate closed, the pressurized air has nowhere to go. The boost builds up in the intake tract between the turbo and the throttle plate; without an outlet, the pressure can bounce back into the turbo's compressor wheel and stall it, and potentially damage the turbochargers.
The DV vents air out of a portion of this intake tract. You have no doubt noticed a "Pssh" sound coming out from under the hood of a turbocharged car at some point--this is the sound of the valve doing its work. It's not unusual for people who have modified their cars to set up their valve to vent the air to atmosphere to make this distinctive sound louder, but this is a bad idea for two reasons. First, in most cars this air has already been "counted" by the engine's computer, so it will inject more fuel to account for this air, and when the air does not appear, the engine will receive too much fuel. Secondly, if the valve is allowed to recirculate the air back into the intake tract ahead of the turbo, as it is designed to do, it helps keep the compressor wheel spinning. By keeping the wheel spinning, when you crack the throttle again the wheel has less catching up to do, and turbo lag (the delay between throttle application and the arrival of boost to the engine) is reduced.
The diverter valves are connected to the engine's intake tract by two hoses, one just before the turbocharger's compressor wheel and one just after. It also receives a vacuum signal from a line that connects to the intake manifold, after the throttle body.
The DVs that come stock in the C5 A6 work fine for a stock setup, although it is made of plastic and can fail over time. An indication of this is if your engine won't hold boost like it used to. Without a boost gauge to monitor your engine it's hard to quantify this, but the loss of power should be noticeable.
The valves are located at the front of the engine compartment, next to the intake manifold. Replacement is easy and should take no more than an hour.
As noted, the valve has three connections--two larger hoses and one small one. Installation of your new DV is essentially a matter of disconnecting these hoses from the old DV and installing them on the new one. The DV has no mounting bracket as it is a lightweight component. In effect, the short hose between the DV and the lower intercooler hose serves as its support.
Keep in mind that when your car was serviced before, parts may have been replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have, so be prepared with different size sockets and wrenches.
Vehicle models change and evolve as they grow older, so the vehicle shown in our illustrations may vary slightly from yours. If something seems different, let us know and share your info to help other users. Questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.
The diverter valves are located at the front of the engine next to the intake manifold as shown here (green arrows). You'll need to remove the engine cover in order to access them. See our article on Engine Cover Removal for more information.
Both diverter valves are removed by loosening the hose clamps on the larger hoses (green arrows) and pulling them off. The last step is to pull the smaller vacuum signal hose (purple arrow) off the valve. The new valves fit in place of the old ones. Take the time to inspect the hoses leading to and from each valve. If they are worn or cracked, they must be replaced.