Why do I have more boost when I disconnect the vacuum hose that goes from the DV to the intake manifold. (See the picture)

I have no boost gauge so I have to go approximate, but i'd say when it's disconnected I have 90-100% of boost. With my DV connected like 60% boost and with another DV that I have laying around I get like 40% boost.

Does that mean my DV id dead ? Or the N75 selenoid next to it ?

thanks

 

What kind of Dv are you running?

 

What the hell... Why are you doing this? Do you understand what is happening if you disconnect the dv vac line and run it like that? You are basicaly killing the concept of it's purpose, which is to save your turbo from getting stalled when you let off the gas. The vac line opens the valve inside the dv when you let off the gas so boost isn't backlogged from the throtle plate right back into your turbo. When the valve opens it recirculates the trapped boost between the turbo and throtle plate back into the intake plumbing... If you really want to diagnose your DV then get a vacuum tool to see if it holds steady with vacuum applied. If you are using an aluminum forge style valve- they don't break, and springs hardly ever wear out. Your N75 is probably fine.. I wouldn't pay attention to the effects of what happens when you disconnect that vac line and run it like that, because that is just wrong. Get a boost gauge if you want to know if you are boosting correctly.

 

It's the stock DV..

 

So I think its betetr to reconnect it

 

yes yes yes

 

But could a loss of boost be sign of a faulty DV ?

Then if I replace it, is it worth putting a hi-perf DV or a stock one if I keep the original turbo (but chipped)

 

yup yup yup

 

If you plan on getting a chip just keep any eye out in the classifieds for a used Forge style DV. Even if you don't get chipped it's nice to know that you are replacing a wear part with an invincible part. Stock DV uses a rubber diphragm that eventually tears and leaks - causing boost loss or inconsistent boost. Aluminum Forge style valves use an aluminum piston.

 

Is the picture you posted of your car and what year is your A4?


Could be a bad N249 valve, but only if you own a 2000+ A4.

No way the valve could be bad with the vac line connected but be good with it not connected. Its either good or bad, working or not working.