1996 Audi A4
125k miles

Getting fault codes p300,301,302,303. Replaced coils and plugs. Same thing. The car isn't running bad, it just idles real hard and the RPMs drop a bunch.

So I started inspecting hoses. I found a nipple which has nothing running to it. It looks like it's what I understand to be the throttle body. From the diagram under the hood I think it is the Intake Manifold Switch Variable. Where would this vacuum run to?






Also, while inspecting I found something that looked like it should have something connected to it. Doesn't resemble any of the other nipples I found but it is right above the drivers side front tire.

 

First thing is to determine if any hoses aren't connected at one end. If all ends are accounted for, then turn on the engine, put your finger or a piece of paper over the nipple end by the carb and see if there's any vacuum present. If there still isn't vacuum when you give it gas, then you're fine and don't need to worry about that nipple. As another test, you can plug it temporarily and see if your performance gets better.

As for the one by the tire, likely it's either a grease gun zerk fitting or an inlet for the air bag suspension system. Optionally, a brake system bleed valve. Hard to tell by your photo and not seeing your car and its options.

 

No suction or pressure. Could it be a bad vacuum solenoid?

I am checking all vacuums and it seems as if a lot of them aren't responding, even with the engine revved up.

The EGR has a red tube running to the vacuum solenoid, no suction or pressure. The throttle body has a blue tube with no suction and pressure. It seems the only suction I am getting is a feeder tube from the top of the vacuum solenoid that feeds right back into the lower part of the solenoid. WTF?

 

Could be a connection point for a sliding rod, doubt it's vacuum or solenoid.

Some will respond, many won't. On carbeurated vehicles, more suction toward the bottom of the carb

If you really want to chase it, get a Motor or Haynes manual and go for it.