Hello all, I am not new to the car game but this is the first audi i have owned. It is a b5 a4 with a gt28 elimintaor kit. i own an auto repair shop and a friend of mine asked me if i was interested in buying his car in a non-running state. for the price that it was offered for i could not resist but to buy it. so after a week of overlooking the vehicle and getting back up and running i realized that the rpms are choppy inbetween 2000 and 3000 rpms ONLY in 1st and second gear. after doing some research on the forums i found that others had the same problem which lead them to a faulty n75. in my case the n75 valve is connected electrically but all 3 vacuum lines that are supposed to go to it are not there. I practically have an n75 valve just hanging on a wire in the engine bay.

can someone please tell me where these lines connect to and or if they are needed. also has anyone had problems with their n75 valves causing a similar problem to mine. thank you.

 

Ones to your inlet pipe, bottom ones to your waste gate actuator, ones on your charge pipe before your intercooler

 

now is there any other reason other than to avoid CEL's that these lines are not connected?

 

It's possible that your friend was using a boost controller.

The N75 is an electric solenoid that goes between the boost source on the compressor housing and the waste gate. It's operated by the ECU and controls when the waste gate opens, thereby controlling boost. Some people choose to replace this with a manual or electronic boost controller. In that case, the N75 needs to stay plugged into the wiring harness in order to avoid check engine lights. There's usually a piece of vac line looped around it though. If your car has a proper tune, there's no reason why you couldn't re-connect the N75. The longer port on the N75 goes into the turbo inlet, and the two shorter ones go in between the waste gate and the compressor housing. This drawing isn't specific to our cars and is also obviously not drawn to scale, but it should help you understand how the plumbing works and what the N75 does.