About two weeks ago I did the timing belt on my '96 a4. When I had everything apart I noticed that the boost line running to the DV was so brittle that it had cracked off right before the DV. I replaced the boost line and installed a boost gauge while I was at it. The gauge is peaking at 5psi and holding 4-5psi. So, I'm pretty sure the car is in limp mode. There is however, no CEL. It holds -10 vac at idle. I've read that it should be closer to -18. The rpm also bounces sometimes and when it does the boost gauge reads zero for a second before the rpm recovers.

Would having no pressure going to the DV or having a large boost leak put the car into a soft-limp mode with no CEL? I don't know if it has any codes. I'm new to the world of VolkswagenAudiGroup and most of my experience is with Mopar and Subaru so I don't have any VAG readers and no current access to an ecu reader.

I don't believe that there are any boost leaks. Maybe the braided tube that runs through the firewall between the heater core tubes. I didn't want to try and replace it as I heard that its extremely hard to reattach the tube to the recirculation fitting.

So im thinking bad n75. Any ideas. I should probably get a VAG-COM right?

 

Just to be clear, you meant to type 97 as listed under your name not 96 right? There were no 96 1.8Ts.

5psi is definitely limp mode, 10 inches of mercury at idle points to a fairly large vacuum leak.

A DV that is not functioning can absolutely throw you into limp mode with no codes, I've been there (due to my own stupidity some years ago.)

Before you start throwing money at the car you should definitely get a $10 VCDS cable off ebay and scan for codes. You should also build a boost leak tester to look for leaks.

If you do all that and don't get any good results (which I doubt) I can send you a known to be working N75 to test.

 

Oops. Yeah, it's a '97.

Out of curiosity, what should the idle vac be?

That's for the info. I'll be sure too invest in a VCDS cable.

 

Idle vacuum should be -18 to -22 with the car idling at temperature and climate control turned off (ie, no load on the engine).