i have a 98 a4 1.8t when you drive down the road the boost spikes to 10 psi and then falls to 4psi. there are know codes in the ecu. i runs like its going into limp mode. heres the thing, if i power brake it it will spike 10psi and hold it all day( if i held it that long). the duty cycle while power braking is 100 percent and that means 10 psi. while driving the duty cycle goes to 5 percent and the means 4 psi.... any help would be appreciated. by the way its all stock!

 

Could be a bad wastegate or n75 valve.

 

wastegates are fun to replace id rather shoot myself than do that again

 

It almost sounds like an issue that has been discussed before:

https://www.audiforums.com/m_233990/.../tm.htm#234119

My solution was MBC, almost 3 months and still no more boost loss issues...

D