n75 valve question

i am haveing an overboost condition on my 1998 jetta tdi it spikes my boost gauge which reads up to 20lbs. and holds it there past 2700 rpm if im on the throttle . i have a mity vac and i tested the vnt actuator and it seems to move freely. at 20lbs it is fully open. ive heard of n75 valves sticking and causing low boost conditions. my question is can they stick the other way and cause a full boost condition all the time? thanks in advance for any input on this..

Wierd, you don't even have a vnt mechanism on the 98 jetta, you should have a wastegate turbo and the chances are the wastegate is stuck shut. You need to move the actuator with pressure instead of vacuum on your car, the wastegate works in a different way, if you are able to suck vacuum through it the actuator is probably knackered and needs replaced.

Seatman said:

Wierd, you don't even have a vnt mechanism on the 98 jetta, you should have a wastegate turbo and the chances are the wastegate is stuck shut. You need to move the actuator with pressure instead of vacuum on your car, the wastegate works in a different way, if you are able to suck vacuum through it the actuator is probably knackered and needs replaced.

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sorry for long absence on this post. but yep i tested with pressure. the mity vac i have does pressure and vacumn with the flip of a switch. the nt75 valve was the culprit. makes sence to me now. they are a 2 way valve. mine just happened to get stuck overboosting.... rather then underboost which is usually the norm for these valves when they fail. so it ya didnt know.. now you do thanks.

andrew k said:

sorry for long absence on this post. but yep i tested with pressure. the mity vac i have does pressure and vacumn with the flip of a switch. the nt75 valve was the culprit. makes sence to me now. they are a 2 way valve. mine just happened to get stuck overboosting.... rather then underboost which is usually the norm for these valves when they fail. so it ya didnt know.. now you do thanks.

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I was under the impression that they usually gunged up when not activated - which would leave the wastegate closed (full boost). The VNT's, however, are different.