Q: boost buildup without N75 active?

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Oct 27, 2006 | 08:31 AM

bagodoosh

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Q: boost buildup without N75 active?

trying to diagnose a mild stuttering that happens while keeping the throttle constant, i logged my commute to work one morning. it's only about 5mi and the engine barely warms up. so i try to keep the RPMs under 3000 and no boost (at least according to the boost gauge).

what i noticed was that i'm building boost, yet the N75 is not even active. at some points the boost is up to 4psi and the ECU's not requesting any.

i was thinking at -5 or 0 PSI the BPVs are probably closed, but isn't the boost primarily controlled by the N75/wastegate?

i dont understand:
1) where the boost is coming from
2) why the ECU is not throwing a deviation code
3) why the boost gauge & MAP readings differ

<img src="http://us.i1.yimg.com/us.yimg.com/i/mesg/emoticons7/39.gif">

<img src="http://bagodoosh.home.comcast.net/s4/Boostage.gif">

Oct 27, 2006 | 12:49 PM

PSmalls::AnalRetentiveNumbNut

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Wastegates are shut until you hit ~5 PSI.

Oct 27, 2006 | 03:56 PM

Mihnea

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Joined: Sep 2001

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Gauge and MAP read different because

MAP sensor is before the throttle body, so you could have some boost there but still vacuum in the manifold because of the throttle plate position.

HTH,

mihnea