Avant died and reborn again...

Bernie Benz b.benz at charter.net
Sat Mar 6 16:31:16 EST 2004

> From: Phil Rose <pjrose at frontiernet.net>
> At 3:05 PM -0500 3/6/04, Tom Nestor wrote:
>> Classic case of sticking bypass valve. Same thing happened to me several
>> times before I was able to track it down. Basically the valve hangs open
>> after accelerating and comming to a stop. The engine stalls and then would
>> restart a few minutes later, sometimes running rough after the restart. It's
>> definately worth taken a look at.
> It may be a BPV problem, but it certainly isn't a "classic case"
> insofar as Craig described issue. As you indicated--the classic
> symptom involves braking & sudden deceleration immediately before the
> stallout, but that wasn't mentioned in Craig's post. Anyway, the
> failure of the BPV valve isn't because it "hangs open"; rather it
> stays closed (i.e., provides no bypass), and that causes the throttle
> plate to become (too) tightly closed by the boost pressure.
Phil, I'm not sure that I buy your suggested reason for stall after sudden
throttle closure under boost with a failed closed BPV.  The throttle plate
is a pressure balanced valve, i.e. equal plate area on either side of the
throttle shaft center line, thus it's position is unaffected by differing
pressure drops across the valve.  More likely IMO, just that the ECU is not
programmed to control idle in a timely manner, while the ISV is still
subject to boost pressure.  If this throttle body had a throttle air bypass
adjustment aka the 7A, this problem could be adjusted out.


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