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Unread 21st February 2018, 06:54 PM
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gregjet gregjet is offline
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Join Date: Aug 2015
Location: Bundaberg, Qld. Australia
Year: 2014
Model: xv
Transmission: manual
Posts: 171
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"(an aside) - the intake runners are all part of the intake manifold - connected at the plenum. All cyls are pulling vac on that line."
INCORRECT
The brake line intake is a considerable distance down the intake runner near the head. In the Common plenum the average vacuum will be pretty mauch the same. But the nature of how the lower pressure ( not actually a vacuum) is produced by negative wave pulses traveling back from the valve face means the pressure in one runner will be a function of the valve opening of that cylinder. The pulses occur fast enough for the pressure in the runner to be different from not only the other runners but also the plenum ( 330kph approx).
If the valve is stuck open enough it will be sending both positive AND negative pressure waves along the runner. That will raise the overall pressure in the faulty runner and reduce the pressure differential in the plenum. That will inturn reduce the air velocity through the throttle body and cause furthur engine problems in the other cyl.
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