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The Serious Freshening

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Engine—Manifold Vacuum Test

Lynn in Bugsby, September 5, 2005

Completed: 2006-08-27 — Started: 2006-08-27

INTAKE MANIFOLD VACUUM: Bugsby's intake manifold vacuum is about 19 inHg during a normal idle (left). Periodically the Steward vacuum fuel pump operates and the idle vacuum drops to about 10 inHg (right).

Work Completed:
Lynn installs a "tee" in the vacuum line between the intake manifold and the Stewart vacuum fuel pump to attach a "wolf whistle." Unfortunately the small diameter line does not move enough air to operate the whistle, but it provides an ideal point to measure the intake manifold vacuum. Lynn observes that Bugsby is pulling about 19 inHg (inches of mercury) during a normal idle.

Lynn notes that periodically (about every 60-90 seconds) the vacuum abruptly drops to about 10 inHg briefly (about 5-10 seconds) then returns to the nominal level. He believes that this signals that the Stewart vacuum pump is actively pumping, using the manifold vacuum to draw fuel from the gas tank into the fuel pump. At the same time the idle noticeably roughens. Lynn expects that this is just the normal operating behavior of the fuel pump.

Work Remaining:

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Last updated: May 24, 2009