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Vacuum Gauge - What is it telling me? 200

posted by  Volvodad on  Mon Jan 26 06:16 CST 2004
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last visit: Thu Oct 23 16:18 CST 2014
[RELATED]

Four cylinder engines are more prone to this than 6's 8's etc. The intake pulses from the cylinders are less frequent and can be distinct enough to make the gauge unsteady. My MGB does this even when it's running fine. An experienced MG technician told me they all do it because of the very small volume of the inlet manifold, then he merely pinched the vac. line enough to smooth out the reading.

But all three of our Volvos (all 4-cyl) have very steady vac readings at idle. As TurboGreasel mentioned, you may have the gauge tapped in close to an individual intake runner so it is being affected by one cylinder's pulses. Try teeing it in to the vac line that goes to the ignition control module. That one comes from the "plenum" portion of the manifold and should give a smooth reading. BTW - both of our LH-equipped engines show a steady 19 inches of vacuum at hot idle. The K-jet car is considerably less, which a Volvo "expert" told me is typical.
--
Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)






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