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first you need a gauge, then you take off one of the vacuum lines from the little tree, or you can go directly from one of the ports on the manifold (infront underneath) is the easiest direct source. then plug in the gauge and start your engine. as long as its between 16 and 20 and holds steady you are ok.
 

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Your vacuum reading can be anyplace it depends on TB size and camshaft. What you will be looking for is a steady reading if your using a stock cam. What are you looking for to be using a vacuum gauge. If you wondering about the health of a engine you want a cylinder pressure gauge.
 

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Discussion Starter #4
Car is stock except for h-pipe/no cats and two chamber flowmasters.

-Engine has been idling bad. Idle goes 850-1000 then 400-500(about to die)then catches itself, levels out and repeats.

-Checked IAC and the plunger moves freely. Plan on checking TPS.

-Finally, want to check vacuum to see if there is a problem. Wiped dirt and buildup off driverside valve cover(the back) then checked later and noticed fresh oil that looks to be from the valve cover. Thus, was told to perform vacuum test.

Thanks for replying and answering question. Any suggestions on the bad idle would be greatly appreciated.
 

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MAP sensor going bad will cause the idle up and down problem also. Check for a vacuum leak from the vacuum tree to all the sources.
 
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