I installed a vacuum gauge tonight. I tapped it off of a "t" I put in near the intake manifold on the line to the cruise control. When cold and at idle the needle vibrates like nobody's business, fully 50% off the guage face. As the engine warms the vibration lessens in amplitude by about 50%. Yet it still beats out a path across the gauge face. It is almost invisible it moves so quickly. As I give it some gas it evens out. When warm, and under load it seems to hold near steady, no vibration anyway.

Anyone have clue what I'm supposed to read or understand from this activity?

I found this http://www.centuryperformance.com/vacuum.asp but it seems to be a domestic engine oriented place. Maybe the bricks are different.

Thanks in advance. Get it? ;)
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1985 244GL - St. Joseph, Missouri

As others have said.

-Get a better vacuum source near the plenum so all 4 cyls. can contribute equally.

-Pinch the hose until the needle settle down.

-Get a long (12) length of hose for your gauge.

DEWFPO
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1998 S90 066,800 and 1995 964 150,200

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.
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)

I would pull the gauge and snip a piece of vacuum line and go under the hood. I teed into the small line that comes right off of the top of the intake manifold that controls the vent selection.

If you unplug that hose and slip your tester hose and gauge you should get a consistent reading. If not you should consider that you have a bad gauge. I have about 16-18 at idle. I range 7-25 while driving.
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1990 240- 245K- Original Owner-M-47, VX cam, IPD sprt springs, bilstiens, spt exh, euro lamps-turbo whls,- still slow but handles well- Near Houston

Well you measure the air pressure between the throttle valve and inlet valves (the inlet manifold pressure).

At idle or under low load the engine is sucking for air, while the throttle is near closed, a low pressure (vacuum) is the result. When the engine is under load it gets plenty of air as the throttle is wide open, the pressure is high or near outside air pressure. Install a turbo and the pressure is higher than outside air pressure.

There is some pulsation as the pistons move up and down and the inlet valves open etc, but it should not be that much. My guess is that you see some air resonance due the other items hooked up to the vac hose you chose.

You could have some serious leakage past the inletvalve, blowing exhaust back into the inlet manifold, but your engine should run really unstable...

There are spare threaded holes in the manifold, go the junkyard and unscrew a fitting from a B2XX engine, fit it on your car and connect up. Should be steady.

Reg,

Jorn

If your guage is plumbed in one of the long intake runners, it will vibrate quite a bit. Close to the throttle body is better.