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mwebb has it right.
An automatic in gear will have a slightly lower reading than in neutral.
The flutter does not exist in my echo with the source in the booster line.
Most boosters have the check valve in the booster where the hose connects.
If you connected it after the check valve the vacuum would not drop after you shut off the engine.
Typically a fluctuation in the needle indicates a weak cylinder, not producing the same power as the other cylinders.
If your idle is rough then that would be another indicator of a mechanical problem.
Most problems would give you a MIL malfunction indicator light, unless they were very minor, below the threshold of the point where the mil limap would come on.
There are many different reasons for a fluctuating vacuum guage, if it is because something is not right with the engine.
weak valve spring
valve not sealing properly
low compression on one or more cylinders
head gasket
intake manifold gasket
plug misfiring
bad ignition signal
injector not spraying properly
thats just some of the problems that could give you a fluctuation.
the purge line for the fuel tank will give you a fluctuation that would probably change somewhat depending on the fuel level in the tank, since the evaporative emissions control system maintains a slight vacuum in the tank.
full vacuum would collapse the tank so its a regulated vacuum that is very small compared to the highest manifold vacuum you would see.
regards
Mech
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