Originally Posted by antman
As a matter of fact a vac gauge is one hell of a diagnostic tool that many people overlook.
Here are some of the things a vac gauge can tell you:
- gauge reading steady 17-22 in Hg indicates normal engine in good condition.
- gauge reading low but steady indicates late ignition or valve timing, low compression, stuck throttle valve, leaking manifold gasket, etc..
- gauge reading steady but dropping regularly indicates burnt valve or improper valve clearance.
- gauge reading dropping gradually at idle indicates obstructed exhaust.
- gauge reading slowly dropping to zero as engine speeds up indicates obtructed exhaust.
- gauge reading fluctuating between 15 and 20 in Hg at idle indicates stuck valve or ignition miss.
- gauge reading drifting indicates a minor intake leak at the manifold.
- gauge reading fluctuating as engine speed increases indicates weak valve springs, worn valve stem guides.
- gauge reading vibrating excessively at idle but steady as engine speeds up indicates worn valve guides.
- gauge reading vibrating excessively at all speeds (when not under boost) indicates a blown head gasket.
- healthy decelerating engine reading should jump to 21 thru 27 in Hg as open throttle released.
These rules hold true for just about all 4 stroke gasoline engines, regardless of manufacturer or configuration (I4, I6, V6, V8, etc..)