Vacuum gauge engine diagnosis

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by: Cobalt327
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[edit] Engine diagnosis using a vacuum gauge

[edit] Diagnostic vacuum readings

[edit] Chart

Vacuum Chart Web2.jpg

[edit] Table

Smooth and steady idle
(800 to 1200 RPM)
Between 17 to 21 inches Engine is in good condition, but perform next test to be sure.
Open and close throttle quickly Jumps from 2 to about 25 inches Engine is in good condition.
Smooth and steady idle Steady, but lower than normal reading Worn rings, but perform next test to be sure.
Open and close throttle quickly Jumps from 0 to 22 inches Confirms worn rings.
Steady idle Intermittent dropping back 3 to 5 inches and returns to normal Sticky valves. If injection of penetrating oil into intake manifold temporarily stops pointer from dropping back, it's certain the valves are sticking.
Steady 3000 RPM Pointer fluctuates rapidly, increasing engine speed causes more pointer swing Weak valve springs.
Steady idle Fast fluctuation between 14 to 19 inches Worn intake valve stem guides. Excessive pointer vibration at all speeds indicates a leaky head gasket.
Steady idle Constant drop Burnt valve or insufficient tappet clearance holding valve partly open or a spark plug occasionally misfiring.
Steady idle Steady 8 to 14 inches Incorrect valve timing. It must also be remembered that vacuum leaks and/or poor compression can result in a low vacuum reading.
Steady idle Steady 14 to 16 inches Incorrect ignition timing.
Steady idle Drifting from 14 to 16 inches Plug gaps too close or points not adjusted correctly. Damaged plug or coil wire

[edit] Resources

Finding vacuum leaks

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