Though little understood, the vacuum gauge is probably the best single indicator of your engine's health you can get.
A Little Background
When your car is idling-whether it's fuel injected or carbureted-the throttle plate or plates are restricting the amount of air the engine can breathe in. The pistons are attempting to "suck" the mixture past the throttle. (Of course, in reality, it is atmospheric pressure that is attempting to "push" air into the engine as the pistons travel downward on their intake strokes.) When throttle is closed, vacuum is high in the intake manifold, from the throttle plate(s) to the combustion chambers. By contrast, at wide open throttle there is relatively little restriction to outside air entering the intake manifold, so vacuum in the manifold is very low.
A vacuum gauge reads pressure differences from atmospheric pressure, so the reading is zero in our "normal" sea of air. By convention, vacuum gauges in the US read "inches of Mercury."
Reading the Gauge
Unlike a fuel gauge, the vacuum gauge will keep you entertained with its instantaneous, wide-ranging movements.
When you floor the accelerator pedal, you can watch manifold pressure (another word for vacuum) swing from strongly negative to nearly zero (atmospheric pressure). When your engine is "on the overrun," like using engine braking down a steep hill at high RPM, you'll see really high vacuum readings. Naturally, turbocharged and supercharged will show very different results, with readings swinging into the positive at high speed. IAP's vacuum gauge is not designed for turbo or supercharged vehicles. Your vacuum gauge is also a sort of "poor man's" fuel mileage indicator; when vacuum is low, you are burning more fuel.
Absolute readings are not as useful as changes over time. That is, if you establish baseline readings under a variety of circumstances, you will know what to look for if your engine begins to deviate. Everything else aside, a high vacuum reading tends to indicate a healthy engine.
Having said this, we can make generalities about the readings you can expect. Note that engines with performance camshafts tend to read lower vacuum. Readings are also lower at higher altitudes; the rule of thumb is approximately 1 inch of mercury for every 1,000 feet of altitude gain.
The following readings will not apply to turbocharged engines, or cars with a separate venturi for each cylinder (like Weber DCOE or Dellorto carbs). All readings are inches of mercury (in. Hg.).
|ENGINE STATE||VACUUM GAUGE READING||INDICATION|
|Steady idle (800-1200rpm)||Gauge steady, 17-22||Normal & healthy|
|Steady idle (800-1200 rpm)||Intermittently drops several needle divisions||Sticking valve or broken valve spring|
|Steady idle (800-1200 rpm)||Steady, low reading, 8-14||Small vacuum leak or valve timing off; could have low compression/worn rings (verify with a compression or leakdown test).|
|Steady idle (800-1200 rpm)||Steady, low reading, under 8||Vacuum leak (check brake booster, vacuum lines, etc.)|
|Idle (800-1200 rpm)||Needle drops sharply on a regular rhythm||Burnt valve, or a valve with clearance too tight|
|Idle (800-1200 rpm)||Needle drifts up & down, along with rpm drift||Mixture off or small vacuum leak|
|Idle (800-1200rpm)||Vacuum gradually drops||Excessive exhaust back pressure (plugged muffler or catalytic converter)|
|Idle (800-1200rpm)||Intermittent fluctuation||Ignition miss; sticking valve|
|Idle (800-1200rpm)||Steady, above 22||Ignition timing may be too advanced|
|Open & close throttle quickly||Drops to about 2, jumps to about 25||Healthy engine|
|Open & close throttle quickly||Drops to 0, jumps to about 20||May confirm worn rings (especially if idle shows only about 15-20) Verify with a compression or leakdown test.|
- Mark Lee