Vacuum Gauge Diagnostics Note;Connect Vaccum Gauge After Throttle Body,Leave All Vaccum Hoses Connected.

Normal Engine at Idle When the engine is warmed up and working correctly, at idle the needle of the gauge should be steady and within the range of 17 – 22 inches of vaccum. Normal Engine operation when Throttle Snapped Open and Closed Vacuum should drop to 3 and then gradual climb to 25 – 28 before settling back at the previous value. This indicates that the valves and rings are OK. Poor Rings or blow-by at Idle The needle will be steady but the reading will be lower than normal. Poor Rings or blow-by when Throttle Snapped Open and Closed Vacuum will drop to near zero and then gradual climb to about 22 before settling back to previous value. If you have a Valve hanging open The needle will drop occasionally about 4 inches of vaccum from its normal idling reading. Burnt Valve The needle will regularly drop by a greater amount than with a sticky valve ,If showing 6 inches of vaccum you.have a Leaky Valve The needle rapidly drops two or more inches each time the valve should be closed. May also indicate a Lean misfire. Loose Valve Guides Fast vibration of the needle between 14 and 19 inches of vaccum.If you have a Weak Valve Springs When revving the engine hard, needle varies between 14 – 22 inches of vaccum. As rpm increases, a wider variation occurs in needle movement. Leaky Intake System Needle reads low ( below 5 inches of vaccum)and idle will be rough and unstable.. May also indicate late ignition timing or incorrect camshaft timing. Leaky Head Gasket Between Cylinders Needle floats regularly between 5 and 19 inches of vaccum. May also indicate intake manifold gasket leak at an intake port or a faulty injector. Blocked Exhaust High reading at first but will normaly stay under 3 inches of vaccum.Also motor will be very flat, no power. Clogged PCV or Intake Manifold vaccum Needle moves slowly through a wide range of readings. Also check idle air/fuel ratio. Conclusion Obviously vacuum gauge readings cannot pinpoint all problems, but they can save you a lot of time and money in chasing down major issues. Altitude Factor If looking at the readings, it’s important that you remember that the vacuum readings will decrease as your altitude increases. So if you live X amount of feet above sea level, remember to drop 1 inch of vacuum for each 1000 feet you are above sea level.