Vacuum gauge

The vacuum gauge is inexpensive and can be found in almost any shop. However, it probably is one of the most underused and misunderstood of any diagnostic tool.

In order to put the vacuum gauge to good use, we must understand how it works and what the readings can tell us. A vacuum gauge measures the difference of pressure in the intake manifold and the actual atmospheric pressure. Vacuum is a pressure that is below atmospheric pressure.

For instance, zero on your vacuum gauge would represent 14.7-psi at sea level. As the engine is cranked, the piston of each cylinder will create a partial vacuum during its intake stroke. This results in lower manifold pressures. Any leaks in cylinder sealing will increase manifold pressure. A cylinder that is not sealing properly will not produce sufficient compression pressures. We need to raise combustion chamber pressures and the resulting temperatures for reliable ignition.

An engine in good mechanical condition, depending on its size, will typically develop somewhere between 17 and 21 in.Hg at 1000 rpm.


Low Vacuum: A low steady vacuum reading at idle could indicate a problem with an external vacuum leak. We could also have late ignition or valve timing. If adjusting the ignition timing to specification does not increase the vacuum gauge reading, the valve timing should be checked.

Cranking: During cranking speeds, we should develop between 3 to 5 in. Hg with the throttle closed. This is a good test for an engine that will not start. A reading of zero would indicate there is an internal engine problem. A quick test here can save a lot of diagnostic time.

Base Idle: You may want to make a quick check to see if someone has played with the base idle screw of a fuel-injected vehicle. Hook up your vacuum gauge to ported vacuum on the throttle body at idle. There should be almost zero vacuum.

Restricted Exhaust (Catalytic Converter): When the engine is unable to exhale properly, a positive pressure will develop inside the cylinder each time the exhaust valve opens. This increases the pressure inside the intake manifold as the intake valve opens. The end result is lower manifold vacuum.

Run the engine at 1000 rpm and record the vacuum reading. Increase engine speed slowly to 2500 rpm. Exhaust backpressure, depending on the amount of restriction, will increase with engine rpm. If the vacuum reading at 2500 rpm should drop more than 3 in.Hg from the reading taken at 1000 rpm, the exhaust system is most likely restricted.

Worn Piston Rings: When piston rings are sealing properly, manifold vacuum will increase above a normal level whenever the throttle is quickly snapped closed. The closed throttle with high piston speed will create a large pressure differential in the intake manifold. This increase in vacuum is usually between 2 and 5 in. Hg as long as the rings are sealing properly.

Induction Restriction: When there is a restriction in the induction system, manifold pressure will not equalize with atmosphere. With slow piston speeds and a wide-open throttle, the intake manifold and atmospheric pressures should equalize. If the vacuum gauge fails to drop off to a zero reading, a restriction in the induction system is present.

Air/Fuel Mixture: An air/fuel mixture that is either too rich or too lean creates lower than normal vacuum. One way to determine if low manifold vacuum is the result of a mixture problem is to inject propane into the induction system. If the mixture is too lean, the engine speed and the vacuum reading will increase. If the mixture is too rich, the vacuum reading will begin to drop as soon as the propane is injected.

Valve Seating: An intake valve that is not sealing will cause a momentary drop in manifold vacuum. As the pressure in the cylinder starts to rise, it will leak past the intake valve. This will result in a large pressure increase in the intake manifold. These pressures will cause the needle on the vacuum gauge to drop 1-7 in. Hg each time the cylinder fires.

An exhaust valve that is not sealing will dilute the incoming mixture and cause a misfire. The vacuum gauge will display a lower manifold vacuum without any fluctuation.

Worn Cam Lobes: If the exhaust valve only opens partially, or not at all, the resulting pressure increase as the intake valve opens will cause a sharp drop in the vacuum gauge reading (pressure increase). This may appear as an oscillation of the needle on the gauge.

When the intake valve is unable to open properly, a volumetric efficiency problem is created. The vacuum gauge will display low manifold vacuum.

Broken Valve Spring: If the valve stays open too long as the result of a broken spring, a positive pressure is created. This can be seen on the vacuum gauge as substantial needle fluctuations each time the valve attempts to close.

Sticking Valve: A sticking valve will cause the needle to drop each time the offending valve hangs open. This is similar to a leaking valve, except that the vacuum reading will not drop at regular intervals.