Vacuum Gauge How-To

From BMW E34 Encyclopedia

Overview[edit]

That is an interesting test that is often overlooked. The vacuum gauge is one of the easiest tools to use. Not to mention the cheap price... What else can you ask for?

Procedure[edit]

  1. Take your BMW for a spin to warm up the engine. Turn the engine off and use a tee to tap a vacuum line. Connect the gauge to the tee
  2. Start the engine and take a close look at what the needle in the vacuum gauge will do
  3. The ideal reading would be rock steady 17-22 in.Hg (inches of mercury) @ 1000 RPM

Reading the Results[edit]


Low steady vacuum reading (15-20in.Hg) at idle

This could indicate a problem with an external vacuum leak, late ignition or valve timing, low compression or stuck throttle


Needle fluctuates from 15-20in.Hg rapidly at idle

This could indicate a sticky valve or ignition miss. Weak valve springs might cause fluctuation at higher engine speed


Gauge needle drifts rapidly at idle

Small intake manifold leak or bad injector O-rings


Gauge needle stays steady then drop regularly to recover and repeat over and over

Gauge needle stays steady then drop regularly to recover and repeat over and over Burnt valve(s) or improper valve adjustment. The needle will drop when the defective valve operates


Gauge needle stays steady then drop to recover and repeat irregularly

Sticking valve causes the needle to drop each time it hangs open. The reading will not drop at regular intervals like for a leaking valve


Gauge needle drops gradually at idle

Chocked muffler or obstruction in the exhaust system. A restricted exhaust (like the cat.) will not allow the engine to exhale properly causing an increase of pressure inside the intake manifold. The result is a lower manifold vacuum