Question

What causes my vacuum gage to vary with the engine speed? We just replaced the air pump.

Jesse K.
Chicago, IL

Answer

Jesse,

I always preach about fixing the system and leaving the vacuum regulator alone. Only in this case, that very well may be where your problem exists.

When an air pump fails there are a lot of carbon parts (rotor & vanes) breaking and grinding going on just prior to the flexible coupling separating. As long as the rotor continues to rotate all the dust and carbon particles are vented out the exhaust port of the air pump. However, a point comes when the broken rotor and vanes will bind in the housing causing the coupling to shear. At that instant, (pump not rotating) the positive air pressure you have in the engine compartment combined with the vacuum remaining in the gyro instruments may cause some carbon dust and small particles to travel “up-stream” towards your gyro instruments. The first obstacle they have to pass of course is the vacuum regulator.

Check your vacuum regulator for any debris that might have gotten caught between the bottom diaphragm washer and the diaphragm seat. If the diaphragm is held open and cannot oscillate, your system vacuum will increase and decrease with engine/pump rpm.

Always clean out the inlet hose when installing a new pump. Any carbon dust or debris left in the line from the failed pump will travel into the new pump as soon as you start the engine. Pumps and time are expensive. Spend your money on only one pump and your time in the air!

Other causes and symptoms of a typical 2H3-[ ] vacuum regulator failure can be found in the Aircraft Pneumatic System Data section of our web-site.

In this case, I believe your solution is a simple regulator cleaning or replacement.

For a complete (down-loadable) outline on how to test your system for proper operation see our section entitled “Data Sheets”.

Let us know what you find and

Fly “Heads-Up”,

R. Heysek
Aerotech Components, Inc.

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