Vacuum Pump Reprise

["Smith, Kirby A" <kirby.a.smith@lmco.com>]

Hello vacuum pump affectionados

At the risk of generating duplicate messages or having this message ignored
as a result of the duplication situation, I want to inform those list
members who fear they have to disable their vacuum pumps to stop rapping
noises that occur in some pumps, that disablement is not necessary.  This
last weekend I noticed my vacuum pump was getting loud again, so for the
third time I removed it, opened it, and cleared out the excess oil.
Reinstallation confirmed that this eliminates the rapping.  In my case, a
complete cleanout seems to be effective for about a year.

My theory, based on my observations, is that oil builds up in the pump
passages due to some defective valve or seal until partial hydraulic lock
occurs, keeping the pump piston from returning fully to its state with its
spring extended.  This causes a clearance to form between the cam-riding
push rod and the piston extension rod.  Now, instead of slight piston spring
compression when the cam height is at a minimum, there is a gap.  This gap
allows the push rod to be thrown at the piston extension, causing a knock or
rap.   Note that I believe this gap occurs dynamically; it is not evident
when the pump is removed. 

A rebuild kit might eliminate this yearly chore, but I am not aware of one.
Cleaning out the pump requires less than half an hour, and avoids the risk
of not having vacuum assist under some engine conditions (which is why Audi
put it on the engine) as a result of removing the push rod as some have
suggested on this list.

                .... Kirby   (Kirby A. Smith)
                              2 x 1988 90q
                          New Hampshire USA