pump->bomb, round 2; overpressure?

Bernie Benz b.m.benz at prodigy.net
Wed May 22 12:49:45 EDT 2002

Comment below.

> From: Brett Dikeman <brett at cloud9.net>
> Date: Wed, 22 May 2002 11:20:35 -0400
> To: quattro at audifans.com
> Cc: 200q20v at audifans.com
> Subject: pump->bomb, round 2;  overpressure?
> So the rebuilt hose burst after about 3-4 hours of operation.
Was this a hose that you had rebuilt by your local hydraulics shop?
If so, find another shop, or DIY.
> Same exact failure mode as the last; trickle at first, and then a
> nice big stream.
You wouldn't expect it the other way around would you?
> So the official hose is now sitting next to me, waiting for
> installation.  Not only did it come early(Airborne is not exactly
> known for their punctuality) but it appears I got the "dee-lux" kit.
> I've got:
> -the lower hose
> -two banjo bolts
> -all 4 replacement o-rings
> -plastic restriction tube with attached brass collar/fitting
> The cute part is that the old hose was suspiciously missing said
> plastic tube(brass piece was still there, of course)...probably ended
> up buried towards the end of the old hose...and maybe the lack of
> this little restriction tube caused the hose to blow from pressure
> spikes?
Your "maybe" is not a possible failure mode.
> Also, while I tried hard to get the hose lined up exactly the same
> way, I've heard that funny bends can make 'em fail too?  Naturally
> one installs the hose with the joint between them loose so the two
> can twist, but...
> What most concerns me is that some sort of pressure relief valve on
> the pump has gone and the pump is overpressurizing the line...and
> this(much more expensive) factory replacement hose will be casualty
> #3.  Any opinions on this?  As I joked to my father, "over-pressure
> and over-delivery are not exactly the normal problem with Audi
> hydraulic pumps...", but you never know.
There is an overpressure relief valve within the pump that limits the peak
output pressure and thus the max pressure delivered to the rack,
but it has nothing to do with the normal hose pressure.  Hose pressure is
determined soley by the rack's demand for power steering effort.  When
steering assist is not in demand hose pressure is near zero with full pump
flow capacity thru the rack valving and back to the reservior.  Do you like
to hold the steering wheel against the stop while parking?  A big NO-NO!
> Last question...which hose does the long end of the plastic
> restriction tube go into?  The lower or upper hose?
I have not seen this "delux" hose kit of which you describe, and was not
aware that apparently, from your description, this replacement kit is a two
piece hose sepperable at a middle joint.  But from my experience with the
one piece crimped OEM hose assembly, the plastic tube and restriction is a
part of the acoustic filter built into the upper hose.  The pump outout must
flow thru this plastic tube into the lower hose section.  The restriction is
formed between the two hose sections, and the annular volume of air captured
in the upper hose section acting together forms the acoustic filter.

> Brett
> --

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