[urq] Fuel Pump surge - still having problems

Bill B. ur.quattro at comcast.net
Wed Apr 16 21:10:44 PDT 2008

Build a fitting that puts the fuel return output into the pump inlet hose,
rather than the tank, ACE Hardware, you are not alone, we are having the
same problem when we kick the boost on our 034 EFI system, and yeah the idea
just struck me, I will hang at the local Ace hardware this weekend and try
it, 14mm on the inlet to the pump? 8mm on the return line, Correct? Bestust,

-----Original Message-----
From: urq-bounces at audifans.com [mailto:urq-bounces at audifans.com] On Behalf
Of Ben Swann
Sent: Wednesday, April 16, 2008 1:47 PM
To: urq at pacbell.net; urq at audifans.com; quattro at audifans.com
Cc: 'Ben Swann'
Subject: Re: [urq] Fuel Pump surge - still having problems


Let's just say, I have enough evidence that this is basically cavitation
going into the
pump.  You can visually see what is going on.  I have no doubt that
basically the pump
is drawing faster than can be supplied from the tank.  I have been over this
and ruled
in and out many things.  There are times when I have been able to
temporarily eliminate
the problem too - basically allowing an open feed into the pump - similare
to your
suggestion of using a test tank.

As you mention below, CIS keeps a constant system pressure, and that is
higher than the
system pressure I'm using.  The difference is EFI tends to be more of a
higher flow,
lower pressure as CIS tends to keep the system pressure high and flow not
that great.
This is one reason I think raising my system pressure may help.  But I still
think I
need some sort of buffer - eg. Surge tank or really big pressure

I was doing some research and this appears to be a pretty common problem -
more with
converting from carburetor to EFI, but also got a lot of hits from CIS to
EFI - a lot
more of this going on with the older rabbits and such.  There is a reason
all the later
cars went to an in-tank and or/ two pump system. 

I am 90% certain that the problem is pre-pump sucking/cavitation.  Now I
need to find
the best solution.


[Date: Wed, 16 Apr 2008 07:20:30 -0700
From: "urq" <urq at pacbell.net>
Subject: Re: [urq] Fuel Pump surge - still having problems
To: <urq at audifans.com>
Message-ID: <02ea01c89fcd$07218b10$1564a130$@net>
Content-Type: text/plain;	charset="iso-8859-1"

Man, it sucks to hear that your problem remains ... (pun intended)

A couple things come to mind ... 

 * A low pressure system should be less prone to cavitate.

 * It seems to me in the CIS situation that the pump supplies the same
amount of fuel at
all times, if you don't need as much as the pump provides the rest goes back
to the
tank.  This would tend to speak against demand-based cavitation.

 * I think many of the in tank pumps were added because the tank sits low
... a full
tank in an urq should have plenty of gravity behind it.  I am assuming that
there's at
least a half a tank of fuel during the testing ...

 * Have you let the pump run without impediment into an empty fuel
container?  It seems
to me you would hear the most cavitation with the pump running open loop.  

You might want to try using a fuel container to feed the system for a test
... maybe
even better to get a fuel cell that you can sit in the trunk for a test.  

Steve B
San Jos?, CA (USA)]
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