[urq] Fuel Pump surge - still having problems

Ben Swann benswann at verizon.net
Wed Apr 16 21:16:15 PDT 2008

Well that is what the pressure regulator should be doing.  The regulator is set for
around 45 PSI whereas on CIS system pressure is usually over 60 PSI.  I can tune the EFI
to accommodate the higher pressure - would actually result in more power too.
Essentially what you say that I'm "flowing way too much fuel to return it to the pump
unused" is how it is done usually.  If not I'd like to know.

Still - I'd like to find out how to keep the cavitation from happening.  I may give the
line configuration one more try.  I can run some even bigger line, perhaps a hard line
with large ID - right now is rubber line with 5/8 ID - should flow fine right, but at
some point during running, the fuel starts to bubble and cavitate during running.  There
is no vacuum on the tank, so that is not what is happening, but I do notice it seems to
happen more as the tank goes to below 1/2 - still a lot of fuel in the tank tho.

If anyone knows of a good setup available that uses a low pressure pump into a small
reservoir aka. "surge tank" which then feeds slightly pressurized fuel into the main
high pressure pump, please let me know.  I'd rather not have to fabricate something, but
here is a setup I found that describes what I'm after:

Also, if anyone has encountered and resolved this problem I describe I'd like to hear.
Maybe someone has done an EFI system like mine that uses stock tank feed and not having
the problem.

It sure would be nice to have this resolved - I'd like to take this car to Carlisle this
year!  So close, but it just isn't right yet.


-----Original Message-----
From: Louis-Alain Richard [mailto:larichard at plguide.com] 
Sent: Wednesday, April 16, 2008 10:45 PM
To: 'Ben Swann'; urq at pacbell.net; urq at audifans.com; quattro at audifans.com
Subject: RE: [urq] Fuel Pump surge - still having problems


Maybe you can install a restriction on the return hose so the all the
circuit will be pressurized from the pump to the restriction but with less
flow overall ? 

Maybe I'm missing something, but I can't imagine that a tuned engine would
burn more fuel than what can be provided by a fully functional stock system
(the tank and its output and vent pipes). So if your pump is "faster" than
what the tank can provide, hence the cavitation, it must mean that you're
flowing way too much fuel just to return it to the tank unused.


-----Message d'origine-----
De : urq-bounces at audifans.com [mailto:urq-bounces at audifans.com] De la part
de Ben Swann
Envoyé : 16 avril 2008 16:47
À : urq at pacbell.net; urq at audifans.com; quattro at audifans.com
Cc : 'Ben Swann'
Objet : 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)

More information about the quattro mailing list