Re. [urq] Need some help with my fuel flow issue

Ben Swann benswann at
Tue Oct 21 09:26:32 PDT 2008


Finally someone else has come forwad with this problem - we can start a 12 step program
and call it EFI-Boosters anonymous.

If you are indeed experiencing the same problem I had, the fuel is vaporizing prior to
entering the pump due to the restrictive feed from the tank.  I tried many "fixes"  and
workarounds to resolve this, starting with removing and flushing the tank - multiple
times in fact.  I tried installing larger pre-filter, new pumps, bigger lines, straight
line feeds into the pump, removing the accumulator, installing new accumulator,
variations upon themes, etc.   After much trial and error, I concluded that the fuel
flow was just to much for the 3/8" feed line that is sort of "kinked" as it leaves the

This was not a problem with CIS, since CIS runs a head pressure of around 80 PSI, the
fuel flow is restricted and return flow back into the tank is more or less a trickle.
With EFI you regulate the pressure to about half of what the CIS pressure is.  The end
result is the pump is drawaing fuel too fast from the tank and once a bubble starts,
there is a run away cavitation.  Even raising the regulated pressure to around 60 PSI -
high for EFI did not resolve the problem, but did reduce the effect a little.  My
experience was the car would typically start off running fine, but a few minutes after
warmup, it would lose top end power - lean out or starve the pump for fuel.  Ultimately
the cavitaion gets so bad, the car barely accelerates enough to get you home, or as I
experience driving to Carlisle - not able to climp up a mountaind - bucking and kicking
all the way.  I did find a temporary workaround that got me to the show and back and
worked for awhile to get me around town.  I found if I reversed polarity on the pump for
a few seconds and then back, the problem would go away for a while.  I was pulling over
evey 15 min or so going under the back set and doing this procedure to get me there and
ultimately wired it up so I could do this from the driver seat.  

So what is the real  fix you are asking.  Either: 1. install an in tank pump, or fuel
tank with in tank setup.  2. Dual pump - high flow low pressure pre-pump into a surge
tank with high pressure pump feeding the EFI - this is what VW/Audi did on a lot of cars
at some point..  Or   3. what I did:  I fabricated a surge pipe.  The theory of this is
simple, but implementation took some doing.  In short:

Fabricate a 2" diameter obtuse angle L pipe.  One end will just about reach the feed and
return line - you need only about 2" mac of line to connect the fittings to the tank -
any more will kink and add potential flow restriction.  The other end will just about go
straight into the pump - using only about 3" of line to connect the surge pipe to the
pump.  The pipe I contructed with 2" copper, 45 deg. Ell connectors - street fittings
and normal fittings to get the proper shape.  The ends were caps that could be soldered
onto the final pipe construction after much fit check and verifications.  Now the fit
check process revealed there is virtually no roo to do this and why it took me so long
to work it out.  For one, if you have the original dual handbrake cable setup then you
need to change over to the newer style and cutoff the part of the braket that mounts the
diff lock actuator - cuttoff the part that the cable go through and then there will be
enough room to pass the 2" pipe - justbaerly, but enough and in the end looks rather
elegant- kindof like the factory would have made.

The end fittings - used barb fittings drilled and tapped into the end caps and soldered
for good seal.  The feed from the tank should be on the low side of the endcap at the
tank side and return line at highest possible point so as any possible vapor is returned
immediately to the tank.  The pump side has the pump feed line lower than the return
line.   I oriented the pump fairly much in stock position, but reworked the retuen line
into a new accumulator and I cut the brand new  factory line and married with fuel
injection grade hose and good quality clamps.   The pipe configuration now pretty much
odffers unrestricted flow into the pump.

How it works - Fuel is gravity fed pretty much directly into the pipe which act as a
reservior - a "fuel capacitor" if you will.  The  2"  pipe has pretty much no
restriction in flow to the pump.  Now this in and of itself should have worked to solve
the problem, but I did not trust that it was the final solution.  I could have run
ruturn line outside of the surge pipe, but reasoned that if the ruturn line was put into
the pipe then it would keep the pipe slightly under pressure and with the return fitting
to the tank being at the highest possible position then any air/vapor would be returned
immediately back to the fuel tank.


Now I am finding all the other latent issues with tuning the car with Megaquirt as I hit
2.5 bar and beyond.  I've already wiped my turbo and blown hoses as well as AIT fittings
out.  The nice thing about megasquirt being speed density, is you can blow hoses and
have leaks all the was to the throttle body and still get home - just won't be getting

As with the thousand other projects I'm into - big and small, I do my best to post
results and picutres, but it all takes time.  In the mean time if anyone wants pictures
of the completed tank, LMK and I'll do my best to get them to you.


[Date: Sun, 19 Oct 2008 20:48:38 -0500
From: Eric Harten <audi90sq at>
Subject: [urq] Need some help with my fuel flow issue
To: urq urq <urq at>
Message-ID: <2B29AA6E-644C-421A-A4E5-7D01AB2456ED at>
Content-Type: text/plain;	charset=US-ASCII;	format=flowed;	delsp=yes

OK, I need some help with an apparent fuel flow issue.  After my recent move (car sat
for two months), the UrQ started running lean under boost.  I tried adjusting the 034 to
add more fuel, but I seem to be hitting a wall.  AFRs start off good and then lean out
to 15s under full boost.  The AFRs ran around 11 under 20psi of boost before the move,
so I know the system is sized correctly.  I have noticed that my fuel pressure drops
10-15 psi when the car starts to lean out.  I just replaced both fuel filters and the
034 high flow Bosch fuel pump is only 2 years old.  It does seem a bit noisy though.
The FPR is manifold referenced and seems to be working fine, but I am not sure how to
test it.  Is there a screen in the tank that I could have  
missed?  What could be causing my fuel pressured drop under boost?   

Eric Harten

1983 UrQ (034)]

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