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

John Cody Forbes cody at
Wed Oct 22 09:23:43 PDT 2008

Why not re-plumb the return line into a T right where the gravity feed goes 
to the pump? If the return was fed into the feed hose instead of back to the 
tank it would reduce the ammount of flow required from the restrictive bit 
of the feed line.

See this crappy 10-second MSPaint drawing for clarification:


Ben Swann wrote:
> Eric,
> 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 tank.
> 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 boost.
> 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.
> Ben
> [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?
> Thanks.
> Eric Harten
> 1983 UrQ (034)]
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