[urq] More Weird UrQ Behavior - fuel starvation
Schaible, David
David.Schaible at jrspharma.com
Mon Mar 29 07:05:14 PDT 2010
I'd like to see the pics if you get a chance to shoot out some emails.
On my TDI using veg oil, I tee'd the lines from the fuel rail return to help feed the pump (it's an external vac pump) different application but same principle, some of the Mercedes used a lift pump to help feed the injector pump (more of what this is about is pump life rather than cavitation but we get flow issues when cold b/c of viscosity)....anyway just a case where a few similar very simple mods help, proved by 50k+ miles in it's current configuration.
-----Original Message-----
From: urq-bounces at audifans.com [mailto:urq-bounces at audifans.com] On Behalf Of Ben Swann
Sent: Sunday, March 28, 2010 11:09 AM
To: 'Grant Lenahan'
Cc: afinn1 at gmail.com; urq at audifans.com; 'Quattro List'
Subject: Re: [urq] More Weird UrQ Behavior - fuel starvation
Yes, but a CIS system maintains a high head pressure 70-85 PSI system pressure with very
little return flow to the tank.
EFI typically runs at much less pressure - typically around 45 PSI, and much higher
return flow into the tank or simply higher flow demand into the pump than the tank
outlet can supply and that results in cavitation. I observed this quite clearly when I
used a high flow transparent fuel-filter before the pump.
I found the pump was cavitating even at idle, but because of buffering in the lines,
damper, etc. the cutout was most likely to occur at higher load than when not under
load.
I also think there may have been some problem even with the CIS system before I did the
conversion, but was not as noticable. The later cars went from a 3/8" diameter tank
oulet to nearly 5/8" feed and I'm sure that helped. Ultimately all the cars went to an
intank pump - no inlet restriction to the pump because it is submerged. The in-tank
pump also stays cool - the external pump tends to get hot, especially when being fuel
starved.
I could go on about reasons why I believe this problem exists. There was a lot of
people telling me I was F.O.S., etc. In the end, for whatever reasons irregardless of
explanation, fluid dynamics, etc. the solution I came up with fixed the problem. I
tried many other things prior to doing this. The only other alternatives involved
cutting the tank and installing larger feed or in tank pump, or installing a fuel cell,
and I did not want to do any of this unless absolutely necessary.
The "fuel capacitor" surge pipe/tank is an inexpensive, simple solution and works. It
is especially applicable to the Ur Quattro because of the limited space to install a
solution like a surge tank, tandem pumps, etc.
I'll create a web-page on this when I can and let folks know. In the mean time I found
4 pictures I took that help explain it.
Ben
-----Original Message-----
From: Grant Lenahan [mailto:glenahan at vfemail.net]
Sent: Sunday, March 28, 2010 10:48 AM
To: Ben Swann
Cc: afinn1 at gmail.com; urq at audifans.com; 'Quattro List'
Subject: Re: [urq] More Weird UrQ Behavior
I'm a bit lost on this pressure vs. flow discussion. It may be terminology, but for a
given motor, at a given RPM and throttle opening, to maintain a given fuel/air mixture,
a specific volume of fuel is required. This translates into a given *average* flow,
although that will vary based on ho intermittent it is (most modern FI is essentially
the fluid equivalent of pulse width modulation).
So ho does the motor now demand more flow?
Grant
On Mar 27, 2010, at 10:56 AM, Ben Swann wrote:
> Andrew,
>
> I'll lay 95% odds you are starving the pump. Changing to a bigger
> badder pump won't fix the problem. I had this frustration with my EFI converted '83.
>
> What is happening is the tube coming out of the tank is insufficient
> to provide adequate flow to the pump. The fuel delivery on the CIS
> cars is designed to have a higher pressure and less flow. When the
> pressure is dropped and flow increased for EFI it basically sucks the
> fuel faster than the 3/8" bent tube can deliver. The result is
> cavitation at the delivery side of the pump. The problem gets worse
> as the car runs for a while. I hit on the problem by finding I could clear the
problem by reversing polarity on the pump for a second - this drives the air out of the
lines.
>
> There are a variety of permanent fixes ranging in complexity -
> pre-pumps to surge tanks, in-tank pumps, modifying the bottom of the
> tank for a much larger fitting (welding a gas
> tank!) and fuel cells replacing original tank, etc. Look up surge
> tank and related and you will find this is a common problem,
> particularly with drag racing and EFI converted cars.
>
> I toyed with this for over a year - drove me nuts. Many on the list
> probably remember my frustration. Pulled the tank multiple times,
> cleaned the in tank sock, swapped pumps out - the nice 044 pumps and
> more expensive pumps only made the problem worse. I actually had
> better luck with a WALLBRO and the OE pump with smaller inlet worked better than the
later CIS pump made for NF/NG!
>
> The pre-pump filter made things worse, and I removed it which helped
> some, but the problem persisted. I did a lot of research and much trial and error.
>
> I did not want to remove the tank unless I absolutely had to. So my
> fix was to fabricate what I call the "fuel capacitor". It is simple
> in concept, but devil in the details since there is not much room to
> put something between the pump and the tank - angles, clearance,
> kinked lines, etc. It is a 2" copper confabulation - kind of a 45
> Deg. Angle between two 6" pipes making one angled pipe capped at both
> ends. The pipe just fits in between the pump, mounts, brackets and
> tank. I had to cut out the unused portion of the difflock cable
> assembly - already removed and being replaced with pnuematic actuator. If you need
to maintain the cable system then this will be much harder to implement.
>
> There is one large outlet that sits at the lowest point in the pipe.
> I should have added a drain fitting because of the lack of pre-pump
> filter and probably need to drain it every now an then - I plan to add
> one later, possibly with a fuel water seperator that sits sloghtly lower than the
assembly.
>
> I toyed with a variety of fuel feed and return schemes, but simplicity
> prevailed. Both the inlet and return are installed in the "fuel
> capacitor". I need to review what I did exactly, but IIRC there is
> one large fitting at the utmost top of the tank - nearly jammed into
> the feed line with a very short line. It was difficult to get it installed without
kinking, but once done, there is little to go wrong.
>
> For the return IIRC it it actually teed into the surge pipe offset at
> slight angle near the top of the pipe. I was not sure if it would
> work, but makes sense that it does and in fact adds additional but
> equalized pressure feed side to not only keep the pump from cavitating, but also keep
the feed line from the tank clear of debris.
>
> In short it works, plain and simple. The only problem I had was using
> non fuel grade line which decided to rupture one day - thank God it
> was in my driveway and not on the highway. So use good hardware and plan it out for a
good reliable solution.
>
> If you or any listers want picture of the assembly, LMK and I will
> send them. I plan to put this on my site, but these things tend to take low priority.
>
> HTH.
>
> Ben
>
> [Date: Fri, 26 Mar 2010 12:21:43 -0700
> From: AF <afinn1 at gmail.com>
> Subject: [urq] More Weird UrQ Behavior
> To: urq <urq at audifans.com>
> Message-ID:
> <ed47d8be1003261221w2a4c5064v1f7af3a500956166 at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> So, at the past 2 track events I've attended with my V8 UrQ I have had
> a weird issue with engine cutout at high rpms, usually in 3rd gear,
> but on the straightaways in 4th gear. They seem to occur during any
> throttle position, like cresting a hill, or accelerating out of a
> corner, which is somewhat unnerving. There are no fault codes thrown
> at all, which makes me think it's a fuel supply issue. Here's the
> weird thing, I would say it a fuel pump relay issue, but wouldn't the
> engine stall (that's what my old UrQ and Coupe GT would do on the highway), but in
this case there's no power until I depress the clutch after slowing a little and
downshifting, then the power is all back.
> The engine never stalls and shuts off! I confirmed it's not a rev
> limiter issue by redlining with moderate throttle input and no issues
> occured. It's definitely at higher RPM though. Does anyone have a
> thought? I thought about relaying the fuel pump, but wouldn't the
> engine simply stall with no fuel supply, or will it simply keep
> running with the car in motion and clutch out without shutting down or turning on all
the warning lights on the dash?
>
> Any help is appreciated. Thanks.
>
> Andrew Finney
> 1985 UrQ 4.2L ABZ]
> _______________________________________________
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>
Grant Lenahan
glenahan at vfemail.net
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