[urq] More Weird UrQ Behavior - fuel starvation

[Grant Lenahan]

OPK, so it sounds like ( for mysterious reasons) that the EFI systems have a higher proportion of fuel returned, thus a higher gross flow for any given net fuel usage. Strange, IMO.

Grant
On Mar 28, 2010, at 11:09 AM, Ben Swann wrote:

> 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
> 
> 
> 

Grant Lenahan
glenahan at vfemail.net