[urq] More Weird UrQ Behavior
Grant Lenahan
glenahan at vfemail.net
Sun Mar 28 07:47:32 PDT 2010
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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