Some posted they replaced the return line with a larger line as required, my question is what is your return line attached at the tank? Did you fab up a larger input for it?

Thanks

 

The correct way is to drop the tank and have a radiator shop intall a bigger return. The wrong way is to use one of the vapor return lines and the original return line into a Y that goes to your return line. Note make sure you pick the correct vapor line.

 

LOOK, my experience is with a ~250 some odd horse 350, and then a 400 horse 355....and a DPFI L98 style induction, the stock return line at 1/4 inch diameter works just fine.....as well as the stock 3/8 supply line.....
been that way daily driven all kinds of weather since '95......never a problem traced to those lines, not once....
have had problems with water in the tank/fuel...but that's Florida....groundwater incursion is a huge problem here....

GENE

 

Everyone I know that tried to use the stock return line ran into tuning problems by not being able to get the feed fuel pressure where they needed it.

What I did was this: I ran a whole new 3/8" line for feed. I added a sump to the bottom of the tank to draw fuel from there. For the return, I used the stock feed line which was a 3/8" line.

If you don't want to move your feed location from stock, and plan to run a new return, then just take the sending unit off the tank, remove the small return line at the sending unit and have a larger 3/8" line installed in it's place.

 

I have the stock small return line going into the stock port in the tank. I have TPI and have never had a moments trouble with it.

 

I installed the larger return line as recommended in the instructions from Holley. The vapor return fitting on the gas tank easily threads off and I bought a fitting with a 3/8ths end on it rather than the 1/4" of the vapor line.

 

Guys, all I know is the basic theory of operation which is if the mani vac is minimal, the regulator closes and pressure rises......meaning if there even IS a line restriction enough to bother anything, it's NOT at WOT.....

so the question is...what does it do at idle??? my mind says...ok ..3/8 in the feed, and 1/4 in the return with injectors pixxing fuel...seems fine to me...

so with regulator hooked up normally, and engine running at idle...can I adjust the FP for any pressure possible to make any sense.....?? meaning can I get as LO as say 36 PSI??? answer is yes...pressure on the pump certainly can rise from a restricted line, normal idle is around 38 or so....and WOT is around 40-42 or so....

so where is the greif????

mine works fine, that's all I know....

someone with a 500 hp BB and DPFI will have a differant experience I"m sure....

With engine off, I jumper the fuel pump to running...easy on MY car for a reason...
at any rate, that fuel pressure is around 39-40 psi....and is adjustable all over the map....nuff said....

GENE

 

The grief comes from systems that run lower pressures. Mine has a pump about 2-3 times more PSI than I need (22psi). So, it sends at say 50 PSI. At the regulator I am only taking 22 psi, so everything else (all the fuel not needed to supply my 22 psi) has to go back down that return line. If the return line has a 5 psi restriction then I am getting 5 PSI on the back side of the regulator and getting 27 psi at the injectors. If the injector timing and pulse width are correct, I now am forcing too much fuel for the engines needs.

All I know is this, with Holley and Edelbrock fuel injection systems they say you must have as close to zero return line pressure as possible. A lot of the systems out there do not use regulators that are based off vacuum. Anything over 4-5 psi is a "no go". Of the 5-7 people who I know that tried to use the stock return line, they ALL had problems getting a stable idle due to fuel pressure build up in the return line.

Other systems such as the TPI based system may vary. I am just quoting from my experience, the experience of others and information from Holley and Edelbrock. They all say 1/4" is too small, 5/16" is minimal with a low number of bends and 3/8" is preferred.

 

:lurk:
keep it coming!!

 

Well all I can say is my idle is smooth, and when pressing for acceleration I used to have a slight hesitation/stumble on light throttle--traffic..., which upon replacing the computer, it went away..runs fine now....so if the vacuum dropped when pressing the throttle under load, the vacuum dropped, thereby restricting the fuel flow on the return line, and thereby raising the fuel pressure.....

only thing I notice is, I can only get about 45 lbs of pressure from the pump under blocked return line conditions...stall .....and I think that's not enough....
need over 60 I think..maybe the source of my slight stumble????

GENE

 

Some will get away with a small return line, some won't. It is not so much the pressure but more the flow of the fuel pump.

It's actually very simple... The return line has to be able to flow the amount of fuel the pump supplies, minus the fuel entering the engine. Electric fuel pumps pump the full amount they are rated at all the time. So a common perf pump is 255 liter/hr (~67 gallons/hr), and since an engine uses only a fraction of that at idle almost all of it will be returned to the tank (again at idle). If the return line is too small pressure will build up in the return and the fuel rail pressure will not be steady.

If you have a regular OEM TPI pump that supplies less than a perf pump your odds of being ok with a small return line are much greater.

 

Yeah, I think you need more out of your pump. Post your question over at the EFI forum on http://www.chevytalk.com . It's where Zwede, myself and a few other C3'er get our EFI help from.

 

I am totally with bseery on this one. I had tons of problems untlil I changed the return line. I whole heartedly believe those that did not have problems did not have problems, and I sure as heck wished that I was one of them. I just dont understand how they got off so darn easy :lol:

Heres another way NOT to do it. Y the return line into the intake of the fuel pump. It actually works as far as lowering the back pressure, but the fuel cycles through the engine compartment to much and starts to actually boil from all the heat. I am sure I tried every way there is to get around it, so if you think you got something, feel free to ask me.

 

Another solution is called a "surge tank". In this setup you run two fuel pumps. You can even still run your stock mechanical pump.

Pump #1 (mechanical or low pressure electric pump) pumps fuel from stock fuel tank to a "surge tank" either in the engine compartment, in a fender, in the nose -- somewhere to safely put a small tank (1 gallon or so).
Pump #2, the high pressure pump to the injection system pulls fuel from the surge tank and feeds the fuel injection system. The un-used fuel, the return line, then dumps into the surge tank. The surge tank has an overflow line that goes from the surge tank back to the stock fuel tank for when you are sitting around idling a lot.

That replaces the need for a larger return line all the way to the tank, and also takes care of any pick-up problems associated with a stock fuel tank.

I know some people have done this, but I passed on this engineering feat for just running a large return line and putting a sump in my fuel tank.

 

Well, in all honesty I have a multiplicity of problems with my entire fuel system.....
tank too small,......
tank has bottom feed, so the fuel pump is tied to the frame rail....not a real problem, most likely...
possible but undemonstrated fuel return line problems....
tank developes pressure sometimes...mostly in summer, and only on occation....noticed ONLY when taking top off to pump gas....
obviously the float in the stock tank evap emissions controll valve...that black box on the top driver's edge of the tank...that float sticks sometimes.....
I suspect a full tank keeps it there long enough for the tank fuel to develope pressures from heater/returning fuel, and ther fore it never gets to 'relax'.....
air pressure now forcing that float ball up against the stop....
I suspect what I really want to do is move the regulator to the rear of the car and route the return right there into the tank, and use my 1/4 inch return line...to run the vacuum signal to the regulator....tons of fun.....

another thing is of course I suffer from the lo tank...hard corner stumbles engine from fuel starve syndrome also....meaning another some sort of stump setup to solve THAT problem too....
what I really want is another tank with the proper setup,....a larger one...
kill all birds with ONE stone.....that's why I was thinking of the later say '82 tank...but am told it's totally differant and would take an act of God to install....
chances of finding a tank in junkyard with any hope of fitting and being larger are pretty slim, I"m imagine....SO, the thought is at this point.....dropping the tank, getting yet another '72 style tank...cutting them both apart, installing the sump, modding the top filler with '82 tanks guts, sealing the bottom port, and splicing the two together forming the 'newly created' tank into extending forward to the point under the rear vent grills, making allmost 25-30 gallons capacity....nice increase, and get over with all the problems at once....

GENE

 

man Gene, that sounds like a beeotch!
with all the aftermarket companies dedicated to vettes and all the griping we do about this issue, you'd think they'd have something for us!
Let us know if you find an easier solution...

 

Why not just use a tank setup with stock sending unit and pump from an 82 vette? Will this fit and work with TBI or TPI aftermarket systems? I was thinking of doing this with my 71 BB. I want to install a TBI setup and have been wondering what to do with the tank and pump. Has anyone done this?