Fuel Tank Return Line

[Steve Jones]

I’m making some changes to my fuel system and will be going from a “dead head” style fuel pressure regulator to a “bypass” regulator. In theory, a bypass regulator is easier on the fuel pump, resulting in a longer life expectancy. Longer fuel pump life translates to less time parked along side the road with no fuel pressure!!
In order to do this I’ll have to weld an aluminum return tube into my tank, which is also aluminum. Here’s the question, 1) should the return tube (entering the top of the tank, of course) extend down to somewhere near the bottom of the tank to return the gasoline into the bulk of the fuel or 2) should the tube just penetrate the tank wall and spray the returned fuel down into the fuel in the tank. I don’t want to set this up wrong, and I can see potential problems with either method. With method 1, there might be a potential siphon problem under the right conditions since the pressure regulator will be lower that the tank With method 2, I wonder about the effects of all the aeration of the fuel that would be taking place. Will that result in a loss of “kick” in the fuel due to evaporation of additives or whatever?

Anyone know what the correct tube length or return point should be?

Any production car with fuel injection has a return line from a bypass filter dumping excess fuel back in the tank. Where does it enter?

Thanks in advance,
Steve

[Bob Putnam]

I don't think it makes much difference one way or the other. The only factor that I would consider is to route the return so that there's the possibility that the fuel gets cooled a bit before it re-enters the tank.

[Richard Hudgins]

Steve,

A couple of points.

1. Do not put the return fuel into the same area as the pickup. The fuel is heated by the pump and the routing through the engine compartment. Put the return as far away from the pickup as possible to reduce local heating at the pickup.

2. If you have a foam filled tank you can just let the fuel dump onto the foam. You will not get aeration. But, if the tank is open, you will need to aim the fuel at a sidewall to keep aeration from occuring. A 30 degree angle to the wall works well. This allows the fuel to contact the wall and spread. If you just allow the fuel to dump straight into the tank, you will have aeration and vapor increases.

3. Bob is correct. If you can cool the fuel on the return leg you will be much better off. We regularly use a air/fuel cooler on the return line to pull the heat out as much as possible. Porsche (on the 928) has a return cooler that is plumbed into the AC system.

[Mike I]

Bob,
As long as we're on the subject, I'd like to pick your brain as well (or anyone else who can help). I'm running a Holley 802 Max Pressure "blue" pump (factory set at 14 psi) with 3/8" ID fuel lines, into a Holley standard pressure regulator (range of 1 to 4 psi) set @ 3 psi for use with Weber IDFs. The regulator is mounted directly to the firewall with each outlet feeding one bank of carbs and I can attest to Steve's concern for a "dead headed" regulator causing premature pump failure since I can readily hear and feel the pump straining against the reduction in flow, especially at idle when flow is at its lowest. I know I can isolate some of the vibration and noise with rubber between the regulator and the firewall, but that won't eliminate the strain on the pump. I probably need to talk to the Holley Tech people about this, but I thought I'd check here first since the question has already been asked. I'm considering switching to the standard pressure pump, which is factory set at only 7 psi, to lessen the pressure differential. If I were to tee in a "restrictor" return line (smaller than 3/8" ID) between the pump and regulator, and plumb it into the fuel filler breather line to be returned to the tank, would that effectively work like a pressure relief? If that will work, how small of an ID would give me the pressure reduction I'm looking for? I'm sure there's a calculation that will tell me that. Any advice you can give me will be appreciated. Thanks.

Mike

[SFfiredog]

Hi Mike,
I too like the idea of a return style fuel system for the reasons you stated. The way I set up the system is simple. I run a -8 fuel line up from the fuel pump to a small fuel log. I then route the fuel lines to the carb(s). I run a 2-port fuel regulator with the inlet side screwed into the end of the log. on one side of the regulator outlet I install the fuel pressure gauge and on the other outlet I run a -6 return line back to the tank. I set the fuel pressure (which is now acting as a restrictor to the return line) to the proper setting. Works like a champ.
HTH,
--Mike

[Jack21]

Steve,

Assumption is carbureted engine. Fuel injection engines (gas & diesel) have/need fuel return lines.

Unless Murphy just has a thing for you and fuel pumps, I don't think you need a return line for reasons stated. If you look at the way a fuel pump works, you're not pumping against a resistance and prematurely wearing out your fuel pump. If that were the case, there would be a lot of guys (and a lot of lawsuits) with bum fuel pumps.

I do know the feeling though. There's a spot on the PA Turnpike NE Extention called fuel pump rock. Broken body casting. That was a long time ago, and it hasn't happened since.

When the pressure at the carb matches the pump output, the pump stops pumping. Bleed this pressure back to the tank, and the pump then will work harder defeating your purpose.

When you shut the motor off on a hot summer day, particularly if you have AC, the heat from the engine can boil the fuel between the pump and carb causing vapor lock, and excess fuel percolating out of the carb into the intake. Can make engine seem "flooded" (which it is) upon hot restarting. Are you getting something like this? If so, do a fuel return line.

In this case, you need a very small orifice, about a #60 jet size, to bleed the pump-to-carb pressure back to the tank while the engine is NOT running to prevent vapor lock and percolation. Many carbureted engines with AC and were prone to this, had this feature.

Most fuel tanks already have provisions for fuel return lines. If yours doesn't, you want to have the tube near the bottom so liquid fuel goes into liquid fuel. The rationale for this is different, however. Fuel can build up a static electricity charge as it is a poor conductor. The liquid-liquid transfer minimizes this.

[gsharapa]

Interesting to see this subject posted, as I have a Superformance with a 514 and 1050 dominator that has the hot start condition. Ford had the same problem with big block 460 trucks and they cured it with a bypass valve and return line to the tank. It actually went in at the top of the tank.

I tried to set this system up with a restrictor as mentioned above and could not keep the fuel pressure up. When it warmed up it dropped from 7 psi to 4. I did only have a .091 hole so maybe a .060 would work better, but not sure.

Race car shop here in Dallas recommended using a bypass valve.

I currenty have a Holley blue pump, number 6 line from tank to filter and then to the pump. Fitting on tank will not allow anything larger. Then I ran # 8 line from the pump up to a fuel log to the dominator. At the end of the fuel log is the B&G bypass valve with a # 6 line going back to the tank. I tapped into the tank at the fuel sending unit to see if this would all work, prior to putting new feeds in the tank. I can set the bypass valve at any fuel pressure and in theory it is suppose to open and keep my fuel pressure constant at my setting (7 psi) and return the unused hot fuel back to the tank. Works well at idle and cool engine but after a cruise and a bit of acceralation it also drops my fuel pressure down to about 5 psi. I keep adjusting it up to 7 psi a few times. Let it sit over night and check dead cold and fuel pressure is at 11 psi which is too high. Not sure what the problem is? Is the blue pump too small in volume? Holley Techs say blue pump is fine for this application but it's either the pump relief valve or the bypass that is bad. It did cure my "Hot Start Concern" but created a much bigger problem now. I'm thinking about putting a bigger pump on and see if that cures it or just go back to the dead head fuel pump/regulator setup. An easy way to cure the hot start leaving the dead head system is to shut your fuel pump off a min or two prior to shutting the car off on hot days. No gas to vapor lock. Wish I would of thought of that simple idea prior to doing all of the above. Any thoughts?

Bob,

Don't return the fuel to the fuel vent hose as that will shut off the air to the tank and shut down your fuel pump altogether. Tried that once and know from experience!

I think you could tap into the actual fuel filler hose and based on what has been talked about above, that might be a good area to in at if you don't have a tank option. Seems to me the fuel would hit directly against the inside of the tube and just drain down into the tank. Filler tube usually is on the other side of tank from the pickup so this might work. Has anyone done this?

[kountzecobra]

Anytime I have added a fuel injected engine to an earlier car, we ran returh line to filler neck. Aeriation is not a problem.

[Mr.Fixit]

For Gsharpa,
Hot start can have many causes. Does it crank easily if you remove the coil wire? How much inital timing advance do you have? Does it smell like gas when you try and start it. Did you clean off the grounding tab along the framerail to which the motor ground cable is attached? With that BB you'll want a battery with a bunch of cranking amps.

[Mike I]

I did a little research last night. My HP Holley carb book has a pretty good breakdown on the "blue" pump and how it should be installed. It has a built in pressure relief that internally routes high pressure fuel back to the inlet port, which is designed to not exceed the max limit of the pump. I think this is what Jack21's comment was directed at. According to the book, what some racers have done is remove the relief valve screw and insert a fitting to plumb the bypassed fuel back to the tank thru a restricted orifice of no larger than .060. The explanation for doing this is that the pump inlet then only receives "fresh" fuel, makes sense to me. There is no other mention of a need to relieve "dead head" pressure, so as far as I'm concerned, my installation just sounds worse than it really is. And a rubber isolator between the regulator and the firewall should help cure most of that.

Mike

Also, the fuel breather on my car (E-M) runs from the top of the tank to a nipple near the top of the filler pipe. If I were to add a return, I would think that is a good place to tee into.

[gsharapa]

My Hot Start is fuel not cranking, battery etc. I've done a bunch of calling today to B & G, Holley etc and I've come to the conclusion my Holley Blue pump (old one) is really not intended to be used with a return line or bypass valve. I just purchased an AERO Equip fuel pump that is suppose to work with a return line and by pass or dead head. $ 183.00 with tax. Going to install and I'll post my results.

[Jack21]

Low $$ solution. Fram makes a fuel filter with 3 hose connections, 2 ea., 3/8" inlet & outlet, and 1 ea., 1/4" outlet. Install between pump and carb. Plumb the 1/4" outlet to your return line. Install a short piece of brass rod in the return line drilled with a 1/16" orifice, and you're done.

Neither pump or carb are affected with engine running, but pressure is bled off when engine is shut down.

If you're getting too much pressure when engine is running, install a pressure regulator between pump and carb to keep pressure in the 6 - 7 lb range at the carb.

[gsharapa]

Well I installed the Aero Equipment fuel pump and the B & G bypass and the fuel pressure still drops under acceleration. Went back and actually ran 10 gauge wire on the pump thinking the amps may be dropping and slowing the pump down. Everything seems to work better but fuel pressure still dropping under acceleration. Last step I'll try is to restrict the return line (#6) and see if that keeps the pressure up there. The way the B & G bypass valve is suppose to work is you put it on the end of the fuel log after the carb which lets the carb get high volume. Set the pressure at 7 psi and when the pressure starts exceeding that the bypass valve opens. Should work but pressure keep dropping. Have # 8 line going up and # 6 going back. If the restrictor does not work then I'm going back to dead heading it and just shut the fuel pump off a 30 seconds prior to shutting off the car..........
Trying to fix the "Hot Soak" and created a much bigger problem!

[zimmy]

Jack21:

Do you have a prt number or source for the Fram filter that you mentioned?

Zimmy

[Steve Jones]

Thanks for the suggestions guys. Sorry I haven't replied until now but, right after I posted the initial question, my company sent me out of town on business and I just got back. My existing setup is two Stewart-Warner fuel pumps mounted in the trunk, similar to the original 427SC. The location is less than ideal and on hot days they can cavitate. I've decided to go with a single Mallory fuel pump mounted below the tank level, where it should be (I'll leave the SW's in the trunk for appearance sake). I like the design of the Mallory, I believe it uses a gear rotor design similar to an oil pump as opposed to the rotary vane design used by most others, including the Holley. I'll use the Mallory bypass regulator as well. Ordered them today from Jegs. I may try to find a way to plumb in the SW's in parallel so they could be used in case the Mallory pump ever quits (what can I say, I'm a belt and suspenders kind of guy).
On the return tube I'm adding to the tank, I've decided to 1) extend the tube to the bottom of the tank 2) aim the fuel away from the pickup and towards a bulkhead and 3) drill a 1/16th inch pinhole in the return tube just inside the tank wall as an air bleed hole so that I can disconnect a line upstream if I have to without worrying about siphoning the tank contents out on the garage floor.

Think this setup will work? If I put the existing pumps in parallel as a backup system, should I isolate them with a checkvalve or a manually operated ball valve?


Steve

[ls1ac]

Cutting the end of the return line 60 deg. will help the stream stay together and not splash. A bulkhead fitting will eliminate the need for welding. (Not easy on a used tank)

[Richard Hudgins]

Steve,

By putting your return line at the bottom of the tank, you are going have a problem with variation in fuel pressure.

With a full tank you will have X+ pressure, with a low tank you will have X- fuel level pressure.

The difference will not be large, but it will introduce a variable that you do not need. Of course as you are putting in a bleed hole, this may take care of the pressure change due to head.

With carb setup, not a big deal in my opinion. But with FI, can cause many problems in programming as small pressure variations cause big problems.

[gsharapa]

Well I give up. Tried the restrictor and bypass valve and still to much variance in the fuel pressure. Set at 7.5 and run up rpm to 4,000 and fuel pressure still drops down to 5. Guess that dominator just wants a lot of fuel.

Traded B & G bypass valve back in for new B & G high flow fuel regulator and going to dead head fuel again. I really did all this to eliminate the hot start/soak condition. Guess I'll just turn the fuel pump off early on those hot runs! What a hazzel I created here not to mention the fear at WOT the fuel pressure would really drop.

[My427SC]

Return Line Kit