I was about to change my fuel pump in my car this evening and I didn't realize my fuel pump has a return line. The Holley fuel pump I bought (nor any of the aftermarket high flow pumps I see) have a return line in them.

Is the return line necessary or can I just cap it off?

Also I'm assuming the return line is the smaller line?

 

It helps avoid vapor lock by keeping the fuel moving in the supply line.

The Carter 6626 is a 120 gph pump and has the return line. Fits perfectly, never failed me in my ZZ4 clone configuration.

(Oh, yes, the smaller 5/16 line is the return.)

 

Yeah, I saw that carter and honestly I didn't trust it cause it was so cheap lol. So I went with a holley =/ Thar cart is on summitracing for 24 dollars... The next closest high flowing fuel pump is a Holley 80 gph that is 73 dollars.

What exactly is vapor lock? I've heard it mentioned before but don't know what it is. And if it only helps avoid it, does that mean it's not required? What are the chances of this vapor lock happening without it?

 

Vapor lock is when the fuel is heated up to a point to where it becomes a gas, not liquid. Since the fuel pump can't pump gas, it can't get to the carb.

Vapor lock is frequent in hot climates, stop and go traffic, or racing. Basically anytime the fuel can get' hot (like around your headers).

I have the Carter M6626 and it works great for my built up engine.

 

Hmm that does worry me some, cause my fuel lines are only a couple inches from one of the header pipes.

But I don't know... I just can't bring myself to trust that carter as quality high flow fuel pump for only 24 dollars. It's just one of those things that seems to good to be true lol. The guys at the race shop today told me I didn't need the return line when I called and asked them about it... But at the same time they are probably used to making sure fuel lines aren't near headers and not thinking about stuff like that. I don't know how much chance this vapor lock has to happen, nor how much the return line helps reduce this chance.

How will I know if vapor lock is occurring? Can anything really bad happen from it (like an exploding fuel tank or something crazy like that)? Would it help if I did the holley fuel pump without the return line if I put heat wrap or something around the fuel lines where they are close to the header?

 

As stated, vapor lock is when the gasoline in the supply line turns from liquid to vapor. The fuel pump can't suck vapor. The carb can't run on vapor, even if the pump gets vapor to the carb. Without liquid gasoline in the carb bowl, the engine dies (or won't start).

When the gasoline turns from liquid to vapor is a function of temperature and pressure - the lower the pressure, the lower the temperature the gas will vaporize. Since the mechanical pump sucks on the tank, the pressure in the supply line is lowered, meaning the gas temp that will vaporize the gas is lower. This is a bad-bad situation, so keeping the fuel as cool as possible in the supply line is critical.

Get over the low price of the Carter. This is one case when paying more gets you less. I seriously doubt you'll need more than 120 gph.

This whole vapor lock thing is why the factory went to a pusher in-tank electric pump. Guess what happens when the supply line to the mechanical pump is pressurized? First, the temp required to boil the fuel is raised. Second, the fuel will be pushed through the mechanical pump and returned to the tank, keeping it cooler, and the mechanical pump can get what the engine needs to the carb.

 

Thanks for the info. I've decided for now that I'm just going to heat wrap the fuel line and run the holley. As long as I'm not going to blow up my gas tank it's not a big deal, I can cross the bridge of not getting fuel to the carb when it's hot if that happens. A few mechanics I've asked around here and the race shop think it is very unlikely to happen though. I'm just putting the heat wrap on it as an extra precaution since it is somewhat close to a header tube (within ~4 inches). If I do end up having a problem with vapor lock, then I'll order the carter and try it.

 

I've wrapped the fuel lines on both of my cars with heat tape. There just isn't a lot of room to run things in these cars. Which is also why they tend to vapor lock.

 

Yup happened to me a couple weeks ago I to bought a after market high end fuel pump without a return and my car left me sit in the city at rush Hour becuase it vapor locked on me.

The after market was supposed to be 90gph but would not keep up. I also bought the Carter M6626 that another member reccomended it to me and it works great. The pump keeps up with the car now.

And on a side note (someone with more info might agree). By not haveing a return line there is a chance your PSI might get to high and bypass the needle and seats in your carb.

 

That can happen even with a return line actually, and is why I'm installing a regulator as well.

The return line should not be returning fuel until the PSI goes above what the pump is rated for. Which in my case is higher than what the carb wants on the stock fuel pump. Carb wants 5 PSI and the stock fuel pump that I have is 7.5 psi.

What I might end up doing is getting a regulator with a return line component on it if I decide that I need it and stay with the more reputable holley fuel pump. The carter may indeed be fine, but even if I get past the fact that I don't trust it, it's just not very pretty!

 

I'm a function-over-form guy. I'd pay more for an unchromed part if the chrome wasn't part of the part's specific function.

You can put a return reg after the Holley if you like. It will have to be set lower than the pump's output pressure in order to do anything. It wouldn't be very effective.

Mechanical pumps control pressure one of two ways: For pumps without a return port, the spring isn't strong enough to push the fuel out above the designed outlet pressure (contrary to what most people think, mechanical pumps pump when the arm returns, not when the cam lobe pushes on the arm); or, with return, the return port has a relief valve that lets fuel flow back to the tank when the outlet pressure reaches the design pressure.

 

I'm a function + form guy! If a part does what it's intended and I can pay 40 dollars more for one that does the same thing and looks great, I pay more

I'm going to be running a regulator either way, so might as well have a pretty pump and a regulator with the return on it if I decide I need one.

Right now what I'm trying to figure out and am slightly worried about, if I put the regulator on I have now, the pump does 6 to 9 psi, I want to limit it to 5psi. Is that going to be enough excess pressure to damage the pump or the regulator? What I'm scared is going to happen is the pump is just going to keep pumping gradually building more and more pressure behind the regulator until the regulator breaks due to to much pressure, or the pump breaks due to to much back pressure... With it being a mechanical fuel pump it's going to just keep pumping or trying to pump regardless =/

I'm going to call the race shop where I got the regulator and pump tomorrow and see if they have a return style regulator. If they do I'm just gonna go ahead get it and put it on and be done with it.

 

Just go get a stock pump and be done with it. It's PLENTY for what you've got. No sense dinking around with all that "chrome" crap, 99% of which doesn't fit right, won't last as long, doesn't work as well, causes more trouble than it solves, and within about 2 days of putting it on, "looks" FAR WORSE than anything else, because it shows up EVERY SCRATCH or blemish so clearly.

I have to count myself among the "function" crowd... if there were 2 identical parts sitting in front of me and one was chrome, I would pick THE OTHER even if the chrome was THE SAME PRICE. I count myself as, not easily distracted by shiny things alongside the path. (Dorothy....) I'm just not into impressing 8-yr-old kids in parking lots. And besides, my hood stays shut for MONTHS at a time; since I don't put unreliable "chrome" crap in there, I don't have to work on it. And the people who get spanked at red lights can't see it through my rear bumper anyway so what difference does it make.

 

Lol, most of the "chrome crap" is just as reliable as the non chrome. The chrome is just a coating, underneath that chrome it looks pretty much just like the cheaper ones without the fancy chrome. As long as you don't buy some no name brand chrome piece.

Judging a parts worthiness on whether it has chrome on it or not is ignorant. And just because you don't open your hood to show off your hard work doesn't mean no one does. My car is part a restoration project, part a pet project, part a simply have fun project. So I like for my stuff to look good when I show it off, whether it's to 8 year olds, 25 year olds, or 60 year olds.

I really need an answer to my pressure question though, within the next couple of hours preferably. The local race shop doesn't have a regulator with a return on it, so if I want to get one this weekend I will need to order it from summit with Saturday delivery soon enough to get it shipped out today.

 

You'll wind up paying more for a return regulator as the Carter pump. The Mallory, alone runs near $100, and you still have to buy the hoses and fittings.

Take a look at how close your fuel lines probably are to your headers. Bet you've got around an inch, or less, of clearance along several inches of fuel line. The return pump will keep fuel flowing through here and keep it from boiling (at least while the car's running).

I've had to insulate my fuel lines, not because it boiled while running, but because it would boil when shut down from heat soak. So I'd do both: insulate your lines and run a return style pump.

 

I know the price of the return regulators, yes they are around 100. But I'm willing to pay that. I just need to know if it's necessary or not due to the pressure that I asked about earlier. I'm not planning on using the carter pump, I'd rather pay more for a return style regulator. I would have to have a regulator with the carter pump anyway because it still pumps out to much pressure for the carb specifications. Granted, yes I could use a cheaper non return style regulator, but if I have to run one anyway I want the good looks with it.

 

This pump here says it only goes up to 6psi, but has no return

http://www.jegs.com/p/Edelbrock/Edel...46606/10002/-1

 

I already have the pump im going to be using. It is as i said earlier 6 to 9 psi. I need to know if that is enough excess pressure to cause a problem with a non return regulator set to 5 psi.

 

It's unlikely you'll see that max pressure at your carb, those are measured under ideal (ie. not on a car) conditions. I would run it with just a guage on it before installing a non-return regulator. I suspect it will be fine.

Return system would still be ideal.

 

Thanks naf, I'll try that for this weekend and just order a return style regulator from summit with regular shipping instead of paying extra for Saturday delivery. Which regulator do you think would be better?

I was looking at these:
http://www.summitracing.com/parts/HLY-12-841/
http://www.summitracing.com/parts/HLY-12-842/

They are basically the same thing except one has a note that it "Features idle bleed for precise control of idle pressure.". I don't really know what that is and if it's something that I should worried about having or not.

 

Ok, well I got the fuel pump finished without a regulator for now.

Pressure 4 inches from the carb is ranging from 6.5 to 8ish. So I'm still going to need to get a regulator probably, I'll just make sure to get a return style one.

The good news however is the fuel pump did indeed fix my hesitation problem. And it seems to have a definite power increase. So I'm guessing my old fuel simply wasn't supplying enough fuel. Now I just got to finish tuning the carb properly. I'm going to be doing an intake manifold swap this weekend so I'll work on tuning the carb more after that.

Oh also to note, I was running for about a full hour at normal running temps (fan kicking on and off occasionally, and running it hard down the road a few times during that), messing with the carb some, and didn't experience any vapor lock problems without the return line. I don't know how long those take to show up though.

 

Mine didnt vapor lock till i was sitting in traffic. after it cooled down started right up drove straight home with no problems.

 

Here you go.

Auggie

 

Hrmm... sorry maybe I missed something but what is this for exactly? Looks like a diagram of a fuel pump, but not really sure of the purpose of posting it lol.

 

This diagram shows that the large spring does the pumping and when the needle closes on the seat in the carb the pumping stops. Now if there was a fuel pump return line the pump would continue to pump fuel through the pump keeping the pump cool and averting vaper lock. I tryed two or three differant pumps and finley settled on the stock L69 pump for about $24. I have a warmed over 355 on the juice with a 750 Q-Jet and shift at 5700 rpm with no fuel starvation problems.

Auggie

 

I'm still not following. You are talking about a needle closing on the seat in the carb, but this diagram looks like it's just showing how a mechanical fuel pump works.

The rod from the crankshaft pushes down on the arm which lifts the diaphragm up sucking up the valve / needle whatever is and sucking the gas in. When the arm springs back out the diaphragm lowers pushing down the other valve / needle and pushing out the gas.

I don't see anything that would cause the pumping to stop...

 

When your engine is ideling fuel demand is very low so the needle and seat in the carb closes and the fuel stops flowing. As you know you can't compress a liquid so the diaphram in the pump stops moving. But if there was a way for the fuel in the pump to escape (return line) the diaphram would continue to move and pump fuel back to the tank. When the carb float level drops and the needle in the carb opens the fuel will be pumped to the carb. Thats why the return line is much smaller then the main fuel line.

Auggie