there is a "return" line connected from, i suppose, the space above the top of the fuel pump diaphragm to the top of the gasoline tank (rear pass side top). it looks to me like its main purpose is in the event of a pump diaphragm failure, fuel would not exit the vent normally located above the diaphragm on earlier model fuel pumps and hit the ground potentially causing a fire. rather it would make its way to the fuel tank instead, but at a comparitivly low flow rate.

if this is its true function, then it is not really a "return" line in the sense that fuel is always flowing, but only for a failed pump case. looking at a typical return fuel line on vehicles with fuel injection from the factory, the line also returns to the gasoline tank down low.

is this correct?

 

In a word, no.

 

the pump supplies more gas than the carburator needs. the excess is sent back to the tank through the return line.

 

Basically it it supposed to stop the fuel boiling in the line to the carb by constantly flowing cool fuel to the carb. It is bled off through a small orifice.

 

What kind of pump is this?The stock pump return is on the bottom. ???

 

So does that mean that without a return fuel line you are ALWAYS wasting gas and getting worse gas mileage?

Is the excess gas burned off or what?

 

It means that if your floats aren't working right, and the pump gives you too much fuel or pressure without a return line it would spill out from the vents like a geiser. Seen it happen several times with older carbs that had stuck floats.

 

So if the floats are working properly then there is no wasted fuel loss without the return line? I have never had any performance issues, but have always wondered if my fuel economy is affected by the fact I have no return line.

 

Generally not, but that really depends on how much pressure your pump creates and how solid the carb is. If it's not pushing more fuel through because the pressure is high, then yes, you're fine. You should have a factory return line on the frame, they usually just get capped off when people change carbs or fuel pumps. Wouldn't hurt to hook it back up.

 

My understanding is that the mechanical fuel pump flows more fuel than the carb needs at idle, so without a fuel return line you might be wasting fuel at idle. I am not really 100% sure, but I think it is much less of an issue at higher rpm. We have also been told many times that the fuel return line keeps the gas cool and helps prevent vaporlock.

 

It would be a good idea to test your fuel pressure at the carb. The rent a tool at Autozone that you install in-line, and it tells you what PSI your fuel is at. Most carbs cannot withstand more than around 7PSI.

 

My first question, is this the original engine? Sounds like you have an engine or at least a fuel pump out of a boat. On a boat that is a vent for a failed pump diaphragm.

 

OK that would make sense.

 

i've heard that word before, got any others? it just makes no sense to send the "return" fuel back to the top of the tank. that's just dangerous. (edit: it's dangerous because it discharges to the top of the tank without any other devices to prevent static charge from lighting the fuel off) so how does the pump know how much to "return" anyway?
the example is a stock 74 L82; other years may be similar; yes the connecting tube is on the bottom of the pump, but could it be routed internally to the top of the diaphragm?
yes, is for an original 74 L82 engine. but what you said is the only thing that makes sense to me too. i may have to cut open a fuel pump to find out i guess unless someone else has already. thanks

 

Since you've already got it all figured out and concluded that the factory set up is dangerous, it must mean that the entire GM engineering team was a bunch of moronic irresponsible baboons. I guess all the other replies explaining how works was wasted effort.

Well at least you said 'thanks'.

 

I Can Open The Fuel Cap And Watch The Gas Flow Back Into The Tank. LIKE ITS MADE TO DO.........

 

I am sitting here reading this gettina a nice chuckle...

OK, Fuel return line has more that one unnction, as someone above said, it keeps a fresh, cool supply of fuel supplied to the carb to help ward off fuel boiling anf vapor lock. It also provides a return path for excess fuel pressure generated by the pump and not needed by the carb and thereby helps provide a more stable fuel pressure. This allows for lighter and therefore more accurite needle valve settings in the carb and a more manageable fuel level in the bowl.
As for the static buildup "Dangerous" fuel management, thats pretty much BS. In order to build a static charge, and thereby present a possible spark, the components involved (tank, line, pump etc..) would need to all be at a different potential. In a car they are at the same potential, the tank is grounded to the frame as is the engine, pump, fuel line etc...
No potential difference, no spark, no Boom.

If you have a return line you should run it, it is part of a lot of things your car coes not absolutely need, but like BASF, it makes it work better

my humble .02

 

The only reason I don;t run my return is because my electric pump has a feature that shuts it off when the fuel consuption drops off. Meaning, if the bowls in the carb fill and floats curb the fuel flow, the pump will shut off until it's needed again. VERY handy when it hasn't been started for a while because all I have to do is turn the key to ignition and wait for the pump to stop. Then I know it's good to go.

If I didn't have that electric I would put the return line back into action.

 

reminder: the title of the thread indicates the intent to clarify the function of the return line, that's all i'm trying to do. i picked the position that seemed most defensible to pose the situation. i expected some to disagree. in fact, i don't have it all figured out, and i know that you didn't really mean to speak on my behalf with flawed logic.
i think that my comments generally support that the original designers are not "a bunch of moronic irresponsible baboons", however, a good deal has been learned on the matter of flammable fluid flow in the past 30+ years. btw, some folks may be able recall issues that chevy and ford have had with fuel tanks and fuel delivery systems in recent history.

and no posts that i've read have been wasted effort, i read them all. thanks for your help
First testimonial supporting this position!!! my vehicle doesn't do this, but runs fine. how does the factory fuel pump know/determine how much fuel to return? There must be an orifice or some special design feature since no regulator exists.

am glad this is getting some chuckles. i agree with your comments for the case for a real return line. i've read some of the aftermarket system descriptions which read essentially the same. but how is a factory fuel pump designed to handle the return flow rate, orifice, other? the aftermarket seems to prefer "back-pressure" regulators.

i suggested that if the fuel returns to the top of the tank, falling to the top of the fuel tank level, that just seems dangerous. that comment is based on current process industry standard handling of flammable fluids. it does matter whether the transporting lines are grounded, but it also matters how fast the fluid is moving, how fast the fluid can dissipate a static charge buildup, and how far the fuel free falls before hitting a grounded surface. i agree that the factory return line as functioning normally (whatever that is) has not presented any "lighting off" issues.

Durango_boy, did you just plug or cap the return line at the tank? or near the fuel pump?
Thanks again

 

The stock system uses an orifice in the pump and a smaller line for return than the supply line. No moving parts, no springs, no calibration or maintenence required.
As far as your concernes afout the static, forget about it, the fuel is going to fall a maximum of what? 14"? And all of that is inside od a grounded metal fuel tank, no place to develope a static difference of potential and it is not at a delivery rate high enough to even think about.
It is safer than the delivery systems at gas stations, they even have a Static Warning at the pumps and there are no problems.
One last thing, nearly every car on the road has the same type of system.