Wasn't able to find any definite answer anywhere I tried looking, and hopefully this is the correct forum to ask...
I was just curious if ALL 1971 Chevelles have (or came from the factory with) a fuel return line on them?
Mine is plugged (capped off) because it has an aftermarket pump which doesn't have a port for use as a return. I don't recall any of my other Chevelles having one, but it's been quite a few years since I had one, so....
Mine has one, but I don't think it's a return line :noway:, but rather a line going to the front mounted charcoal canister. I don't think any of these carburetor equipped cars had a true return line like today's fuel injected cars...
My car also has the vent box mounted behind the rear seat...
Hope this helps,
If your vapor lines are capped and your charcoal canister is history, make sure you have a vented gas cap. Stock with vapor lines and canister was nonvented. I used to get a massive vacuum suck when I popped the cap for fuel till I figured that out. Don't know if it would ever be enough to collapse the tank, but I imagine it was also making it harder for the pump to draw fuel.
First Guess: Just the ones factory-equipped with A/C.
1/4" tubing, considered a VAPOR RETURN rather than the sort of return used with a fuel pressure regulator where the return line is flowing nearly all the fuel back to the tank.
The OEM vapor return is typically restricted in the fuel pump to a .060 orifice, or near that size.
Primary purpose of the OEM vapor return system was to reduce tendency for vapor lock of fuel pump due to excessive under-hood temperature.
You may be on to something Schurkey. My car is factory AC car and I also found this replacement pump just now...which definitely has a return/vapor line on it.
First Guess: Just the ones factory-equipped with A/C.
1/4" tubing, considered a VAPOR RETURN rather than the sort of return used with a fuel pressure regulator where the return line is flowing nearly all the fuel back to the tank.
The OEM vapor return is typically restricted in the fuel pump to a .060 orifice, or near that size.
Primary purpose of the OEM vapor return system was to reduce tendency for vapor lock of fuel pump due to excessive under-hood temperature.
Rather than vapor lock issues, I think the vapor lines are primarily to reduce emissions into the atmosphere from evaporation, which is why the gas tank has a nonvented, i.e., sealed cap. The system works by sucking fumes out of the gas tank, through the vapor canister, into the intake, according to specified parameters (mostly on startup I think). I've read somewhere that a parked '65 Mustang emits more pollution just sitting there than a new Honda does while running.
Not sure about other year Chevelles. Here is what I do know about my 1970 LS-5/ 4-speed. El Camino. The " KEYWORD for 1970 is the " TAILPIPE ". The 1/4" line that is attached to the long canister fuel pump is a VAPOR line and has nothing to do with vapor lock.
Here is how a 1970 LS-5 Emission system works. First " NO " vapors of any kind are sent into the Atmosphere. Engine oil vapor fumes they have= TWO EXITS,
The right valve cover has a hose to admit the fumes into the air cleaner and they are BURNT OFF and exit the TAILPIPE. The left side valve cover has the PCV valve. The left side fumes are sent into the carb. base, BURNT OFF and exit the TAILPIPE.
Fuel tank vapors. The 1/4" line. 1970s had many different FUEL VAPOR systems: Calif. cars had the Charcoal canister in the engine bay. Some had it behind the rear seat. Some had the fuel tank vapors sent to the engine fuel pump, Then they were pumped into Carb. and BURNT OFF. How about the " AIR" systems on the solid lifter engines.
My one owner LS-5 /M-22 El Camino has almost nothing as far as emission equipment, Just the vapor line to the fuel pump, WHY?
The emissions standards for " TRUCKS " were not covered by the same standards as Automobiles.
The GENERAL was trying lower emissions by any means possible, Trying many different ideas. WHY all the different emission items on all the cars?
The Government continued to DROP the standards. The Auto makers were trying to keep up. The government became so HARSH.... The auto makers said: To hell with all of this, We will build small V-8s & six cylinders and forget the " High Performance Market ".
That is what I have put together about my 454 emission equipment. I have it close not perfect. I ordered my 454 in Feb. of 70, It was built in March of 70. As of today, I have owned it for 50 years/ 4 months / 2 days.
Trivia: 1970. 454 and others had a CLOSED vapor & fume system. All fumes & vapors were burnt off and went to the atmosphere via--- TAILPIPE.
Bob
Of course it "had to do" with vapor lock. The fuel pump was equipped with a "vapor return" system so that vapor could be blown back to the tank along with a minimal amount of liquid fuel. That minimized the vapor reaching the carb.
Fuel tank vapors. The 1/4" line. 1970s had many different FUEL VAPOR systems: Calif. cars had the Charcoal canister in the engine bay. Some had it behind the rear seat. Some had the fuel tank vapors sent to the engine fuel pump, Then they were pumped into Carb. and BURNT OFF.
Mistake in direction. The vapors did not pass from tank to fuel pump to carb. They were pumped from fuel pump to tank, preventing (or at least reducing the amount of) them from getting to the carb. This is how they combated "vapor lock"...by providing a return path for the vapor to go back to the tank. The plumbing was via 1/4" tubing and hose, but the fitting in the fuel pump that connected to the 1/4" plumbing was heavily restricted--perhaps .060 orifice. Liquid fuel volume down the "vapor return" plumbing was limited, while vapor passed easily.
AIR has nothing at all to do with fuel tank venting, or the vapor return system. It reduced CO and HC in the exhaust by providing additional oxygen (air) to the hot exhaust stream; the unburned and partially-burned exhaust could complete it's combustion. Makes a mess of fuel economy, and the exhaust manifolds become much warmer than is good for them.
Again, the vapor plumbing was a RETURN back to the tank, it didn't provide a path for vapor to get TO the fuel pump.
Tank venting to equalize pressure (and control hydrocarbon emission when the car venting system was equipped with a charcoal canister) was entirely separate from the vapor return system.
Just another thought: Big Block Chevelles that had the factory 3/8" fuel line, Or NO VAPOR line,, 2 line fuel pump. I guess they did not vapor loc? TRIVIA: When I built my FAKE/ FRAUD-1970 SS 454 Sta. Wgn. To make the engine bay LOOK like a real LS-5, I installed a long canister fuel pump. To make the fuel system match my Real " LS-5 fuel system".
I found a piece of 1/4" brake line about 6"// 8" long, smacked the end shut, With a small 1/4" piece of fuel hose, I attached it to the long canister fuel pump and shoved the 6" piece of brake line into the frame beside the factory fuel line. Now the fake vapor line is beside the O.E.M. fuel line and it all looks FACTORY.
I am guessing my long canister fuel pump has the "return line"? Blocked off. With the 1/4" line blocked off, I have driven my FAKE SS 468 over 46.000 miles on very hot days in Goodlettsville Tenn. with the A/C operating, Its been 13 / 14 years and no fuel issues yet.
Bob
Just install a tee on the supply line to the carb so there is continuous flow back to the tank. The closer you can mount the tee to the carb the better..
A return line is almost mandatory with todays ethanol blended fuel since it attracts moisture..
Just install a tee on the supply line to the carb so there is continuous flow back to the tank. The closer you can mount the tee to the carb the better..
I have absolutely no idea how return plumbing has anything to do with ethanol or moisture.
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