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On a 69 350/350, there is a line that returns to the fuel tank either directly from the fuel pump or from the fuel filter. (It's not nearby, and my memory is sketchy.) Anyway, the question is this: what is the function/purpose of that return hose? Mechanical or environmental? Additional question: (not necessarily for extra credit)--I know that the LT-1 cars of 70-72 don't have those, and that's one of their identifying features. I seem to recall something in the archives, but can't find it, that the difference was related to the quadrajet vs. the holley. Can you help me with this one?
Thanks!!!!
Answer to function/purpose of return line is for fuel vapor according to the Figure 10-fuel line installation illustration in my Chevrolet Service News of March 1970 which supplements the 1970 Chevrolet Service Manual which includes the 1969 Corvette but not the 70 Corvette. My 68 327/350 has the return line coming from the fuel filter and my 70 has the return line coming from the fuel pump.
it relieved the pressure between the fuel pump and the carb after the engine was shut off, and if you remove this return line you need a different fuel pump because of the increase in fuel pressure. this return line is a calibrated leak to the fuel pressure side of the pump. i think they were mostly on the Qjet equipted engines. also when this return line is in the system you need the Qjet fuel filter with the check valve or the carb float bowl can be drained down making for hard starting if the carb used a "windowed" needle seat.
if the line is connected to a return line type fuel pump it will pump gasoline out on the ground because it is to be connected to the fuel tank so the excess fuel go back into the tank.the line recirculate the excess gasoline to the tank
Yes, the line is designed, primarily, to return liquid gasoline to the tank and not vapors. It's often referred to as a "vapor return line" but it's really a FUEL RETURN LINE. It's function has absolutely nothing to do with emissions control. Evaporative emissions control (and, "vapor return lines") did not begin for Corvettes until the 1970 model year with California emissions.
Also, beginning with the 1970 model year, the external fuel filter and attached fuel return line were eliminated from the Corvette and the fuel return line was incorporated into the function of the fuel pump. The exception was 1970 through 1972 Corvettes with LT-1. These models had no fuel return line, although 1970 with California emissions and all 71-72 did have the EEC's "vapor return line".
Although some folks think that the fuel return line was used for Q-Jet equipped cars and was not used for Holley-equipped cars, that's not exactly the case. There was one Holley-equipped car that did get the fuel return line (from the fuel pump). Which one? 1971 LS-6.
Clem, can you explain a little more regarding the Qjet filter check value and "windowed" needle seat. Is the check value incorporated into the stone filter in the carb?? What does a "windowed" needle seat look like compared to a non windowed seatr needle value?? I am not familair with these and I have a 69 327 that is giving me fits with the fuel bowl going dry after shut off.
I have been under the impression, and I can't recall where I gathered it, that the fuel return line (on the passenger side) was to aid in reducing vapor lock. It returned a portion of the fuel back to the tank where it would cool before being drawn back to the fuel pump.
The vapor return line (on the driver's side) is involved in controlling the evaporative emissions, as you point out. Nice trivia question, by the way.
Yes, one of the functions of the passenger side fuel return line used on most 68+ Corvettes is vapor lock control. I think that the line has other functions, though, and vapor lock control might not even be the primary function.
In 1970, the line's origin changed from the GF-432 filter to the fuel pump. However, I don't understand enough about the 70+ "long-cannister" style fuel pump's "operational nuances" to understand the reasons for the re-location.
A fuel bowl going dry in a Q-Jet is a notorious symptom of the soft plugs in the bottom of the fuel bowl section of the carb leaking. There are FOUR soft plugs in the bottom of the fuel bowl of Q-Jets----2 "large" ones and 2 "small" ones. These can be accessed by removing the throttle body assembly from the carb. The soft plug area then needs to be SCRUPULOUSLY cleaned, roughened with a file, and treated to a generous application of a gasoline-impervious epoxy like JB Weld. After curing, it may be necessary to file the epoxy in certain areas so that the throttle body can be re-installed properly. If you do it right, the fuel bowl will be leak-free. "Forever".
This doesn't mean that if you don't start the car for a few months that it will start right up with a full fuel bowl. Gasoline is still a VOLATILE compound (or, it wouldn't work) and it will EVAPORATE from the fuel bowl, given adequate time.
I don't know the nuances of fuel pump construction either. Seems to me the vapor lock function would work as well with the return line from the fuel pump as from the fuel filter. The area of high risk of vapor lock is the low pressure side (read suction side) of the pump. Keeping the fuel cool(er) reduces the chance of fuel boiling, which is what vapor lock is.
The pressure side of the fuel pump is far less likely to experience vapor lock because the boiling temperature of the fuel increases under pressure - just like the coolant in the pressurized cooling system.
Do you have any idea what other uses the fuel return line could serve?
At the moment, I don't. I recollect, vaguely, hearing some explanation of the system which, at the time, seemed very authoritative and sensical. The problem is, I forgot what it was. Part of it involves something to do with the Rochester carb. For 68-69, the system was not used on Holley-equipped cars. For vapor lock control, alone, I would think that they would have used it across the board. It could just as easily have been included in the fuel system of those cars as the others. The reasons that Clem described may have been at the root of it.
Thank you for the suggestion Joe, I am removing the carb tonight to plug the heat cross over under the carb and will now also be checking the fuel bowl plugs.
This condition is aggravated on '68 300hp and 350hp Q-Jet engines (and on '66-'67 300hp and '66-'69 427/390hp engines) by the "hot-slot" in the carb pad under the front of the carb, which eventually softens the lead bowl plugs in Q-Jets, which then leak raw fuel and can cause engine fires - on the '66-'67 Holley applications, the heat can distort or crack the baseplate. This design was the cause of the biggest carburetor safety recall in GM history in late 1969, and was abandoned. It's a good idea to plug the two holes that allow hot crossover exhaust gases to pass through the "hot-slot" in the carb pad, for carb longevity and safety, as shown in the photo.
Something to mention, as there seems to be a lot of new people asking questions on this board lately, that may not have a whole lot of mechanical experience. IF someone here decides to change the fuel pump to one without a fuel return line, make sure the return line to the tank that you remove is not left open, but is raised up and plugged off securely. The fuel tank can siphon back through the return line. The same as if you had a split in the line at the fuel pump or a bad fuel pump. It CAN and HAS emptied entire fuel tanks, on many garage floors. Just something to be aware of. FWIW
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