I have a 1969 cutlass supreme. It had a 350 4 barrel from the factory. When I got it, it had a one line aftermarket fuel pump. I tried to get another one, but it was back ordered. I bought a factory style fuel pump for the 455 that's in it now and it has the return line port. My car doesn't have a return line. Was it supposed to? Will it really affect me if it doesn't? I was going to plumb a new line today, but saw nothing obvious that was sticking out of the tank sending unit. One supply line and two into one vent that points straight up. Am I missing anything and will it matter? I'm not sure if I am risking vapor lock. Thanks

 

Not all cars had a return line, the easiest thing to do is block the return line on your new pump. If your car didn't vapor lock before, it won't vapor lock now.

 

You're always risking vapor lock if you drive in heavy traffic on hot days without a return line.
That's one reason why all cars have had return lines for years.

Back in 1969, however, there was no such consensus, and so your car was not equipped with a return line when it was made.

Others will know the exact variations in each year, but, in essence, in the 1969-'71 range, bigger engine, higher power and air conditioning all increased the likelihood of having a return line, while smaller engine, less power, and no air conditioning decreased the likelihood.

Odds are you would never have a problem as currently configured, but adding a return line, in an engineering sense, would be an improvement.

How motivated are you?

- Eric

edit: Eric was more succinct.