Fuel Pump/ Vapor lock issues

[dph42]

Good evening all and Thanks for the add! I need a little help from some experts. I own a  pretty decent 1969 Camaro that I have had for about a decade. Regular "Bow Tie"with a 350 and some mods. It has been sitting for about a year so I drained all the fuel out, fresh gas and started it up.  After a bit the filter went dry and it died.  Replaced the pump with a Carquest aftermarket pump and could not stop a leak from the center of the line. Chinese junk I Imagine.  In the mean time I am working on a 1970 that I plan to sell that is having the same problem. During my research I have found that the fuel is boiling, Vapor Lock! Anyway back to my '69, it has a blocked off return line that I plan to hook up with a 3 line pump as soon as I replace all the rubber splices. That is supposed to cure the vapor lock.  My question to you is do any of you have one with a return line? I have a couple of other shells and they are all single fuel line cars. I can find nothing about '69s having a return line.  I even went through my assembly manual today and no mention of a return line. All the 3 line pumps I look at  are mighty tall and I am afraid I may have clearance problems I would appreciate any input You might have. I don't know what I am gonna do on the '70, I already installed heat sheath on the last section of fuel line next to the headers and still the same problem, runs for about 8-10 minutes and then you can watch the fuel boil in the filter and there is no return line.
Thanks in advance for any help you can give!
David

[Randy Shanks]

David,
I have a '69 SS w/ 396-325 hp that has a return line.
I think the return line was used on the 396 w/ a quadrajet carb.
Look on page 279 of your AIM
The filter with return line is really expensive.
Good Luck
Randy

[KurtS]

On Camaros, all 69's with Quadrajets have dual lines.
http://www.camaros.org/diffs69.shtml

[Kelley W King]

Are you sure it is not just bubbles? Your fuel pickup sock might be bad. If you blow in the fuel line to the tank you can tell.

[dph42]

Thank you all for your reply's!

David,
I have a '69 SS w/ 396-325 hp that has a return line.
I think the return line was used on the 396 w/ a quadrajet carb.
Look on page 279 of your AIM
The filter with return line is really expensive.
Good Luck
Randy

I will check my manual when I get home tonight thanks for the info.  I have seen those filters on websites before I am sure I can find them again.  In order to use the return line must I use that filter or can I run the return right off the pump?

Are you sure it is not just bubbles? Your fuel pickup sock might be bad. If you blow in the fuel line to the tank you can tell.

I am about 99% sure.  It looks just like the 70 I am working on here at work as soon as the pump reaches about 120 degrees it starts, big bubbles first and then smaller until nothing but vapor.  There is a video on YouTube that shows exactly what mine are doing, I will try to post a link. You mentioned blowing in the line to check the sock, How can I tell by doing that if you don't mind sharing.

https://www.youtube.com/watch?v=cadNfSNi_Oc&t=142s

Thanks again for all your help!
David

[KurtS]

I'm not convinced. If that was the case, every old car would have vapor lock all the time. And if you live in AZ, SOL?
I see it, it's real, but what's the root cause? Bad fuel pump, bad lines, bad pickup? I can think of several things that could cause the restriction or cause low pressure which *allows* the vaporization.

[X33RS]

Living in AZ and driving our classics daily, fuel supply is something to pay attention to.

The return lines are a wonderful thing.  If a car is factory equipped with one I definitely use it.   It does aid in keeping the fuel a little cooler, constantly circulating and not sitting stagnant in the fuel line possibly absorbing heat.   I run stock mechanical pumps on our stock or near stock vehicles.  Care has to be taken to make sure the lines aren't laying on a heat source and have ample space for some air circulation.  I see some pretty cobbled up fuel lines on classic cars.

Our 69 Z (obviously) was never built with a return line, however with all correct factory fuel lines, new sock in the tank, new stock pump, heat cross over open and functional for factory divorce choke etc.....  My wife daily drives it, and we reach ambient temps in the 105 range during the summer months.  I've never experienced any hint of vapor lock with this car.  We always run 91 octane in it as well.  I do run a 1/4" thick gasket under the carb just as an attempt to help with any possible heat soak, even if it's a marginal attempt. 

Gas is another point worth mentioning.  The lower the octane you run, the more susceptible you are to heat soak and vapor lock issues.  The flash point of the fuel is much lower.   The gas formulations are also quite different today than they were even 15-20 years ago and tend to lend themselves more susceptible to heat.   Because of this a car that is marginally getting by could possibly see heat soak problems off and on depending on the station you use or the octane you pump in.

There are ways to combat it.  Some will block the heat cross over if living in warmer climates, which does help to keep the carb cooler.  I like to run cooler thermostats as well.  The cooler I can keep things under the hood as a whole, the better off I am.

Newer cars never see the issue because they run fuel systems that push fuel from the back with pressure at 58 psi or higher.   Electric fuel pumps can be retrofitted to classics to help combat the issue, but frankly I haven't really found the need for that as long as everything else is functioning properly.