Fuel issues which no one can figure out...

[Stoutstanding]

Hello all,
First time posting to this website. I have a 1972 GMC sierra grande 2500 that I have fuel delivery problems with. I keep losing prime on the inlet side of my pump. The truck has a 1997 454 with an Edelbrock electric fuel pump located on the passenger side frame (beneath the tank). Here is the series of events that led to my problem: My gas cap was loose and spilled fuel so I replaced it with a new vented cap. After driving it for a couple miles I vapor locked the system. I unscrewed the cap, relieved the vacuum and reprimed the pump. Since then, I have had constant issues with my pump loosing prime on the inlet side. I thought I may have ruined the pump, so I got a new one from Summit. That didn't help. I then decided to change all the rubber hoses and filters in case they were the issue. I also checked the sending unit, the lines, sock filter and tank to make sure it was clean and had no air leaks. Everything checked out fine. Tried driving it, but lost prime again and had to siphon the line to get fuel. I have talked to mechanics and done extensive research, but I have no idea what is causing this. My tank is in the cab, behind the seat. The sending unit goes through the cab floor, attached to a rubber hose, then the 100 micron filter, rubber hose, pump inlet. There is no return line nor a vapor line on the truck. The outlet side of the pump goes to the a filter then the carb. If anyone has any experience with this type of issue please let me know, I am at wits end! Thank you very much.
Chris
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[LongBox]

Change the cap. Sounds like it does not allow the tank to breathe in to eliminate the vacuum. Old mechanics adage: "When something goes wrong, always check the last thing you worked on first,"

[Stoutstanding]

Originally Posted by LongBox

Change the cap. Sounds like it does not allow the tank to breathe in to eliminate the vacuum. Old mechanics adage: "When something goes wrong, always check the last thing you worked on first,"

I put the old one back on after I vapor locked it. It's so loose that there is no vacuum issue with it.
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[Grumpy old man]

Remove the fuel filters and try it again , any special reason your running one in front of the pump and another after the pump ? I would just install a regular metal filter under the cab and replace the pump to carb line with steel tubing and be done with it .is there a reason a standard mechanical pump won't do the job ? I mean Hey , it's a 2500 your not going racing , If one filter won't do the job of keeping the fuel clean it's time to clean the tank .

[Stoutstanding]

Originally Posted by Grumpy old man

Remove the fuel filters and try it again , any special reason your running one in front of the pump and another after the pump ? I would just install a regular metal filter under the cab and replace the pump to carb line with steel tubing and be done with it .is there a reason a standard mechanical pump won't do the job ? I mean Hey , it's a 2500 your not going racing , If one filter won't do the job of keeping the fuel clean it's time to clean the tank .

The truck had both filters on it when I bought it, so I kept the setup the same since it had been running great. The filter before the pump is a metal type, 100 micron (pump requires min 100m filter). The block has no spot for a mechanical fuel pump, that year was fuel injected and there is no plate where one would usually go. Losing prime before the pump is usually a break in the line or something that allows the fuel to stop being gravity fed. That is the part I can't figure out.
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[geezer#99]

You could install a fuel regulator that has a return line back to the tank. The pump could be deadheading and heating up causing a vapor lock.

[toolboxchev]

Originally Posted by LongBox

Change the cap. Sounds like it does not allow the tank to breathe in to eliminate the vacuum. Old mechanics adage: "When something goes wrong, always check the last thing you worked on first,"

Thats one I am adding to my list of tricks.

[Wrenchbender Ret]

Have you removed the sending unit? The tube may be cracked or rotted out on the inside of the tank.

[B. W.]

My first guess is what Longbox said but after reading Wrenchbender Ret's post he has a good point. If all else fails I would replace the cap with a rag & take up smoking!

[HO455]

Maybe you could explain in more detail what is happening to the truck. What are the symptoms that are causing you to believe it is vapor lock?