High Vacuum in Gas Tank

Gilleys 59 willys

Bigger Hammer
Jan 18, 2020
32
90605
First Name
Gil
Willys Model
Wagon
Willys Year:
1959
Hi all. I bought my Wagon not running. Steel tank was rusted through. I replaced the Stock Steel Fuel Tank with a Poly Tank. I also got a new Gas cap. I found the Fuel Pump to be Bad. I wanted to replace it , so I sealed the disconnected line at the Fuel Pump. After I replaced the Fuel Pump, I opened up the line and Fuel came shooting out like it was being pumped. I re-sealed the line and took off the Gas Cap. Massive suction. So then I connected the line to the Fuel Pump. Then I put the Cap back on tight. My wagon started right away, but Gas started to shoot out of the of the Carb. Went back to the Gas Cap and cracked it and got the same Vacuum sound. With the Gas cap loose, started it up again. Still shooting fuel out of the top of the Carb. So, to me , the Carb. is shot. But my question is : Is there supposed to be that much Vacuum in the system? Does the Fuel cap need to be a ventilated one? Thanks for any input.
 

dipthongdude

Gear Grinder
Mar 14, 2020
10
Brick NJ
First Name
Jonathan
Willys Model
Wagon
Willys Year:
1959
I swapped out the old steel tank for a Poly tank. Put on a new "vented" gas cap. Noticed the fuel gauge wasn't going down as usual, when I looked at tank, saw it partly collapsed. Took off gas cap, yes, heard the vacuum, and the tank resumed it's shape. It was a vented cap, but, needed one psi to pull air in. Obviously with Poly tank, that's too much. I just was able to defeat the restriction, and allow free flow of air into tank. All is well. Have no idea about the gas shooting out of carb, but yes, sounds like it need work.
 

dipthongdude

Gear Grinder
Mar 14, 2020
10
Brick NJ
First Name
Jonathan
Willys Model
Wagon
Willys Year:
1959
I defeated it on mine sort of accidentally (I mean having no idea what I was doing, although it was my goal to defeat it!). There was a yellow plastic thing right in the middle on the inside of the cap. Took a screwdriver and pushed around at it until it seemed like something snapped. Anyway, it was then obvious that there was no restriction.
 

RandallN

Bigger Hammer
Aug 25, 2019
113
northern arizona
First Name
Neil
Willys Model
Pickup
Willys Year:
1963
OK I was in Phoenix, the hottest place on the planet, and decided to use a Non-vented gas cap to save on fuel evaporation.
elec. fuel pump.
You definitely need a vented gas cap!
Furthermore, when you put in that poly tank you might think about placing some kind of stiff card stock or thick plastic strips between the tank and those two metal straps that hold the tank in place.
Else your tank will wiggle enough to possibly wear a hole in the plastic.
Those are Toyota Mirrors. God Forgive Me! :)
DSCN4019.JPGWell, the vapor lock that resulted in my plastic gas tank took out the external
 

RandallN

Bigger Hammer
Aug 25, 2019
113
northern arizona
First Name
Neil
Willys Model
Pickup
Willys Year:
1963
Awww Heck. When I stuck that picture in it cut it in half. It was supposed to say:

Well, the vapor lock that resulted in my plastic gas tank took out the external elec. fuel pump.
 

RandallN

Bigger Hammer
Aug 25, 2019
113
northern arizona
First Name
Neil
Willys Model
Pickup
Willys Year:
1963
Ok one more thing. I had one of those old cylindrical frame-mounted electric fuel pumps. I took Jeep out for a longer than usual trip to Tucson and it died about half way there from fuel starvation. I figured out what it was. That old fuel pump had so much suction that before the fuel would no longer flow the top and bottom of my new plastic gas tank were kind of "sucked in" so much that they never went back completely to a flat rectangle. A couple years later I had to remove the tank and re-seal around the sending unit and put it back on because it started to leak all over the ground at a RaceTrak station when I filled it up all the way up to the top of the inlet tube. It still has that curvature to it that you'd never have with a metal tank. If I had it to do over I would have got the metal tank, even though it was twice the cost, but I didn't because it didn't have the bottom drain plug like the original one had. So from my experience I do not recommend either plastic gas tanks or un-vented gas-caps.DSCN2246 (2).JPG
 

dipthongdude

Gear Grinder
Mar 14, 2020
10
Brick NJ
First Name
Jonathan
Willys Model
Wagon
Willys Year:
1959
I agree re: Poly tank vs. Steel.... My steel tank had a leak, I probably should have seen if it could be fixed... or invested in a new steel tank. The poly tank is working fine now, and I don't expect any further trouble, but having it partly pancake on me because of "vented" cap, plus having some difficulty getting the sending unit sealed up so it wouldn't leak were a bit disconcerting, and I didn't like having to install the tank twice (because of initially not getting sending unit sealed well enough)...
 
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