Fuel Cap / Pump Question
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Tuned
Joined: Apr 2019
Posts: 310
Likes: 50
Fuel Cap / Pump Question
After I bought my 77 F250 (w/ EVAP system) in 2017, I replaced the gas cap with a Stant locking gas cap which I thought was vented (I'll have to check that now). A short while after the swap, my mechanical fuel pump quit (so I thought) so I bought an electric pump and swapped the cap back to the original because I thought that I may have gotten the wrong cap and ruined the pump due to it pumping dry. Last fall before I put it up for the winter, I happened to notice that air was coming out of the old pump to carb connection while the truck was running (Haven't taken it out/ replaced it yet). I was thinking that maybe my pump isn't shot after all and I DID actually have the wrong cap which starved the pump causing loss of fuel to carb. Is it possible that a vented cap on a vented system would allow the pump to suck too much air and cause the fuel starvation issue? I plan on hooking the old pump back up to the tank, leaving carb disconnected and seeing if fuel comes out while cranking the engine over, but haven't gotten to it yet. I guess I also should note that the new locking gas cap just pushes on then clicks when the tabs engage....it's not a twist on type like the original. NumberDummy pointed out in another thread that a vented cap will not fit on a non vented system and vice versa. I assume that this pertains only to the twist on type of cap and not the type of locking cap that I bought. So, theoretically, is it possible that the new cap IS a vented cap and my mechanical fuel pump is NOT shot and that was my problem when the truck died? Sorry this is a little long-winded, I'm just trying to be sure I give you all of the information and I explain it as well as I possibly can, being that I'm no mechanic.
Any help would be greatly appreciated!
Any help would be greatly appreciated!
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
A vented cap on an evaporative emissons equipped tank wouldn't cause a problem. That would be similar to running around without a fuel cap.
As far as seeing air bubbles in the fuel supply, check the condition if all the rubber hose connections in the supply line. There's one at the tank, one at the fuel pump and, if your truck's a long bed, one mid-frame. These get old and crack, allowing the pump to suck air.
As far as seeing air bubbles in the fuel supply, check the condition if all the rubber hose connections in the supply line. There's one at the tank, one at the fuel pump and, if your truck's a long bed, one mid-frame. These get old and crack, allowing the pump to suck air.
Thread Starter
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Tuned
Joined: Apr 2019
Posts: 310
Likes: 50
A vented cap on an evaporative emissons equipped tank wouldn't cause a problem. That would be similar to running around without a fuel cap.
As far as seeing air bubbles in the fuel supply, check the condition if all the rubber hose connections in the supply line. There's one at the tank, one at the fuel pump and, if your truck's a long bed, one mid-frame. These get old and crack, allowing the pump to suck air.
As far as seeing air bubbles in the fuel supply, check the condition if all the rubber hose connections in the supply line. There's one at the tank, one at the fuel pump and, if your truck's a long bed, one mid-frame. These get old and crack, allowing the pump to suck air.
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Tuned
Joined: Apr 2019
Posts: 310
Likes: 50
Doubtful. It ran like a champ with the mechanical pump before it died one day and it has been running great since I installed the electric pump. Thanks for your insights! That'll be something to keep in mind down the road.
More Turbo
Joined: Feb 2020
Posts: 633
Likes: 84
From: AL
I’d been battling the same thing. 77 F150 non evap, original gas cap with vent. I cleaned it up, made sure it was not seized( or so I thought). Noticed about 60-70 miles into a tank, the truck just acted like it didn’t want to idle at all. Went to put fuel in it one day and it had built so much pressure it sprayed fuel when I cracked the cap. Ran and idled great after that. New cap in bound.
Mountain Pass
Joined: Feb 2020
Posts: 118
Likes: 2
Gas Caps
Unless you are running an O.E. Vented cap you need to check it. I went through a couple of Auto Parts stores caps before finding my issue....the parts are junk! Mine was not venting at all and kept fighting what I thought was a bowl/needle/seat issue. One day the old truck had sat for a few days and I knew things were right when I left it. Popped the hood to look at something and there was fuel on my intake. I knew it was not the carb and it was HOT outside. Thought about it, went to my tank and so much pressure I could NOT get the cap loose. Waiting until after sundown and it let so much pressure out the tank popped. While I do not recommend this, I personally took a tooth pick, wedged it inside the vent holes of the cap mechanism and twisted it. Basically lodged the vent open. Problem solved. The vents work for a short period and then fail. Anyone with a known good brand speak up. This can give you a bit of hell.
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