Every time i take my gas cap off after driving my truck for awhile, it hisses. It doesn't blow air out, so, wouldn't a vaccume cause the truck to run crappy because the fuel pump cant acheive full flow. Or, is this a normal thing?

 

Given you have the OEM fuel cap...

Check the vent line on top the tank, that runs to the charcoal canister....I'm betting it's plugged...

 

ok, thanks, ill check it. I dont think the fuel cap is OEM. it says napa or autozone on it.

 

Make sure it's a vented cap...

 

Well if found the vent tube last night, and sure enough a mud dobber had made its way in and clogged the end of it up. Cleaned it out, and little to no vacuum even if i take the cap off right after driving. Also, after ive driven it for it bit, it doesnt loose power.

 

So it's running better?

And yes...Vaccum in the tank aka Vapor Lock, will make a vehicle run CRAPPY...Even shut off....


Try sucking water through a straw, with your finger over the opposite end..

Same scenario...

 

it runs much better, "tanks" for the help

 

now another question is, my front tank has been removed since ive had the truck. Does the vent in one tank also vent the other tank at the same time? if so, ive only got 1/2 of the normal ventilation.

 

nope. separate vents for each tank

 

i know im a little late on this thread but i was wondering if the vent tubes connected to little black boxes on the pass side behind the radiator? and which end did you find the blockage? Did you have to drop the tank? I have an 88 250 with a 460 and am having similar problems.. truck runs awesome in neutral then when driving does the same hting as yours. I cant even put fuel in the front tank.. it just keeps tripping the shut off on the pump. the rear tank works fine it seams but i still have the power loss and the misfireing. thank you

 

What you described should be the same tube. Mine was different in that it was a swapped motor and the tubes were cut off a foot from the tank. Originally they should have run to some sort of filter box in motor compartment

 

OK This is a very old post, but how the heck is one to check the vent ON TOP of the tank? remove the bed? What should be the result if you blow on the canister end of the vent tubing, and which one is it? I think I have a clogged vent too. truck runs like ****, extreme vacuum build up in tanks. Thank You Mate.Oh I forgot 1994 F-150 4.9 Liter, dual tanks.