I just replaced the two 3/8" rubber fuel hoses on my old truck, the one that goes between sender and frame, and the other that connects the two long runs. Wonder if gas caps can clog and stop venting well. Reason I ask is that when my tank nears empty, it REALLY builds pressure and nearly blows the cap right off after you turn it. Can the vent stop working on these caps??

 

Short answer...yes.

 

Is it an original or aftermarket cap?

 

Not sure if it's original or aftermarket. It says "pressure" and "vacuum" on it. Looks old.

 

Oh ok just wondering, I just bought a locking vented Stant cap to replace my regular vented cap. The regular non-locking cap was Stant brand and it seemed to work just fine, didn't have any problems. Yours must be an original cap.

 

Yours builds pressure and almost blows off? Mine builds suction pressure and gasps/sucks air when I pull it off. I've heard of tanks collapsing so I rarely run it past 1/4 of a tank.

 

yes, they can go bad. Just replace it.

In Illinois they test the gas cap when you have your emissions test done.

 

10-4. Will replace. Hell, in NC they don't test cars past 35 yrs old and I even wonder why the frig they go back that far. Cars older than 1996 just get a safety inspection-no emission. Leave it to a Yankee state to violate state and citizen rights. Gotta love Jefferson. Gotta disagree with Lincoln and Henry clay.

 

Come to California. I have to emission test my 21 year old Acura. Nothing is smog exempt unless it's 1975 or older. Nice huh.

 

Pressure/Vacuum printed right on the cap is a dead give away that this is NOT a vented cap. If the truck has the "Evaporative Emmission System", then the tank is vented thru the charcoal canister under the hood, and a non-vented cap is correct. The vent line is plugged, if this is the case.
If the truck was not equiped with the "EES", there would not be a CC under the hood, nor a vent line running from the tank to it, so a vented cap would be needed.

 

With Evaporative Emission (ev/em), not only is there a charcoal cannister, but a fuel vapor valve in each tank.

Originally, there was a restrictor in ev/em fuel filler necks that would only allow gas station pump nozzles used for unleaded gas to fit in (leaded nozzles were larger in diameter).

In addition to the filler neck, the fuel cap is also different with ev/em, has 4 "ears" on it, instead of the 2 ears used w/vented caps.

The 4 ears are offset with two ears, for example above at 3 & 9 o'clock, 2 below at 6 and 12 o'clock.

"I have ev/em, but there's no restrictor in the filler neck."

Some peeps back then knocked out the restrictor, so they could use leaded fuel which was cheaper than unleaded. But, if a cat converter was present, the problem was...

The lead in the fuel melted the charcoal in the converter. Within a short time, their vehicles sounded like vacuum cleaners headin' on down the road, because the melted charcoal was forced into the muffler.

Evap Emission was introduced in 1970 and was initially mandatory in sold new in CA (where else!) F100's, E100's, U100's and Passenger Cars.

 

Thanks everyone. To be clear, my truck is 6200 lbs. (GVW) and did not have Cats, AIR, EVAP system or charcoal canister or vent line, so my cap should be a vented one. It looks like I may have the wrong one on there from previous owner. I'm going by Oreilly's at lunch to get a Murray brand 6749. Oreilly's has this in stock. Advance would have to order their brand (Stant).