I do not recall having this sort of thing come up when I had the OEM fuel cap. But recently, being in Yakima for work, I had to get myself some locking caps for truck and trailer. Yesterday, I pulled the trailer home and occasionally I could hear a noise along the lines of a plastic or rubber moving as I was driving. It was odd. Heck at one point, I thought it was the cab of the truck flexing.. haha.So, I make it to the fuel station to refill. I was darn near next to the red line on E. I unlock the fuel cap and twist it off. Tank had a crazy amount of vacuum on it. I looked under the truck and fuel tank was sucked in quit a bit. Filling the tank up to full, I could hear the tank expanding back and it made the exact same noise it did while I drove and when I removed the cap.Is this a problem? I cannot remember if having a vacuum such as this is a good thing or not? I thought caps were supposed to allow a vent?That and I don't remember if there was a difference between a gasoline and diesel motor. As I'am more mechanical savvy with the gas than diesel...

Your tank vent is probably plugged. It is on the top of the tank, but it is possible that you had a vented cap before and now you don't. Most newer gassers require the gas cap to seal for emissions purposes. Sounds like a gasser cap.

I agree with MnTom the tank vent on top on the sender is plugged up.

Mine is the same way and the new fass set at 46 pounds does not help. I ran a few feet of hose up from a vent to always have a good vent.

Well... when I got the cap, I had to tell them what I had so they could match it up to the truck. So maybe its an EPA cap.. :rolleyes:I'll give it a look over. Could anything bad come about from this?

Could anything bad come about from this?

Only the possibility of starving your VP of fuel............

I agree. You need to fix this sooner than later. Pumps are designed with inlet an outlet pressures in mind and any deviations can cause poor pump performance to complete failure.

Could anything bad come about from this?

Damage to both lift pump and VP44. Maybe even the fuel tank as well.

There's some one here... sorry I forget who... is running a marine fuel tank vent according to his signature. A marine tank vent is just a rubber hose to a screened through hull fitting. The vent is ideally located as high as the top of the fill pipe so the nozzle will shut off before pushing fuel (spill) out the vent. It would need an elbow in the top of the tank to connect to. I think modern vehicles have a roll over valve installed. I don't see why suck a vent couldn't be installed in the fill pipe near the cap (inside the filler door area).

I had this happen to my 02 after I installed my raptor. Don't know why but every once in a while it would start hard if I would drive it awhile, shut the truck off and then start it back up. Finally figured it out that there was vacuum in the tank. So I just started leaving the cap loose and never had the problem again. Its going to be hard on the vp44 when the fuel is having a hard time getting out of the tank. I don't know if the roll over valves are suppose to bleed off any pressure/vacuum or not. You could hook a hose to the fitting on the module that just has a rubber plug, then run the hose up as high as possible and coil it to prevent any water from getting in.

My fill has a line which enters within sight of the cap. I wonder if that is a vent & if so could that be T'd to another vent like a marine vent. http://www.ebay.com/itm/PERKO-MARINE-561-BOAT-FUEL-TANK-VENT-W-SPLASH-GUARD-SWIVEL-ELBOW-561-DP4-CHR-/271257225054

My fill has a line which enters within sight of the cap. I wonder if that is a vent & if so could that be T'd to another vent like a marine vent

That is a vent, but not for the tank. That is a vent for the filler so you can fill it faster. If you look at the filler from say my truck and one from your truck Russ, you will see that yours necks down quite a bit and then expands at the tank. Mine is the same size all the way.

OK, so that tank vent that ties to the neck. But if that fill vent was T'd to the atmosphere, it would serve as a vent for tank withdrawal.

OK, so that tank vent that ties to the neck. But if that fill vent was T'd to the atmosphere, it would serve as a vent for tank withdrawal.

Yep, That will work!

Mine has a tee in it so I ran a hose up high and put a differential breather on the end. It will spit out fuel when I top off the tank but I have not figured a way around that yet.

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When I was in the marina business, folks were not so concerned with tiny amount of fuel spillage... but marine type vent was best installed higher than the fill plate (boat gas cap) or certainly with a hose loop up against the desk where the fill was mounted. So the nozzle tip would actually be below the fill. It was possible with a careless fuel jockey to fill excessively so the level was above the vent & small leakage would occur. If one was alert, one could hear the fuel backing up.What I'd do in a truck, is mount the vent up under the fender... higher than the fill... so fuel will come up the fill higher than the vent fitting.

Well, first chance I can get to look at this will probably be tomorrow. I did take a gander under for a moment and noticed the fill line is mainly metal until a little bit before the tank. I would think that if I need to make a vent, it would have to be at this point and then zip tie the hose up above the filling point. I'm gonna see if I can access or see the port on the tank itself and clean it out or whatever it needs to work again, first.Had to make a trip with the trailer today for warranty work so I experienced more of the tank vacuum effect again. It gets pretty noticeable at about half empty...Also, I wonder, you guys think this sort of thing could affect mileage?

Like I said before, I would run with the fuel cap loose to prevent the vacuum. I wouldn't worry about mileage, I would be more worried about damage to the vp44

Like I said before, I would run with the fuel cap loose to prevent the vacuum. I wouldn't worry about mileage, I would be more worried about damage to the vp44

I believe I still have the old cap. If I can find it I'm gonna put that on and see if it is indeed the cap. Problem is, I gotta have a locking cap on, as the areas I work in, people will steal your fuel...