Engine/Transmission (1998.5 - 2002) Fuel Tank under vacuum?

[nuv22]

lately i have noticed that my fuel tank is always in a vacuum state. after driving for about 10 minutes it goes back in to a vacuum. is there a vent that is clogged? Does the engine pull a vacuum for emmisisons like on the gassers? i am just wondering about this because i would think it would be WAY harder for the LP (relocated with vulcan kit) to pump if the tank is under vacuum. any ideas ?

 

[Thomas]

The model year 2000 (and likely model year 1999) Factory Service Manual mentions the following about fuel tank filler tube caps:

OPERATION The loss of any fuel or vapor out of the fuel filler tube is prevented by the use of a pressure-vacuum fuel fill cap. Relief valves inside the cap will release fuel tank pressure at predetermined pressures. Fuel tank vacuum will also be released at predetermined pressures...


The tank on diesel-fueled trucks is vented to the atmosphere via the vent at the rollover valve on the fuel tank. The only need for the relief valves inside the cap (mentioned above) would be in case the check valve/vent at the tank (rollover valve) became blocked. This blockage could be caused by a red cap left on the vent at assembly. This source of blockage has been documented in TSB's. I've never read of a red cap left on the vent stopping the mighty Cummins or totally collapsing a fuel tank-probably because of the vacuum relief valve in the filler cap.
The gasoline-fueled trucks have a similar setup, with the following differences. Trucks with 35 gallon tanks have two rollover valve/vent assemblies. The vents are not open to the atmosphere. They are plumbed to a source of engine intake vacuum via charcoal cannisters.
The gassers need an emmision system on their fuel tanks. The diesel trucks don't.

Inspect the fuel tank rollover valve for restrictions or that the shipping cap has not been left on the end of the valve.

 

[nuv22]

great thanks for the reply i will try to do that maybe i can get my hand up in there. do you think my cap went bad? is it ok to run the truck with the cap slightly loose? i mean there is enough vacuum that when i take the cap off it takes about 4 seconds to equalize. and i hear the tank stretching.

 

[Thomas]

... do you think my cap went bad? ...

You seem to be missing the information in the post. The fuel tank on the diesel trucks is open to the atmosphere via the rollover valve and vent on the top of the fuel tank. There should be no vacuum, regardless of what the cap to the tank is doing.

Check the rollover valve and vent.

 

[nuv22]

i see this but since you said "Relief valves inside the cap will release fuel tank pressure at predetermined pressures. Fuel tank vacuum will also be released at predetermined pressures... it lead me to belive that the cap could also allow air to come in to the tank in the event that the valve was blocked. plus it was just a question jeez! just trying to get any info i can.

 

[DavidC]

Yes you can run with the cap slightly loose. Just watch for sloshing during turns, stops, acceleration. At least it will give you some time to find the problem.



Dave

 

[Thomas]

... it lead me to belive that the cap could also allow air to come in to the tank in the event that the valve was blocked...

Air can come into the tank at some point via the relief valve in the cap. The air coming in via the cap will not eliminate the vacuum - only limit it.

The rollover valve/vent, when working normally, will prevent the vacuum in the first place.

 

[nuv22]

Thanks for the info DavidC. i figured i better ask since i am sure some one here has ran it slightly open before. This will also like you said give me time to find the problem.



Thanks again!

 

[nuv22]

Air can come into the tank at some point via the relief valve in the cap. The air coming in via the cap will not eliminate the vacuum - only limit it.



The rollover valve/vent, when working normally, will prevent the vacuum in the first place.

yeah i know the cap is not the problem like you said the valve is the problem i just figured i would ask about the cap because i want to run it cracked open on the way home, also i asked if the cap could be damaged because the amount of vacuum was quite a bit in the tank... just thinking out loud



Thanks again for the info Thomas