Fuel Tanks Pulling A Vacuum

icanfixall · Sep 10, 2010

Ever since I modified my tanks with the 1/2 inch air line off the tops the tanks are under vacuum. The motor runs but sometimes it will advance the pump and it sure rattles the motor. This usually happens after a cold start and I drive off anywhere up to about 1 miles. If the motor starts and I let it warm up in the driveway it will run up a few times and drop back to idle. Almost sounds like its running out of fuel... Probably is... :dunno

Is there something wrong with my fuel caps not venting to the outside... :dunno

I no longer have the rollover valves. That was removed when I installed the large vent lines to the fill necks....

LCAM-01XA · Sep 10, 2010

I noticed my tanks will develop vacuum as well, I also have the big-rig-pump-compatible fillers with 1/2" lines and no rollover valves. Never experienced an issue with the fuel pressure tho, engine runs the same way regardless of how empty the tank gets - and I got a 33-gal rear tank that I have the habit of using up entirely on a single trip, so that would be lots of vacuum in there. I dunno why that happens tho, the caps are supposed to be venting but they clearly do not do so properly...

subway · Sep 10, 2010

not completely sure what you did but if there is no vent to allow air back in to replace the fuel being used by the motor there is no way to avoid pulling a vacume. i had to add a vent to my in bed tank because of this problem. i actually added a ford roll over valve and some hosing to allow the tank to breath and the problem went away.

LCAM-01XA · Sep 10, 2010

The caps are supposed to be venting tho, with those there should be no need for an external vent... Maybe time to swap the caps for a pair off a carbed Chevy or something else that has no external vents in its tanks?

Agnem · Sep 10, 2010

Gary, have you tried just running around with your caps on half way? Can we assume your vent is below the level of fuel in the neck when your having this problem?

MIDNIGHT RIDER · Sep 10, 2010

Ford factory diesel caps of these vintages DO NOT VENT.

The roll-over valve allows a minimal amount of venting; barely enough to accomodate the engine and darn sure not enough to allow fast fuel filling.

What I did years and years ago was to remove and discard the factory supplied 7/16 vent-hose and roll-over valves.

1/2 hose is an easy fit into the tank grommets where the 7/16 hose originally was.

I TEEed the vents of all tanks together into one 1/2" hose that terminates high and dry, with a downward-pointing "U" shape at the top end, such that rain cannot enter.

I can fuel with the rapid delivery pumps at the truck stops with no problem.

There is no hose whatsoever connected in any way to the filler-necks, save the big hose. ;Sweet

gatorman21218 · Sep 10, 2010

Guess what i'm doing tomorrow

icanfixall · Sep 10, 2010

Well this is interesting news.... I probably will be making a new "vent line" soon. Yes Mel... I do run with the caps loose and it does not matter what level the tanks are at but... It seems the lower the level the more vacuum I can pull...

LCAM-01XA · Sep 10, 2010

MIDNIGHT RIDER said:
Ford factory diesel caps of these vintages DO NOT VENT.

Well mine sure as heck look like they would, cause they got four small holes on the bottom of the cap... But assuming you're correct (cause you usually are), what caps will fit these trucks and are vented at the same time?

RLDSL · Sep 11, 2010

If you are pulling a vac, then you need to get that sucker vented and quick because it's going to get to the point where one day you are going down the road and your IP runs dry. I have run across this before a number of times. Back home it's very common for diesel tank vent lines to get clogged by muddaubbers and you have to get in and poke them out with a wire once in a while. but some vehicles have odd bends in hard lines that a wire won't go through and they have to be cut off to be cleaned out and sections of rubber added.

George D. · Sep 11, 2010

Capps aren't vented the correct name for the valve you removed is a roll over vent valve and is the reason you are pulling a vaccume on your tanks. I have the same problem with my jeep removed it because it was leaking and a new one was like 130bucks didn't cause a problem with the carb but when I swiched ro fule injection I can hear the tank pop out when I pull the cap.

LCAM-01XA · Sep 11, 2010

Rob, like I said even with my large tank it ain't much of an issue, actually it ain't no issue at all so the caps must not be sealing properly... in which I will assist them somehow very soon, and they will be sealing even less - it's easier than redoing my vent hoses.

f-two-fiddy · Sep 11, 2010

FYI, any fuel tank cap that has the warning label on it, is not a vented cap.

The vented cap went away due to emission laws.

Goofyexponent · Sep 11, 2010

My truck.....same story. I usually take a week or two to burn off a tank, and when I pull up to the pump....I can hear the vacuum release when I open the cap.

LCAM-01XA · Sep 11, 2010

f-two-fiddy said:
FYI, any fuel tank cap that has the warning label on it, is not a vented cap.

The vented cap went away due to emission laws.

Yah, that make sense - so will a cap for say '70s F-series fit our fuel necks?

Fuel Tanks Pulling A Vacuum

icanfixall

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LCAM-01XA

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subway

be nice to the admin :D

LCAM-01XA

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Agnem

Using the Force!

MIDNIGHT RIDER

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gatorman21218

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icanfixall

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LCAM-01XA

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RLDSL

Diesel fuel abuser

George D.

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LCAM-01XA

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f-two-fiddy

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Goofyexponent

Mentally Unstable..

LCAM-01XA

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