Fuel Tank Vacuum

[jar84203]

How much vacuum should a fuel tank encounter under idle? I recently replaced the fuel sending unit on my 71 Duster 225 and noticed the fuel cap hissing from vacuum.

 

[jbc426]

None. They are vented. In 1971 many have vapor recovery systems too.

 

[Sedanman]

Does it have a charcoal canister? It may be plugged

 

[jar84203]

It has a vapor-liquid separator (4 lines come out of the tank) and no canister. I ran the car with the original cap and a new one. They both had the vacuum hiss noise.

 

[Marcohotrod]

sounds like your vent (not on the cap) is not open

 

[RustyRatRod]

Vented caps do allow a small amount of vacuum or pressure to be applied before it is released. They have a vent valve that the pressure or vacuum amount must be exceeded before it is released, so it's perfectly normal and possible to hear a hiss from the fuel cap. They are not just "vented" openly to the atmosphere. There is a valve to overcome.

 

[73Swinger18]

Not sure if your vent system has a rollover check valve at the end of one of the lines. If you do have one, I'd check to see if the ball bearing moves back and forth freely in it. I would also disconnect the lines at the tank and blow compressed air through them. I poured a shot of gas in mine and blew it out the other end with a rag tied to it. The gas acted like a slug and cleaned it out pretty well.

 

[jar84203]

sounds like your vent (not on the cap) is not open

Which vent are you referring to? I don't see one mentioned in the FSM.

 

[jar84203]

Not sure if your vent system has a rollover check valve at the end of one of the lines. If you do have one, I'd check to see if the ball bearing moves back and forth freely in it. I would also disconnect the lines at the tank and blow compressed air through them. I poured a shot of gas in mine and blew it out the other end with a rag tied to it. The gas acted like a slug and cleaned it out pretty well.

Ill have to check the vapor-liquid separator lines. It didn't make any noise before I dropped the tank.

 

[jar84203]

I finally made the time to check out the problem. I changed the Holley 1920 to a Carter and accidentally switched the pcv and bowl vent lines. Doh... The vapor line to the tank was getting manifold vacuum.