If they are like the 94-97 PSD's they are vented through the fill hose/fill neck. They use a hose within a hose. There is a soft sleeve type hose within the stiff outer hse. The fuel enters the tank thru the inner hose and the air vents thru the space between the two hoses.
No problem filling, etc. The question is asked because of a problem I have posted on before and have not solved. When running on one tank the truck runs fine but when switched there seems to be I believe a fuel shortage after a period of time. Seems to get worse aS fuel is depleted in tank. Replaced both sending units at same time few months ago, Dont think problem can be there. I was wondering about venting becasue this is the F350 that I added flatbed body and used filler tubes that came with it, as best I remember. The flatbed body came off of wrecked chevy 1 ton. Problem I suspect is either Air in system when running on that particular tank or lack of air when running on that tank. I have checked for everthing to see if air is entering system. I dont know about venting and how that would affect this tank?
thnks , Remote
Since you have a non stock filler without the vent tube, I'd bet that's your problem. Through the years I've actually seen a few of the old metal fuel tanks sucked down and collapsed from someone buying a non vented tank cap and using it. I'd buy a vented cap and see if that helps.
Heavy truck and diesel mechanic (thankfully retired after 30+ yrs)
'91 F-250 SC 4X4, 5spd. ATS Turbo, 3:55 diff.