Since the weather turned here I have been fighting what I thought was a carb icing problem. I have an Edelbrock 600 on a rebuilt 300. The truck is an 1989 F150 that was EFI. I added a Mallory fuel regulator to regulate the 55 psi frame rail pusher pump down to 7 psi. The regulator is a 3 port with a bypass going back into my tank.

I made a hot air intake and the icing problem appeared to be solved for some time. It was close to three weeks that my truck ran fine. While driving to work last week my truck died after about 3 miles. It appearred to be icing. I limped it a friend's house and parked the truck as it was dying by the time I could get into third gear after restarting.

I came back for the truck later that evening and it started fine but it would die almost immediately under load. The fuel pressure gauge was showing 7psi max but it was erratic.

I was able to limp it home and when I went to add some dry gas to my tank later that evening, while taking my gas cap off I was getting a vacuum at the cap and when I finally got the cap off I heard a pop in the tank like it was expanding.

I'm fairly certain I've had a fuel pump go as the pressure gauge shows 0 psi and I've got no fuel at the carb.

I'm leaning towards the in tank pump and will check the flow at the pusher pump to be sure.

My question is why would I be getting a vacuum in my tank and what do I do to avoid this? I changed the front tank due to a leak right after I bought the truck. I don't recall a vent tube but it's been some time since I changed the tank.

Thoughts?