Hey Guys I used the search function and couldn't find anything. So here goes nothing
When I did the motor swap in my truck I put a new gas tank in (1990 f150) and it's supposed to be an 18 gallon tank welllll my fuel gauge was reading a little more than a 1/4 tank of gas so I thought I was fine but as I was waiting at a stoplight my engine sputtered and died and I couldn get it to start. After getting it pushed to the side of the road. And fiddling it with a second. I got it to start as the light turned green luckily there was a gas station very close and it died again as I was pulling in. I filled her up but for some reason it only took 12.53 gallons. What gives? it also has a rebuilt fuel pump

 

It's possible that the tank won't let you fill it all the way. I've seen a few complaints of that here.
Have you scanned the computer for codes? If you determine that the truck is getting awful mileage, I'd scan and make sure everything is how it should be.
Is it an HO 5.0, or a regular one from a truck or Crown Vic or whatever?

 

I'm pretty sure it fills all the way up. Because the plug that plugs to the fuel pump came undone at one point cause of broken tabs so I had to take the bed off and pull the fuel pump and that puppy was full to the top. Was pouring out lol. So I replaced with a brand new plug. And put every Thing back together. It's actually getting great gas mileage. And the only code I have is for the egr valve which isn't hooked up and the tab air system. It's a non high out point out of a Lincoln town car.

 

Your tank may not be venting properly and a vacuum is being created. Do you have a vented gas cap? Is it plugged?

 

I do have a vented gas cap and it is new, but it could be faulty I'll definitely check when I get home. My plan is to put a gas can in the back of the truck and try to run it out of gas. And when that happens if my gas gauge doesn't show empty I'll pull the gas cap off and see if it'll start and stay running and if not I'll have that back up gas can (if I'm actually just running out of gas)

 

Also I don't think the '80s and newer senders work right with the older gauges

 

I have a set of dolphin gauges and a classic instruments fuel link that corrects the ohms. The main thing I was worried about was that I "ran out of gas" and when I filled up it only took 12.5 gallons.

 

I put brand new "vented" fuel caps on my 78.

It stalled several times, but always restarted...until the last time, and I couldn't get it to start. Rode a tow truck home. Turns out the vented caps weren't really venting, and they were causing a vacuum situation on the tanks. I drilled an 1/8" hole through the center of the caps, and the problem went away.

Fine now!

 

PBY, thats the kind of fix I like..... No BS and done deal.......

 

I have the newer fuel cap, that is on a 1990 f150. Same concept though. I'm going to put a hole in it after work and just be done with it. I'll keep this post updated if that fixes the issue. Thanks for the help everyone

 

Did you check to make sure the pickup is at he bottom of the tank? Sometimes new sending units' pickup tubes are not bent right and the end of the tube could be hovering several inches off the bottom of the tank. Since running out of gas really means "I'm not getting more gas into the pickup" you can literally run out of gas when you have several gallons left.

 

I have an electric fuel pump. And a separate sending unit that attaches to the fuel pump. The pickup for the fuel pump is at the base of the plastic housing. It's a hole with a filter sock that plugs into it. And that sits in the little square duvet that's pressed into the bottom of the tank. But I could definitely be wrong

 

Well, that certainly sounds like the pickup is at the bottom of the tank!