Out of gas but tank is full
 

My friend has a 99 ,4dr,with a used/new engine.Since the engine replacement the gas gauge doesn't work and it will stop like it's empty,she puts in a gal.that she carries w/her now and it starts right up.Then the most she can get in the tank is 4 gals.I ran a plastic tube into the tank ,no obstructions.No leaks anywhere.She has a locking gas cap so there shouldn't be anything put in it.The mech.who put in engine cant be found.
I found a similar problem ,this was her fix (Problem has been fixed it was a leak caused by the Neck Clam Filler and Evaporative Emissions System Vent Solenoid )but we have no leaks.
Any ideas before we go to a shop ? Thanks

RE: Out of gas but tank is full
 

What do you mean stops like its empty.. meaning the truck dies out? Or the gauge goes to E? Sounds like the sending unit...

RE: Out of gas but tank is full
 

The locking gas cap is likely incorrectly vented. After a few gallons are used, a vacuum is pulled on the 'non-vented' tank and the fuel pump can no longer pump to the engine. Ask me how I know!

RE: Out of gas but tank is full
 

To bammontanadies like it's out of gas.
To
Mammys CarI'll have her switch caps when she goes somewhere and use the locking cap just when she is parked and see what happens.

Thanks.