ok i have a 1991 f150 with a 302 and i use my back gas tank cause the front one dosent show how much gas you have. everytime i get gas in the back tank i swear the gas is going some where one moment its high and next time i start it its lower??? i dont see any leaks. My old 351w used to get better gas mileage then this thing does.

 

Quoted this from an older post. Could be your problem.

"Vehicle Description: 1990-1993 Ford F-Series vehicles equipped with dual fuel tanks and 4.9, 5.0,5.8, or 7.5 liter gasoline fueled engines. Some vehicles can experience a cross tank fuel flow condition due to a damaged check valve in the fuel pump/sender assembly. Fuel may be supplied from one tank and some or all of the unused fuel may be returned to the other tank."

 

Yep, COMMON issue in these trucks.

 

is there anyway to check for this?

 

Keep the front tank full if it doesn't leak and the pump works, then you can just work off the gauge on the back tank and use the front for emergencies. If the rear tank gauge keeps giving you varying information then you might have an issue with both sending units. Just a guess I am throwing out there for someone else to correct!

 

If it is the cross-flow problem, it's possible it could over fill the front tank.

 

Yes it will flow out of the filler tube if the front is full, in 90 ford put intank high pressure fuel pumps in replace of the low pressure intank pump and deleted the high pressure pump on the frame rail along with the selector valve. To replace the selector valve they put check valves in the high press. intank fuel pump. In a nutshell when that check valve doesnt close all the way some of the unused fuel gets that is supposed to all go to the rear tank leaks or flows through the front tanks check valve (thus cross filling) . The only tank selector is the actual switch in the cab. The check valves in the pump are only closed through the pressure of the rear tank that the pump creates . the only 3 fixes are to unstick or clean out the check valve, or put a inline check valve in the front tanks return lines, or to replace the $110 fuel pump(just the pump not the sending unit). I would just put the inline check valves in which is what they did for that recall at the dealership. Or I guess you could just delete the front tank and delete the lines to and from, and also so rewire the selector switch in the cab or just dont use it. Yes I did a **** ton of research on this subject when I deleted the front tank on my 90 f150 . So trust me, I put the hours of researching in so you don't have to. Believe me it took along time to get enough matching answers from the right people, plus I already did it and it works fine. Peace im out!