I thought I had seen this posted on before, but can't find the posting. Anyway, 1995 F150xlt, 141k on odometer, dual tanks. Replaced front fuel pump earlier this year. I believe the gasoline in the front tank is backflowing, literally, to the rear tank. On recent drives home from work, the gas gauge while on the front tank literally moves by some hidden force. While on the aft tank, the needle barely moves at all. AND, maybe its just my imagination, but when I switch to the rear tank, it looks like theres more gas in it. Can't swear by it tho!! If memory serves, something was mentioned about a fuel delivery module or something? So does that have to be replaced in the front or rear to stop this? Thanks SO MUCH for any and all help. This site is out of this world.

 

There is shuttle valve in the return line of the fuel delivery module. When open it allows the fuel to return to the tank. It is supposed to be normally closed and held open by the pressure generated in the supply line when the pump for that tank is running. If the shuttle valve for one tank fails to close, you will get return flow to both tanks. So if you think fuel is transferring to the rear tank, you should suspect the shuttle valve in the rear tank's fuel delivery module.

Do not rush out and buy a new FDM without testing the existing one first.
Fill both tanks and run on the rear one until it shows empty on the gauge.
Record the mileage.
Switch to the front tank and run till it shows empty on the gauge.
Record that mileage.
Switch back to the rear and see if there is fuel registering on the gauge. If there is then you have a problem.
If there is not you may still have a problem. Re-fill both tanks and record the volume that was required to fill each and calculate the MPG for each tank. If the numbers are way off you could still have a bad shuttle but not bad enough to cause the transfer to register on the gauge.

You can either replace the FDM or you can simply always use the rear tank first then switch to the front when it is empty

 

Thanks StrangeRanger, I thought you were the one who answered the guy on this kind of question. I'd better go out and do this shortly, as gas prices are starting to go thru the roof again. Also, could this have anything to do with the fact that I only changed out the front tank fuel pump?

 

Only to the extent that you didn't change the rear one. The two pumps are independent of one another and changes or problems with one will have no effect on the other.

 

Alright StrangeRanger. Sounds like this experiment will take a while, but will keep you informed of the results when I get them. Thanks for your time and will get back with ya!

 

Thinking about this. if you want to shortcut the process simply fill the front tank but continue to drive on the rear till it shows empty, then switch to the front and drive on it till it shows empty. Switch back to the rear and see what the gauge says. If it still shows empty then any transfer flow you may have is too small to worry about. If it shows fuel present in the rear, then you have a problem. you won't be able to get exact numbers like you could if you filled both tanks but you'll get an answer a bit more quickly.

 

Read this caption & follow the link to see if your VIN is affected:



Note that, even though this one has expired, the 2 safety recalls mentioned in it never expire, and you may be eligible for them. A quick call to your local dealer would be even more informative since they have access to more info than the public Ford website releases.

 

Ok, I promised to follow up, so let me explain as best I can. I completely filled the front tank past the F mark, and the rear tank to just past halfway--All the money I had!! After ONLY 75.8 miles, the front tank already indicates a bit past 3/4th of a tank, and when I flip to the rear tank, it indicates almost 3/4ths of a tank. Do I have flowback going on? Also, I have noticed that the fuel dial fluctuates quite a bit. When I am at a light it would point at 3/4ths of a tank, and when I take off--no jackrabbit either--the pointer would swing to halfway between 1/2 and 3/4th of a tank, and back to almost 3/4th again. Whats going on there?? Thanks again, Steve and StrangeRanger or anybody for any help here. Again, a 95 F-150, dual tanks, 302 V8 5.0 with 141+ k on odo.

 

It sounds like there is some transfer going on but to prove it, you needed to start on the REAR tank and run it down than switch to the front.

 

Hey Steve83 or StrangeRanger, Well I did what you said. I ran the rear tank down to the red line, as I only had a half tank. Went 143 on 9 gallons=just under 16 mph. Not bad. Anyway, then switched to the front and ran it down almost to empty, and before filling the front back up, switched to the rear, and lo and behold I had a little less than half of a 1/4th of a tank.
So guess that proves that I have flowback to the rear tank? And that little of an amount, is it something to be concerned about? And should I plan on replacing the rear fuel pump in the near future? Thanks again in advance for a response and Happy Christmas and a Merry New Year!!

 

1/8 tank is barely enough to know that anything was transferred. I'd refill the FRONT ONLY and do it a few more times until there's no doubt that the level is really rising.

But if it's that slow, I wouldn't shell out the money & eat the rust to change the rear pump. You know what's happening, and you know how to work around it safely: never drive on the front tank when the rear is close to full. Problem solved. When it gets worse, then the improvement will justify the cost & effort.

 

Hey Steve, thats what I was thinking too, as long as it was such a small amount. Just have to remember not to fill up the rear as well as the front or I might have gas running out the back filler, and can't afford that nowadays!!! Thanks for the reply and have a safe and Happy New Year!!

 

Just ALWAYS run off the rear tank after a fill-up and then transfer to the front after you've emptied the rear and you'll have no issues.

 

Thanks Steve83 and StrangeRanger, I knew at least one of you would be helpful on this issue! Am kinda kickin myself for not changing out the rear fuel pump when I did the front. Oh, well, a project for later on when it gives out I guess. Thanks again guys and have a great New Year. Strange, you gotta be freezing being just outside of Akron!!

 

Actually, it's not bad at all, we're used to it and prepared for it. I've been out hiking in the National Park almost every day; I just have to layer up properly. Usually winter doesn't hit us quite this early though. I had a friend who moved to Fargo, ND. She longed for the mild winters here in OH.