I just want to start out by saying thank you to all those who post on this forum. I have never had to post or even register until now because there is such a wealth of knowledge on here that there was no need to in the past. I always found the answers I needed just by doing a simple search. I have finally run into a problem that I can not solve and it pertains to the above post.

I have a 1995 F-150 XLT 4.9 with a manual Mazda trans. I dropped a remanned engine in it that now has about 8,000 miles. It has dual tanks.
My truck runs perfectly on the front tank. Therefore I assumed that eliminated quite a few components that could be causing me grief. Pressure readings are within specs on the front tank. When I started having stumbling issues and hard starts on the rear tank I assumed a bad pump. I ordered an all new rear tank assembly from rock auto, whole shot, tank, pump everything. Installed it, everything seemed to run fine. Then the last couple days the stumbling and hard starts on the rear tank are back. I checked the pressure on the rear tank and they are well below specs, 35psi to be exact. It should be 50-60 psi. I inspected all my fuel lines, they seem fine. I replaced my fuel filter just in case that could be the issue, though I doubted it.
The truck runs like a dream on the front tank from start to highway speed.
On the rear tank it starts hard or not at all and stumbles all the way until you get up past 35 MPH, then it runs fine.
Being as the truck runs perfectly on the front tank I am assuming that eliminates fuel pressure regualtor and elctronic control issues.
So what do you all think, did I get a defective assembly from rock auto or is there something I am overlooking?

 

id call the company that makes the pump and tell them they sent u a falty pump

 

yeah that was what I was thinking,lol. cant get quality parts these days!!

 

i put a fuelpump in a buddys bronco put it all together n nothing drove us crazy checkin everything so i pulled the pump n jumped the wires right off the battery n the pump wouldnt even come on tock it back n got anoutherone problem solved

 

Yeah, it pumps but not enough pressure.have to return it i guess.pain in tn @ss

 

The problem is the FRONT FDM; not the front pump or rear tank assembly. Read this caption, and then the others in that album:



Note that the valve head is the same on all '90-96 FDMs, so you can get the cheapest one available & snap it into your FDM, if it's different.

BTW
My truck has always run perfectly with 30-35psi. It has a '95 4.9L and a '96 V8 tank/pump.

 

Manual says koeo check should be 50-60 psi. That is what the front tank reads. I ruled out an fdm issue as i do not seem to be getting any fuel exchange. Am i missing something?
Thanks.

 

My best guess based on all your symptoms is that the rear pump's low pressure is due to a leak in the front FDM's shuttle valve. Have you checked pressure on each tank while driving? How about while it's stumbling?

You say the rear pressure is now 35; did you check it immediately after installing the new tank, before the symptoms returned? If not, it might have always been 35.

 

Never checked it prior to having issues. I forgoy to mention the front pump and fuel gauge sending unit are also new. Replaced those awhile back because i was not getting a fuel gauge reading. I wont go into the long story on why i had to replace the pump as well as the sending unit. I am definitely going to do some more diagnosis along the lines you are speaking though.
thanks.