91 f150 4.9

So recently I just replaced both fuel pumps and got the front tank working again, everything has been working good for the most part up until a couple weeks ago....When the rear tank gets to a 1/4, she starts to sputter like she is out of gas...ok went to front tank and hit up a gas station. Filled up rear tank and the pump stopped at 10 gallons. (That rear tank has always been weird to fill too even before this issue when gauge would show close to E and I put gas in pump would stop at 14 and gauge wouldn't show all the way full and I was able to slowly fill it up to 16 gallons. MAYBE I AM DOING SOMETHING STUPID.) So my question is what is the issue. Is it possible the fuel pump is failing? maybe some **** in the tank clogged the little screen. Thoughts? I guess I can do a fuel psi test.

I am running fuel pumps from this company OEM Replacement Fuel Pump Assembly *Rear Tank* Ford F150 1990 - 1996


I need to pull the bed again anyways as when I fill the front tank, it leaks. If I run the rear thank with a full front tank the front tank leaks too. So I wonder if cross flow issue is in there too or maybe something else.

 

I think it's an issue with the pump, I noticed even with the rear tank full I still got a weird stumble issue like it was low on fuel. Swapped to front tank and truck ran great. I should still do a fuel psi test to confirm this issue.

 

There is a reason why aftermarket pumps can be as cheap as $50, and the official MotorCraft pumps can be as much as $300. Having said that, I try and use aftermarket when I can to save money. However, if a part has a lot of moving parts, or is expensive/time consuming to replace, I'll always go with Motorcraft when I can. For instance, I also have to replace the FDM in my front tank. The gauge doesn't work, the pump barely works, and there is a small leak in the tank. Later this summer, I'll be taking the bed off to do some wheel arch repair. While it's off, I will be replacing both fuel tanks and FDMs. I'll get Motorcraft FDMs, but will buy aftermarket tanks, straps, fill tubes, etc. it will cost around $1000. Is it overkill to replace the entire fuel system from the engine on back? Yeah, probably - especially when the rear tank is fine. However, given the time that replacing tanks and FDMs take to do properly, it will be worth it as I (hopefully) won't have to bother with it again. The first ones lasted almost 20 years, so I hope the new ones lat as long!!
As for your issue, it sounds like at least the rear sending unit/float is bad, and also sounds like the pump is also not working properly.

 

Sounds like you have a problem with the front FDM check valves.
When running on the rear and it is not running well my guess is it is pumping into the front tank.

The level of the fuel makes no difference as the FDM takes the fuel off the very bottom of the tank except for cooling of the pump motor. So do not drive with the fuel level below 1/4 tank as you will not get the cooling of the pump motor and you will shorten its life.

This problem you are having is why if you take it to a Ford dealer they will replace the FDM's in both tanks to fix the problem. That way they will not get a come back.

The PSI check will not tell you if it is the front or the back FDM for the above reasons.

 

Subford,
Why would fuel not returning to the rear tank (due to bad check valves on the front FDM) cause the engine to sputter? Unless, of course, the rear tank runs dry, but it sounds like if he can only pump a little fuel into the rear tank at the station.

 

I did not say it was on the return line.
If the HP check valve in the front tank is not shutting then the fuel pressure can be low and cause the sputter while running on the rear tank.

 

Ah, so it could be going into the front tank on the way to the engine? Just when I think I understand this fuel issue that I and so many others have, I learn something I never even thought of before. Thanks!

 

Luckily this company I bought from has a lifetime warranty...maybe this is the reason LOL. But damn that is a little bit overkill but hey as long as the truck treats you well.
So would you recommend I call this company up that I bought the pumps from and replace them both again? This may be some of the reason why when the truck sits for 8+ hours I have an extended cranking time, truck starts then stalls, start it back up and good to go. Also as a side note I currently have no codes. I DID get code 181 a week ago while running off the tank. I deleted codes and have not seen it come back yet.


Also I may just say **** it and convert to 1 big tank...on both trucks.

 

When a truck sets and then cranks for a while before it starts an will start right off when the key is turn on and off a few times means the fuel has all drained back into a fuel tank from the pressure line going to the fuel rail.

This could be a bad HP check valve in one of the FDM or a bad fuel pressure regulator or a leaky injector.

 

Word, I know I've replaced the FPR because the old one was leaking and I have upgraded to the 4 hole injectors too.

Forgot to add that on on the front tank I saw a valve that looked something like this on the front tank.

Either way Ill give that fuel pump company a call tomorrow and see what I can do. Seems like it's a trouble free process.

 

Haha - it all comes full circle. Thats my picture I took when I bought a cross-flow recall repair kit for my truck on e-bay. I found out from a poster (I think SubFord) that that kit is not for my model year/engine.

 

That photo is of an FPR and the outside inline high pressure check valves for when you have two new pumps and the FPR has blowed out the HP check valves inside the FDM.

This happened to the 1993 F-seres trucks and that kit was the recall quick fix as Ford did not want to replace the FDM's. The 1993 truck came out with a FPR that allowed the fuel pressure to go to high and it would blow out the HP check valve in the non running FDM. Then when you switched tanks it would blow out the other one.

 

LOL yup just found that thread. Subford was definitely posting in there.
Oh well damn. So I guess the fix to this would be to replace the FDM's then.

I wonder if there is an issue with the switch on the dash? seems to be working fine though.

I know I may not see any psi issues but I am going to rent a fuel psi kit today, just to get some testing out of the way.

 

Here are my findings for fuel psi. I did these tests according to haynes.
Also when I hooked up the tester there was no psi at the rail. Truck has been sitting for about a day.

Front tank
Engine Off (hook wire up to fuel pump test on DLC and turn key to let pump run): 52 psi
Engine Running: 46 psi
FPR disconnected: 56 psi

Rear tank
EO: 35 psi
ER: 42 psi
FPR Disconnected: no change, the needle just vibrated really fast.

When I rev truck with rear tank running psi drops a couple psi. When I rev truck with front tank psi raises a little bit.


Also when I shut off the truck the psi at the rail doesn't drop right away. But it definitely loses psi at the rail in about 8 hours or so.

I'm thinking that rear pump is faulty. When the front tank leaks when full it leaks from the top too. So I wonder if I screwed up something during the install, maybe messed up that o ring around the sending unit. No one is perfect right

 

Yes it sounds like the back pump is bad.
The pressures on the front tank are OK.