Odd Fuel Delivery Issues

[NonZero]

Hi there,

I have an incredibly clean Ford F-150 from 1995, with the 4.9 and dual tanks.

I have noticed an odd fuel delivery issue. After driving on the highway for a few hours, I notice a small engine stutter. It will gradually increase to a full-on rocking motion back and forth as if someone is flipping the ignition on and off on purpose. I can switch to the other tank and it immediately picks back up just fine, but will steadily fall into this range of motion just like before.

I believe I have isolated it to the reservoir switching unit which must be switched out. While doing this I will do both pumps as I don't believe they have ever been done.

Another odd bit: When I switch from the rear tank to the front tank, often there will be more in the front tank than I remember. Am I correct in thinking the front tank could be pulling fuel out of the lines from the rear and causing the issues because the fuel pumps cannot keep up with a constant pressure loss under load?

I am inclined to do the switching unit and both pumps just as preventative maintenance. Does anyone have any other ideas before I do this?

 

[InlineDave87]

In that year there is no reservoir switching unit. The fuel pumps have check valves that take place of the old style fuel switch. When these check valves go bad in the fuel pump it will bleed some of the fuel into the next tank. So yea go ahead and replace your fuel pumps and see if this fixes your problem.

 

[chad]

for me? this whole system is pretty compicated. The only way I
could get a handle of the ele & fuel problems was to get the charts,
schematics and fuel routs. This was onan older rig tho (do things get
any LESS complicated when moving further into the future?). Days w/the
Mechanical switch btwn tanks, big cross rails cross tanks (not along side, etc.
Once done w/new fuel filters’n checkin grounds it really became helpful w/these
guides - right in frnt of me under the lift or @ curb side...
(http://www.fordification.com/ ’61 - 79 ford truck, ford truck enthusasts https://www.ford-trucks.com/forums/index.php)

 

[Frank]

for me? this whole system is pretty compicated. The only way I
could get a handle of the ele & fuel problems was to get the charts,
schematics and fuel routs. This was onan older rig tho (do things get
any LESS complicated when moving further into the future?). Days w/the
Mechanical switch btwn tanks, big cross rails cross tanks (not along side, etc.
Once done w/new fuel filters’n checkin grounds it really became helpful w/these
guides - right in frnt of me under the lift or @ curb side...
(http://www.fordification.com/ ’61 - 79 ford truck, ford truck enthusasts https://www.ford-trucks.com/forums/index.php)

chad, the fuel delivery system is one of the rare things that DID get simpler when it got newer. From '90-on the trucks with dual tanks have only one inlet line, the two tanks T the line together, no valve. And one return line, also T together with no valve. The pump with power has in/out valves open, the pump with no power has both valves shut. The only thing controlling it all is 12V power to one tank or the other.

 

[chad]

ok, saw the change from mechanical switch to ele,
(ona dash) now know what that means.
W/efi needs return fuel lines. It come back to just one tank?

 

[Frank]

ok, saw the change from mechanical switch to ele,
(ona dash) now know what that means.
W/efi needs return fuel lines. It come back to just one tank?

When a tank pump gets battery current, both internal valves open, it pumps fuel and returns into the same pump on the return side. Meanwhile the other tank pump prevents both the supply fuel and the return fuel from backfeeding into that tank with valves internal in the pump. There's no longer any valves in the lines. If the internal check valves in the pump itself fail, then the tank that is not pumping will fill with fuel as it backfeeds from the pump that's on, since the lines are connected.

 

[chad]

got it (I think) “...There's no longer any valves in the lines...”
and so that’s how they do it “...with valves internal in the pump..."

“...If the internal check valves in the pump itself fail..."
which is not sucha bad thing~

Still like to C the way fuel lines are run, where the pump sits in this meassss