1993 F-150 4.9/E4OD. 115,000 original miles. Dual tanks.

In January of this year, I bought a '93 F150 from a friend. He told me that the previous owner, whom he had bought the truck from, had apparently replaced the fuel line from the frame mounted filter to the engine as well as the small inline valve (anti-knock valve? Something to prevent pulses in the fuel lines, I've heard). He had replaced the fuel filter, as well as replace the cap, rotor and wires. No mention was made of either fuel pump being replaced. The front tank has not worked since before I bought it, but the back one does. If I switch to the front tank, the fuel gauge pegs past F and the engine dies immediately.

Long story short, from the day I bought it, in order to crank it for the first time each day, I have to prime the system two or three times. If I try to prime it once and start, it just cranks but doesn't start. Once it starts, it runs fine, albeit with a slight hesitation immediately off idle.

My problem began when it started bogging down when I'd go to accelerate, between 1000rpm and 1800rpm. It has a stumble at idle in park (literally sounds like a pop in the exhaust every couple of seconds), and at around 35-45mph in OD, it bucks bad enough that the truck shakes. Not like a rattle-your-fillings-out kind of thing, but more like every second or two it has a little kickback to it.

I have replaced the following:

- Rotor, due to damage to center contact
- Coil; replaced with Accel 48,000V E-core coil (OE was damaged upon plug wire removal....go figure)
- Plug wires
- Plugs ... I had went to platinums but was not happy with how it ran, so I went back to copper. I had ran the gap at .055 but as the stumbling worsened I went to .045. All plugs looked clean and identical, even the old ones it had in it. No fouling or oil residue. There were small gray spots on the insulator next to the center electrode.
Ignition Module ... was not the cause; tested good at O'Reillys but showed up as having a bad "field" on AutoZone's tester. Stock was gray, replacement was black but it's what they specified (I am aware there are differences)
All vacuum lines have been replaced


I have checked the following:

- Cleaned IAC ... was not very dirty. Appears to function, idle drops if unplugged.
Checked function of EGR valve ... applied manifold vacuum in park at idle and engine stalled immediately. I am assuming this indicates that it isn't leaking exhaust into the engine.
- Advanced timing slightly ... cannot find timing mark on balancer so I have to set it by feel.
- Cleaned contacts in ignition module plug, IAC plug, and TPS plug.
- Poured in a bottle of Lucas Injector Cleaner

Now here's what has me confused:

I put a pressure tester gauge on the truck. At the time of connection I showed 0psi on the gauge. I turned the key to run and the pump kicked on, but the gauge slowly rose to 10psi. I turned the key to start and cranked the truck, and the gauge showed 45psi at idle. If I raised the RPM the needle dropped a little for a half a second then went back to 45. If I unhooked the vacuum line to the regulator, pressure increased by about 2-3psi. If I turned the engine off, pressure held around 45-48psi.

From everything I've read, the later 4.9 fuel system needs to run 50-60psi. The problem is, when I look at the specs on the fuel delivery modules at the parts store, all of them state they put out 45psi max, with a static pressure of 120psi or so. Is this correct for a 4.9 fuel system?

And finally, will low fuel pressure cause a slight knock under load? It is directly proportionate to throttle movement; if I feather the throttle while it's doing it, the sound changes, and goes away below 1000rpm and above 1800. I am pretty sure it isn't rod knock seeing as the engine doesn't make any noise aside from this and some slight spark knock under heavy acceleration.

Thanks in advance for any help. This has me pulling what little hair I have left out.

 

You have two problems.
1. you need to change the ICM to a GRAY one. It will be harder to start with a Black one.
2. You have a fuel pressure problem.

The valve he replaced on the fuel line was for FDM with a bad HP check valve. There should have been two of them, one for each tank and they should be plugged into the FDM.

Sounds like you do have a bad HP check valve in the non running FDM.
I would get rid of the two check valves and replace the FDM in both tanks.
Then you know you have it licked. With the fuel pressure holding with the key off means you more than likely have a good FPR.
But you got to get the fuel pressure up for it to run right.