Help.

Vehicle:
I have a 1989 F150 4.9L 6cyl manual dual tanks

Problem:
Last week, I started the truck & a few seconds later it cut off. I attempted to start it again, it turned over, but wouldn't start. To listen for fuel pump sounds, I turned KOEO. I didn't hear any sound. I checked fuel pump relay connectors and repaired 1 wire I had fixed several years ago. I turned the key on and heard fuel pumps.

I started the truck and the check engine light came on. It has a very rough idle (jumps from 300 -1500 rpm). Then, the vehicle cut off.

To check codes - I hooked up my Code Reader, followed screen prompts, when instructed, I turned key on, depressed clutch, but received a message that the DLC didn't connect. I've also jumped EEC in an attempt to read Check Engine light flashes, but the Check Engine light has not illuminated with either KOEO or KOER.

While driving, the vehicle:
1- Lunges at constant speed
2- Almost stalls, especially from a stop to accelerate
3- Combatting the stall by depressing the clutch and revving the rpm (applying fuel) allows me to slowly accelerate; however, it hestitates greatly
4- The Check Engine light has not illuminated since it displayed at onset (neither while running nor while KOEO)
5- I have switched from front to back tanks without a change

Note: When I purchased the truck in 2008, I had to replace the Fuel Pressure Regulator (FPR). My current issue seems similar, BUT when the FPR died, anytime I accelerated, the vehicle cut off. (unlike now). I mention this similarity to convey the symptoms.

What I've done:
First, I normally have time and patience to diagnose better; however, with several other projects, this time, I didn't do that. I did a "no-no". I threw a few parts at it. I know bad.

1- By jumping fuel pump relay, I've verified all three pumps operating. Rail primary pump and both in tank pumps
2- I replaced the EEC Relay. No change
3- I replaced the Throttle Position Sensor (TPS). No change
4- I replaced the Manifold Absolute Pressure (MAP) sensor. No change
5- When I removed a couple of the vacuum tree hose caps, the engine rpms increase, but this doesn't change the hesitation / stall. I'm not sure of the names for the two caps I remove, but if you are standing at the front of the vehicle looking toward the cab, the vacuum tree is on the back left above the upper intake manifold. there are vacuum connectors toward the front and toward the back. The caps I removed are the middle front (larger) and the back right (smaller).

My gut feeling is that it's electrical (due to the no Check Engine light and OBD cannot detect DLC).

I've attempted to explain the best I can and provide as much information as possible.

This is my first post. I really appreciate all insight and assistance.

Thank you,
Ricky

 

Just a wild guess, but the symptoms sure sound like a clogged fuel filter or some other restriction in getting fuel to the motor.

With not enough fuel, it will rev when you put the clutch in because it is under less load. Heavy load (like accelerating or maintain highway speed) is when you need the fuel.

FWIW, if the truck was recently run very low on fuel, that can put the debris from the bottom of the tank into the filter and clog it up. Known many people over the years that have run a car dry or near dry, only to get a clogged fuel filter shortly after.