I have fuel pressure to the injectors,39lbs on the rail, both fuel pumps have been replaced. Fuel relays are working.Engine turns over strong. It will fire on ether. I was told that the ignition module controls the injectors firing so I replaced the module today. Same thing. Any help would be great.

 

OK. I'll repeat myself for the second time in one day (I hate repeating myself though- but that was another subject). Ignition modules RARELY go bad. It's likely an ignition COMPONENT, WIRE OR WIRE CONNECTOR. Yours seems to be a component. You must first look at the items feeding input to the system if you have fuel pressure. You CAN individually test parts you think are bad BEFORE replacing them. A visual inspection of sensors and wires can go a long way. Look for anything unplugged or with a bad clip/plug. Unplug sensors and look for rusted/corroded wires (Blueish or rust colors). Look for sensors that tell the computer the engine is in motion or that fuel injectors should fire (crank sensor,etc.). You can eliminate things like speed sensor, but the starting circuit is all basically subject to put under the microscope. Usually you'll blow a fuse/fusible linkif something overloads so check fuses and links first (they are major components).

 

Thanks, I was thinking crank sensor . I don't want to start throwing parts at this truck until I have a good idea how the system work. I'm a class 8 truck mechanic but when this happens on a diesel you look at the cam sensor. My little gas job doesn't have one. I really needed to know what tells the injectors to fire and when to fire.

 

Makes you miss those antique deals, huh?

 

I have an "88" F150same engine (4.9) same problem. Turns over fine but only starts when primed. intank fuel pump, fuel filter and relays have beenreplaced.
Note: The truckhad been driven for a few days without a gas cap.

Background: My wife has a habit of running the truck out of gas. The night before the problem she took off and (supposedly) run out of. The next morning when I went to pick it up, I found that the battery had been stolen. Not just disconnected, but ripped out taking the Negative terminal with it.
The reason I`m saying this is becauseI found a disconnected wire near the battery coming from a wire harrness routed in front of the radiator.
"Below is a link showing the wiring in the engine compatment",
It shows 2 wires connects to the negative post. I cannot figure what theses are for. They may have went with the terminal when the battery was stolen,
Does anyoneknows what they are for? Do they have any reference to the fuel relays?

http://www.alldatadiy.com/alldata/AF...20580/89720581