Fuel pump trouble
 

I have a '91 F250 4x4. It was built with a 351 and a C6. I discovered somebody swapped in a 302 disguised as a 351 (upper intake and valve covers are marked 5.8). I've had trouble with the fuel pump not running. It can happen when I'm trying to start it or while it's already running. Any ideas - ignition switch, relay, wrong ECM?

Here's what I know:
302 with the 351 firing order
fuel pump usually starts after a few turns of the key (or many)
relay wires have been hacked and spliced back together. Last time there was no fuel, I played with the relay wires, no luck. Crammed the shift lever as hard as I could into Park, got fuel.

What I've been told:
A 302 with the 351 firing order is a '95 or later. What ECM would work with my C6 tranny? I doubt that it was changed when the engine was swapped.

I should add that the truck originally had two tanks but the side one was taken out. So there's also that switch to wonder about.

Without getting into the specifics of which engine etc., fuel pump should run for 2 seconds when the key is first turned on. The computer controls this via the fuel pump relay. As long as the engine is cranking and the computer is receiving an RPM signal the relay is supposed to stay energized. This is one of those times when a simple test lite or voltmeter connected to the fuel pump power supply, at the pump, will provide a visual picture of whether the power is constant or interrupted. Depending on what result is obtained there, will determine your next course of action.

Well, we already know that it's interrupted, don't we? All the times I turn the switch on and don't hear the pump, or when it quits while driving? I was hoping to just find out what component commonly goes bad, and start there.

If you bridge the DLC to run the pump constant, does it drop out? Or always run when commanded?

I don't know what the DLC is.

You would not believe how many fuel pumps are replaced when the problem was just a poor connection ,usually at the pump. It;s not a good feeling to have dropped the tank replaced the pump and have the same problem. A good part of the time the ground connection for the pump gets corroded and produces a variety of symptoms and that is why tests are done right at the pump and plug/ connections. You can start checking for voltage drop from the fuse on down to the pump in the tank , but you cannot always be sure unless you do a thorough job.

DLC/Data Link Connector or EEC Electronic Engine Control..

Sometimes to 'test run' the pump and 'prove out' the system components like, pump, inertia switch, relay, and power distribution circuits/connections, you can jumper or 'bridge' the EEC/DLC for test purposes only. The trucks PCM/ECM controls pump run times by controlling the fuel pump relays ground. By jumping the test connector, you take the computers control of the fuel relay grounding function out of the diagnostic equation. During a normal system test with a scan tool, the fuel pump is run in the same way and checked. During a self test, if the ground was missing, generally a DTC is set.

On the left side of the engine bay, up close to the firewall/booster you'll see the test connector. May still be snapped into its plastic cover. With a jump wire grounded as show, the pump should run 'continuous' as long as the ignition is ON.

If by adding a ground on the connector corrects the pump drop out/run time issue, chances are the computer isn't supplying a ground or is intermittent to the relay. At least suspect.

If it doesn't correct,, then it helps to diagnose and pin point the area of failure since power distribution can be checked either from the pump forward, or from the relay on out back to the pump while the circuit is/remains active. As well as ground check while testing. If you have power distribution and an active ground at the pump connector, helps to diagnose a dead/dieing pump. Sometimes a stiff thump on the tank with the ignition ON will jar a pump back to life and confirm a sick pump.

Again,,TEST only. For fire and personal safety purposes, a test connection should never be wired and left. Being the relay harness was hacked, could be what was attempted or a test/run point..