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fuel problem on 91, 5.0

Flysure1

Active Member
I put in new fuel pump, ran fine, quit once again then started, was showing it to sell, quit again, 2 days later it started, put it in shed and it quit again, put in new relay and bypassed inertia switch, still not coming on with turn of ign. switch, relay clicks a second time if I stick a test light on hot out, voltage is very weak at tank connection----ground problem?---I can run a hot direct to fuel pump and it runs constant.
 
First of all check all the connections and verify you have a good ground. Start chasing where the path for the power breaks down at. Could be bad ignition switch, bad connections... Is it controlled by any computer units?
 
I could be wrong but I remember some kind of connector under the drivers seat for the fuel pump. Maybe check there for a bad connection.
 
"AzPete" said:
Is it controlled by any computer units?
The computer does control the fuel pump relay. When the key is turned to the ignition position, the computer tells the pump to run for a few seconds to prime / pressurize the fuel rail. It will continue if the engine is running, other wise it will shut off. If you turn the key on and off a few seconds apart, you should hear the pump run each time.

Also, It's been a while since I've read the schematics, but I think inertia switch is wired into the relay's control circuit, not in series with the power / pump... meaning that if you bypass the switch, it will only affect the relay's activation. If the inertia switch is faulty, then it would shut off the relay, therby shutting off the pump. I have the schematics somewhere if you need that confirmed...
 
"stangg" said:
"AzPete" said:
Is it controlled by any computer units?
The computer does control the fuel pump relay. When the key is turned to the ignition position, the computer tells the pump to run for a few seconds to prime / pressurize the fuel rail. It will continue if the engine is running, other wise it will shut off. If you turn the key on and off a few seconds apart, you should hear the pump run each time.

Also, It's been a while since I've read the schematics, but I think inertia switch is wired into the relay's control circuit, not in series with the power / pump... meaning that if you bypass the switch, it will only affect the relay's activation. If the inertia switch is faulty, then it would shut off the relay, therby shutting off the pump. I have the schematics somewhere if you need that confirmed...

I have not messed with this for a while---I think it is maybe a ground problem--there is only one wire in and out of inertia switch and I wired them together--there is a red wire that is hot(runs into the relay under drivers seat) and I assume it feeds the relay, when I turn the key on I get a click in the relay then prob a second or so later another click, there is a blue wire that has low(dim test light) coming out of the relay---there is also low(dim test light) at the very last connection at the tank---I can run 12 volts off of the battery to the pump and the car runs--(also the fuel pump runs constantly)
 
I believe the blue wire is the sense wire that goes back to the computer to tell it whether the fuel pump is running or not. If you suspect that grounding is the issue, then I'm pretty sure you can "jump" ground by connecting the terminal on the debug plug to chassis ground. I made the attached drawing back when I was putting EFI in my car, and am pretty certain this picture represents the factory setup (Blk = GND, Blue = Sense, Red = 12V. I got the circuit out of the Helms manual. The book also shows where to find each relay. I'm not sure where Iput the book, but I wlill find it to verify in the next day or two.
 

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