How do you test power going back to the fuel pump in the tank? I replaced my fuel pump because it was not priming when I started the car. Now it still will not prime to start my engine. I need help to determine where the problem is. Thank for any help I receive.

 

Your first tests should be at the fuel pump relay using a 12V automotive test light . With one side of the test light properly grounded , do you have power on the orange wire ? If no , check the inline fuse . If yes , turn the ignition key to the run position with the light on the tan/white wire to see if it lights for 2 seconds (or so) when the key is first turned to run . Does the light come on for the 2 seconds ? If yes you've got an open connection between there and the back of the car , and if not you must move to the primary side of the relay (we've been on the seconday side with the orange and tan/white wire checks) . With the test light still grounded on one side , do the key on test while probing the small dark green/white wire at the relay . Does the light come on for the two seconds at initial key on ? If yes (and yes to power being on the orange wire but not at the tan/white for two seconds) your relay is most likely bad . If no , your ECM isn't putting out the 2 second prime signal .

That should give you someplace to start , and here is a diagram for your fuel pump wiring showing the wires I mentioned .

 

To do those test do you have to have the fuel pump relay unplugged from the relay?

 

I did the test with both wires to ground and neither one of them lit up when turning the key to the on position. I guess that means I have a bad ECM. The light does light up when you go from ground to the orange wire. So I know I have power coming in on that wire. If anybody can help me with this please let me know what I need to do to get this car going again. Thank in advance for the help.

 

Not positive but could this be vats related as well, as i think its suppose to cut fueling. Not sure if pump prime tho or if injectors

Has engine wiring been modded at all? or ecm been handled in anyway that would damage any wiring coming out? I guess ecms can go bad but i cant say i have seen that happen.

 

The wiring has not been changed at all. That's why I thougth that it was the ECM that was the problem. Thanks for the help. I will keep testing until I find the answer.

 

You have to do the test with the relay plugged in.

You can't be sure the ECM is bad till you test powers and grounds to the ECM, and Ohm the wiring from the ECM to the fuel pump relay. It would also be wise to initiate communications with the ECM to see if it will talk to you.

Also as mentioned it might be a VATS issue - I am not sure if the ECM will perform the prime without the VATS module signal. Is the security light on when you are trying to crank? VATS should also disable cranking but someone may have bypassed the relay in the kick panel. That's very easy to do.

In the VAST majority of cases, the "damn computer!" is not the problem. Not that it's impossible, but people always try to blame the thing they don't understand and it's usually not the problem.

GD

 

87 f body doesn’t have vats. The 2 second prime still happens even if a vats system isn’t working on vats equipped cars.