Fuel Pump On Without Key?????

[jimmy'sjimmy]

Just like the title says, fuel pump stays running without key. Its an 87 TBI 350 with brand new pump and relay. Any reason it would stay on? All i did was turn the key to acc. to get the trouble codes from last time i drove it and the pump stays on, i unhooked the battery and when i hook it back up pump comes on. It usually comes on and them stops after a few seconds. Any ideas?

 

[dyeager535]

Odd. Sounds like the ECM is getting power when it shouldn't.

I'd pull the ECM fuse, see if it happens again.

ECM controls the fuel pump prime (2 seconds) nothing else should be capable of making the pump run for a certain amount of time.

 

[jimmy'sjimmy]

ok pulled the fuse and it quit. there was a 30amp fuse in there!!! could that be the reason???? shouldnt it be a 10a?

 

[dyeager535]

Have to look at the wiring diagram for the truck. Higher amp fuse would not cause what you are experiencing to happen.

Need to figure out why the ECM is thinking the truck is on. I'd be most suspicious of the EFI related items that are ignition hot only. Check that they aren't getting power when they shouldn't be.

Your relay, if it was dead before, wouldn't have allowed the pump to run for prime, and you'd never notice it.

I assume inserting the fuse just makes the pump run more?

This may have manifested itself as a battery drain in the past.

 

[jimmy'sjimmy]

Well the problem before was that there was no juice to the pump. Turned out to be the relay. New relay and all was fixed. Hmmm.....Did i forget to mention that this is my first motor swap??? Maybe i crossed some wires or hooked em up to the wrong partner. It was working fine, drove it from Austin to Dallas 200 miles and no prob just emissions codes. It has been sitting since Sunday and nothings been touched until this evening. I ll have to wait till tomorrow to get after it, its already dark.

 

[dyeager535]

There will be juice to the pump even with the relay gone, the oil pressure switch handles that. It takes 5PSI to trip it (so longer crank time) but the relay is not needed for the fuel pump to work.

 

[jimmy'sjimmy]

We changed the oil pressure switch also when we changed the relay. Just figured that the relay was the reason. I remember hearing or reading that the oil pressure switch will cut the fuel if it goes to 0psi. U think the pressure switch could be the reason? Had to use my original switch and a diff adapter because the one it came with broken, and since it was a 92 had different angle and connector. Ill have to get a good look at all of that tomorrow. Thanks for the help!!!!

 

[dyeager535]

Oil pressure switch *will not* cut fuel IF the relay is working. It is solely as a backup for the relay.

You could try disconnecting the oil pressure switch, see if it solves the problem. It always has 12V going to it, if it's failed closed, it would force the pump to run I guess. But wouldn't really explain the timing of the pump.

 

[jimmy'sjimmy]

Ok so i looked at a diagram and its when i pull the ECMB fuse it shuts off. Looked at the wires in the truck and looks like the tan/white wire from pump that runs up to the "hot fule module" dunno what that is connected to the gray oil pressure wire. The fuel pump relay had fell off the little mount on the firewall and was resting on the headerand is now kinda melted!!!! Anywho the red wire coming out of the harness from the relay has nothing connected to it, so i connected the tan/white from the pump to this. Is this correct or should i have the fire dept on call????? Dyeager was right about the battery drain thing, optima is dead and wont charge so i cant cehck what i wired. Anyone think the way i changed it will work, before i get a new battery????

 

[4X4HIGH]

The red wire on the relay harness is for testing the pump. If you put 12v to that red wire it forces the pump to work.

 

[dyeager535]

You've got the hot fuel module?

I'd try either swapping that with a known good module, or just bypassing it.

IIRC, the hot fuel module somehow kept the pump running a bit so the fuel doesn't sit in the lines and boil after you turn the truck off. If that's how they work, then if the module failed "closed", it would never shut off.

If the sole power wire to the fuel pump runs through the hot fuel module (would seem odd not to have a bypass, like the oil switch for the pump relay) then you'll need to disconnect the module, and test to see if the 12V going to the pump is coming from the module, or somewhere else.