1990 5.0 TBI, auto, bone stock engine.

BL: Fuel pump will not prime or run via relay but the pump works.

I need someone to help because my car has been down for going on 3 months and it’s my DD. My fuel pump does not want to run. I can hear that it isn’t priming or running at all.

The car was giving me issues starting. Pump would not prime before starts and had no fuel on several start attempts, and then would finally prime and start after sitting for a few minutes. The last time it ran I had the same issue getting it started, it then ran for about 3 minutes as I drove at 25mph. The fuel then cutoff randomly while driving and hasn’t run since.

I have replaced the fuel pump (Which was a pain without a lift) because the fuse is good, and I tried swapping relays with no change. With new pump still no prime/start. I continued to troubleshoot by confirming I have 12v going into the relay via Orange wire. I also confirmed I get 12v from ECM for 2sec at KOEOff. I jumped the relay by providing 12v directly
to the Gray fuel pump power wire and the pump ran fine, so I know it and all the wiring works fine. Yet still no prime/start with relay. Replaced relay and still no change!! I have now tried 3 relays (two were in the car already and one new).

could I have gotten a bad relay from the parts store? Or am I missing something else to test? I don’t like throwing parts at the car but I seem to be missing something during my troubleshooting.

Thanks for your help.

Slice

 

1. Find a wiring diagram. What wire provides ground on the trigger coil of the relay?
2. Do you have an ammeter? How much amperage is the pump drawing?
3. How much voltage do you have at the connector plug nearest the fuel pump, when the fuel pump is RUNNING?

Normally, even if the relay was removed with the engine running, the oil-pressure bypass circuit should power the fuel pump. I'm betting that you have problems in the relay part of the wire harness, AND you've got problems in the oil-pressure switch part of the fuel pump harness.

 

I’ve got the wiring diagram, and aside from the oil pressure switch side I have touched all of it. I tested the ground and it checked good with a multi-meter. I did not test what amperage the pump is pulling. However the only way I’ve gotten it to run so far is directly applying 12v to the gray wire. I guess I’ll try putting power the oil pressure switch.

Is the oil pressure switch also the sending unit for the gauge?

two related problems would really make life harder to diagnose. Thanks

 

Yes, the oil pressure sender/switch is all in one unit down above the oil filter. If you powered the gray wire, you also put power to the OP switch. That's not how it works... The OP switch is there to power the pump when the engine is running, if there's a ECM failure. You put power to the gray pump supply wire and the pump ran... did the car start and run while the pump was running?
Sounds like there could be an underlying VATS issue going on. I'll have to look in my service manual tomorrow and see what the flow chart says to check next.

 

I didn’t attempt to start the car when I applied 12v to the supply wire. I had too many wires strung across the engine at the time. I have gotten it to fire on starter fluid despite the relay being disconnected but ran out of it so couldn’t get it running on the starter fluid. I will attempt that once it stops raining everyday. I unfortunately don’t have a garage to work out of at the moment.

thanks a lot for actually trying to help.

Slice

 

My fuel pump issues has evolved
into an ECM issue. I no longer am getting any signal from the ECM. I don’t get a check engine light and I can’t get a Code 12 if I try to pull codes. What wires should be providing power to the ECM? I’ve already pulled an ECM and Prom from the junkyard with no change at all.

 

 

Did you check the fuse next to the battery?

 

So I started checking wires outside of the ECM harness and found some shoddy work done by the previous owner leading into the junction box. He replaced a chewed up wire with one half the size, just twisted the wires, and threw electrical tape on it. Wires started poking through and grounding out. ******* engineering drove me nuts for 4 months! Thankfully it didn’t start a fire.

Thanks everyone for pitching in to help out I really appreciate it.

Slice