Every time I hook up my oil pressure sending unit the fuel pump turns on. It stays on until I unhook the sending unit. This is the second switch to cause this.
Could the fuel pump relay be going bad?

 

Check your FP relay, it may be closed. Just a thought.

 

Ok. Thank you. Two questions
1. Which relay is it?
2. Would it still run fine if the relay is closed?

 

If I got this right,when you turn the key to ON, the FP Relay closes and sends a signal to the Fuel Pump for 2 seconds to prime and then the pump shuts off untill you crank the engine. Then if the oil pressure switch see pressure, the fuel pump comes on and the engine starts. They all work together. So...if the relay is stuck closed, it's sending a signal to the FP to turn on.
Now, on the other hand, you have the V-6. I'm pretty sure it works the same. Not 100% sure, but a stuck relay is a stuck relay.

Anybody else with more detail, please chime in.

 

Ok, thank you.

 

I know I'm a little late to the party here, but that description above is not at all how the fuel pump circuit works.

The way the fuel pump works is simple, but it's in 2 parts.

1. ECM controls fuel pump relay. The ECM closes the relay circuit for 2 seconds at initial power on to prime fuel system and bring up to proper operating pressure. When engine is cranked, with any luck, the ignition module in the distributor or other ignition device (coil pack assembly, crank trigger, etc) sends reference pulses to ECM telling ECM engine is being cranked over. ECM then closes relay circuit to turn on fuel pump and grounds fuel injectors to provide fuel delivery into cylinders and engine starts.

2. Oil pressure switch bypasses fuel pump relay. In the event of either an ICM failure, ECM fuel pump driver failure, fuel pump relay failure, or primary system wiring fault, the oil pump switch closes at 15 PSI of oil pressure to provide power to fuel pump to allow engine to run to be limped to either a shop or to complete another trip to have the primary circuit repaired. Which is why some will note that the fuel pump continues to run after ECM and engine are shut down until oil pressure drops below 15 PSI.

If the fuel pump is running whether or not the ignition is on when the connector is plugged into the oil pressure switch, then the switch has failed and is stuck closed. Switch gets constant power from the same fuse feeding primary fuel pump relay circuit and ECM constant power feed, and outputs into the same relay output wire as a bypass.

 

I am having a similar issue with my car after V6 to TPI swap. The car ran but now all of a sudden it won't start unless the fuel pump relay is unplugged. As soon as I plug it back in when I can get it to start the engine dies. Is this a faulty oil pressure sending unit?

 

Never mind. Posted in wrong thread. Moderators, please delete this post.