Merc. 5.7 MPI Oil switch No start Problem

My 5.7 MPI will turn over but will not run; however if I relieve the oil pressure by opening the oil pressure port on the top rear of the engine near the distributor the engine will start. Only problem is I have oil everywhere.

Serial# 0M620208, 2004 Searay 240 Sundeck. Yes, the serial number is for a 5.0 MPI, I upgraded the engine block and reflashed the ECM555.

I checked the oil pressure sensor wiring, my wiring diagram shows two wires (DK BLU/BLK and BLK/PNK) which are there, but there is also a GRY wire as well.

The sensor is a Mercruiser oil pressure sensor, part#881879-6

My understanding is the oil pressure switch is an on/off switch that supplies power back to the ECM555 and then the ECM applies power to the fuel pump relay.

The fuel pump will prime when the key is turned.

Any thoughts on why it appears the switch is operating backward or reversed?

Thanks

Joe

If your diagram only shows two wires, then that diagram is not applicable to that engine.

your understanding of an oil pressure switch may be what it is...but that's not how the oil pressure sensor works with the 555 ECU...and even if the sensor was bad, the pump should still run...see what strategy is used with the 'updated' program...

Ok, that is the wiring diagram that came with the repair manual. Do you have the proper diagram?

How does the oil pressure switch work with the 555ECU?

Thanks

This is the same diagram as I have. So when I turn the key on both pump will prime plus the engine will run as long as I relieve the oil pressure.

click to enlarge

that is a digital pressure sender, not a switch. the dark green wire in my previous post goes to ground in the ecm and closes the relay to operate the pump. your missing something but its not the sender for the gauge

Ok, I see that now, my wiring diagram is correct, just my eyesight. I did not see the faint grey line going to XDR PWR. By the way, what is XDR Pwr?

So I will trace my wiring to the fuel pump and ECU.

Just to clarify, I was misinformed then about the oil pressure sensor controlling the fuel pump?

that is the 5 volt feed to all the sensors. a 3 wire has 5v input, wiper to ecm, and ground

Ok Got it.

What about the digital oil pressure sender. Does it have any bearing on the starting of the engine?

none...................................