Mercruiser 5.7 Running Issues

[Redneckboatfloating]

Boat was on the lift running just find then it just cut off. Would not restart. Did some checking and realized the fuel pump was not cutting on or getting power. I jumped power to the fuel pump and it starts then shuts down. Its acting like something is shutting it down the minute it starts. I've been told bad oil pressure sensor.

 

[zellerj]

Yes an oil pressure safety switch (OPSS) that is bad, or has a broken wire, will cause these symptoms. You can get them from an auto parts store for about $20-30.
In boats with an electric fuel pump, regulations require that you have a OPSS that prevents a non running engine to continually pump fuel with the ignition switch set to run. While cranking, the fuel pump is powered from the starter solenoid and runs. After the engine starts, and the ignition switch returns to run, the OPSS is closed if it senses oil pressure - usually about 4 psi, keeping the fuel pump pumping and away you go. If it does not sense fuel pressure, the OPSS switch opens and the fuel pump turns off. A broken OPSS is open, so after the cranking stops, the fuel pump turns off, causing the engine to stall, and preventing the fuel pump from dumping gas into your bilge.

 

[Rick Stephens]

Don't really have enough information to answer precisely your questions.

What Merc 5.7 motor do you have? MPI, carbed? They are totally different. Late model with ECU?

Serial Number would definitely help.

 

[Redneckboatfloating]

My fuel pump has always cut on the minute I turn the key and will keep running if the key is left on. So it sound like it wasn't wired correctly to begin with. Its cutting off when I have the fuel pump wired directly to the battery.

Its a Thunder Bolt Ignition and carb?

 

[Rick Stephens]

Only fuel injected motors can legally have the fuel pump on when only the ignition is turned on.

Still need more info to know just what you have. Post serial number, or a pic of the label on the flame arrestor cover over your carb.

 

[Redneckboatfloating]

I'll get the serial number this evening

 

[Redneckboatfloating]

Here is a pic of the serial number and and the carb cover. I also attached a pic of what I think is the knock sensor which I think should have a wire running to it?

 

[Rick Stephens]

Yup, needs an oil pressure switch.

 

[Redneckboatfloating]

Yup, needs an oil pressure switch.

View attachment 365012

Did you see my post where I said I hooked the fuel pump directly to the battery just to see if it would start? It starts then shuts right down. Does the oil pressure switch also cut off the ignition?

 

[Rick Stephens]

Did you see my post where I said I hooked the fuel pump directly to the battery just to see if it would start? It starts then shuts right down. Does the oil pressure switch also cut off the ignition?

Yes, I saw your post. No, the oil pressure switch does not also turn off ignition. However, have to start fixing things as they get identified. The pump always running when the key is on removes the usefulness of hotwiring your fuel pump since it is already hotwired. And it needs fixing by wiring it correctly. As you work through things like the fuel pump electrical system you will find other stuff jerry-rigged by previous owners.

 

[alldodge]

it starts then shuts down.

Either the kill switch (man overboard) or the key switch

Check for 12V on the purple wire with key ON

 

[Redneckboatfloating]

Yep, it was the kill switch. I was cleaning the boat when it shut off so I must have hit the switch.