mercruiser 5.7 hard start fuel pump/oil pressure switch?

[jaybird7]

Hello newbie here, I have a 1995 Rinker Captiva 209 which i believe was repowered in late 90's or early 2000's? with a 5.7 and is carb model. Ive been having hard start no fuel issues which led me to the fuel pump. The fuel pump does nothing but if I take the wire off what i believe is the oil pressure sensor (next to the oil filter) and I briefly jump the fuel pump it kicks on and the engine will start and continue to start each time all day. when it sits for several days it fails again. This leads me to think the oil pressure switch is preventing the fuel pump from running because I get no fuel at all after it sits for days. The switch has 2 wire prongs on it one to the alternator and one to the fuel pump. If it is indeed the switch does anyone know the part number and if local auto parts places carry it? The oil pressure gauge always seems fine so I'm thinking maybe the few psi during starting just isn't registering and allowing the switch to close and let the voltage flow to start the fuel pump? any input greatly appreciated.
Jay

 

[Bt Doctur]

mercruisers are powered by the ign system, volvo`s are powered by the alt with diodes

 

[alldodge]

The starter powers the fuel pump (Purple/Yellow) when the motor is cranking. Once the motor starts and oil pressure builds, then the switch sends power to the pump

Check the purple/yellow wire coming from the starter

 

[jaybird7]

i appreciate the wiring schematic but my wiring is different in places. my starter doesn’t have that many wiring connections 3 not 4 and no purple wire going to it. a big red one, orange one, and the yellow with red. across from the yellow with red is just a plastic bump no feasible wire connection. the fuel pump has two wires one red one black the red goes to the oil pressure switch that has 2 prongs. the other prong on the oil pressure switch has a red wire as well and that goes to the alternator. i’ll add pics
the red and black that’s below my finger in the pic is the red and black that goes to the fuel pump.

 

[alldodge]

I'm thinking the electric fuel pump was added to the motor later. In 1995 the SBC had a mechanical fuel pump unless it had TBI.

The switch has 2 wire prongs on it one to the alternator and one to the fuel pump

It needs a 3 prong switch and would be wired like this




https://www.summitracing.com/parts/...MCRQtRF-da9NEnJY2TLdGKV7SZdu98xBoCSKwQAvD_BwE

 

[jaybird7]

Thank You AllDodge. I will get this switch and wire it according to what you've sent. I'd think this switch is available at a local auto parts store but when I called they insisted I needed a part number (smh) I figured being GM simply asking for a 3 prong oil pressure switch for 95 GM SBC would suffice, guess not. 1/8 npt for one you listed but I've also seen one as 1/4 npt though I think you are correct. I'll let you know how it goes when I get to it.

I guess that means the motor may be original and it wasn't repowered. I assumed repowered because the cover on top of carb says 5.0 and serial numbers don't match motor. The motor is most def 5.7 because the 5.0 head gaskets didn't fit when I replaced them but the 5.7 did. Previous owner was an idiot and perhaps just threw a cover he got elsewhere on it.

 

[alldodge]

I'd think this switch is available at a local auto parts store but when I called they insisted I needed a part number

Yes they would have it at the local auto store, but the issue is unless you find a guy with gray hair, you need a number

 

[harringtondav]

The two prong oil pressure switch will close when it sees 4-5 psi of oil pressure, enabling the fuel pump to run. Since jumping the harness caused your fuel pump to run at key on, I suspect the switch is bad, needing more oil pressure to close. Merc 87-86425A01.

 

[nola mike]

That wiring doesn't look original. Is the black wire from the fuel pump just a ground? I'm wondering if you have anything powering the pump when cranking. If you restart, you still have enough fuel in the bowl that the pump isn't needed during cranking. In between starts the fuel might be draining. I (and many/most others) wire in a relay to power the pump. I used a Carter a68301 3 prong switch.

 

[jaybird7]

the black wire is just a ground. as alldodge said i think the electric fuel pump was added later on and likely someone just used whatever wire they had on hand. the power it was getting was from the alternator during crank (the other prong on the oil pressure switch) but oddly after testing it it was barely 10-11v. i think the solution is the 3 prong switch to take a proper voltage from starter wiring until it fires then from key on. likely the 2 prong switch is bad and isn’t working at the 5psi. i had to order the switch at local auto parts place and it comes today. they simply found a picture of a 3 prong oil pressure switch and ordered it, hopefully they are all the same part in terms of NO NC and C.
if this solves my hard start issue i’ll be very happy. it’s been a struggle starting with starter fluid or direct gas just to get her fired after sitting. should have dealt with it sooner. thank you to all who reply. it’s really appreciated!

after looking online at what they ordered it doesn’t appear correct (they ordered ps127) ps64 looks correct and of course they don’t have it. perhaps i need to just order the carter? i had no idea the switches are so different for the same motors... curious about needing a relay? seems just an in line fuse suffices per wiring schematic? i am not great at electrical but have some ability.

 

[alldodge]

This is one by Airtex and they make the pumps for Merc and others
The one you ordered may work, but its designed to light a bulb when pressure is lost. Most likely it will work, just depends on current rating

https://www.summitracing.com/parts/...gd9VSEMHe7qf-AX3dNUVrVwOi0DxHQexoCgYgQAvD_BwE

 

[jaybird7]

good morning gents, I am happy to report after getting the ps64 (3986857 a more widely known part number) 3 prong ISP marked oil switch wired in that the engine fired right up. I knew once I touched the throttle to prime it and saw it squirt fuel right away it was going to work! it started so much faster and easier whereas before it would act really cold blooded and often die and require starting a couple times and giving it some throttle. it’s had been sitting 6 days. I did install an inline 10 amp fuse too as the wire diagram suggests. oddly unless i’m mistaken the idle rpm on the gauge seems slightly higher though the sound of the engine does not. I could have sworn it was 1k rpm and now is 1200. I don’t recall what the recommended idle rpm is off hand. I also have had an ongoing issue of just a click from the slave solenoid and after a few key turns it engages, I guess now would be a good time to sort that out and be 100% done. probably just the slave solenoid going bad. The 2 prong oil pressure switch that I removed has the original motor paint on it so I guess it’s 25 years old like the boat! Thank you to all that took the time to give input and Thanks again to alldodge for the schematics and knowledgeable input! Good day fine sirs!

 

[alldodge]

Idle should be 650 to 700 rpm for idle

 

[jaybird7]

the gauge must be off then because if i lower it to 1k it just about dies. sounds perfect at 1200. weird

 

[nola mike]

the gauge must be off then because if i lower it to 1k it just about dies. sounds perfect at 1200. weird

Or your timing's off or your carb is gunked up...

 

[alldodge]

Your probable right, but get a loaner tachometer from the local auto store to verify.

 

[Jtom7]

Here we go again, this year the fuel pump doesn’t energize using the 3 prong switch installed last year during start up. If I run direct power it works fine. Someone mentioned a relay or wiring in a power switch to power the pump for start up then cut it off. Can anyone elaborate on that so I don’t screw up the wiring? How the 3 prong switch is always an issue I’m not sure.