1996 mercruiser 4.3lx electric fuel pump only works when cranking

Blueheeler

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Mar 25, 2020
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Motor will start and run after cranking for around 5 second but soon dies from lack of fuel. iI checked and it only has 12 volts when cranking. the boat shop said it could be the oil pressure sensor so i bridged the wires an d it does the same thing???
 

Blueheeler

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So when running i have 10 volts at the fuel pump connection is this enough to sustain fuel pressure?
 

Rick Stephens

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Still sounds like a problem with the oil pressure switch. It is the one on the top rear of the motor Not sure how you can get 10 volts. The pump isn't very high amp load, just a few. And it obviously works or it wouldn't prime when you cranked. As you sound like you already know, there is a wire on the starter that powers the pump when cranking.
 

Blueheeler

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Hi Rick. I think i found a fuel circuit schematic for the boat (96 Mariah) and as you described there seems to be power to the pump when cranking i now see it comes from the starter. I'm not sure after the key is released after starting what powers the pump? It looks like it taps into the alternator circuit somewhere. I show 10 volts at the pump and choke wire and one side of the oil pressure switch with key on and engine off but once started i have 10 volts across both sides of the oil pressure sensor but 12 at the battery. I am charging both battery's right now and will verify voltage again in the morning. Do you know if any part of the system reduces voltage? (Resistor Ect Ect)
 

Scott Danforth

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clean your terminals. you probably have a few dirty terminals causing the voltage drop.
 

Blueheeler

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So i cleaned some wires and it is a little better. I connected the oil pressure sensor wires together and now the fuel pump runs with the key on. What kind of output should i see from the alternator rear terminal? I am thinking the battery's are running low on charge.
 

Bondo

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So i cleaned some wires and it is a little better. I connected the oil pressure sensor wires together and now the fuel pump runs with the key on. What kind of output should i see from the alternator rear terminal? I am thinking the battery's are running low on charge.
Ayuh,..... Once the rpms have hit 'bout 1200 after startin' the motor, the alternator should be producin' 'bout 13.6 volts, or slightly more,....
 

Blueheeler

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Okay thanks for the reply. Unfortunately I cannot see if the alternator is putting out because this morning it will not start I disconnected the fuel line from the carburetor and the fuel pump is putting fuel out during cranking. The carburetor is not dumping any fuel because it will start with carburetor cleaner down the throat. I guess I need to take the carb off now.
 

Rick Stephens

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The purple wire from the ignition switch goes to the electric choke and the oil pressure switch. When there is oil pressure that switch should then run to the fuel pump. The fuel pump then will quit running if there is no oil pressure. The purpose is to stop fuel from pumping when the motor isn't running. This is Merc wiring, not Mariah.

If you are not getting fuel to the carb that is not a carburetor problem.


Photo below is of the wiring diagram plus a modification to add a relay in purple. I didn't have the diagram without that mod, sorry.

wirepump.png
 

Blueheeler

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So it looks like it is a carburetor problem. I just dropped it off at a local Rebuilder. I'll post the results when I get the carburetor back tomorrow. Thanks for the information everyone!
 
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