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Old 09-05-2013, 06:06 PM   #21
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I am going to start tracing the fuel lines back from the carbs to the throttle. Change the fuel filter / separator, and see what happens.
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Old 09-05-2013, 08:13 PM   #22
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I have it down to the fuel pump, as I can find fuel in all tubing until the tubing leaves the fuel pump. Just have to figure out how to remove it as it is in a rusty area and nuts are locked in. Could also be the Fuel Pump Relay, I guess.
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Old 09-05-2013, 08:22 PM   #23
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You never said what engine you have and is the fuel pump mechanical or electric? Electric will be about the length of a soda can and 1/2 the diameter. If mechanical then there is no relay.

As Dan said disconnect the fuel line at the carb and place it into a empty bottle or can and crank the engine, if you get fuel in the container the pump is working.
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Old 09-06-2013, 12:10 AM   #24
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Mike, I have a Merc MCM 5.0 V8. I took the fuel line off the carb already and turned the ignition while the metal tube was just dangling there, and no fuel. There was plenty of fuel in the filter/separator though, so I am thinking fuel pump. Was quoted like $280 locally, and found some for like $179 online. I checked for DC voltage at the leads that feed into the pump, and just go 0.07V when I tried to turn the engine over. I am assuming it should be 12 VDC? So before I buy a new pump, I need to find out what the voltage should be going to the fuel pump when ignition is turned to start, as well as see if I get that same voltage at the starter motor. The starter is new, so I doubt that is it. Do you know what voltage I should get at the fuel pump when I try to start the engine? That would be a great piece of info for me to go on. And thanks for all your help.
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Old 09-06-2013, 12:39 AM   #25
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The pump requires 12V, you can try jumping with some 20 guage wire from the battery to see if it will run. So need to check the fuse then relay. There is also a low oil pressure switch that kills the pump if the oil pressure is low. It is located on the block just above the oil filter assuming you don't have the remote kit.


PS: I supplied a wiring diagram a few weeks ago in another thread, just don't remember which one.
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Old 09-06-2013, 02:12 PM   #26
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HL, just want to make sure I have this straight. MCM 5.0 (GM 305) electric fuel pump with a carb connected to an Alpha outdrive, correct? One more thing that will help me is the year and engine serial number.
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Old 09-06-2013, 03:55 PM   #27
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The pump should get 12V when you turn the ignition switch to "On". If everything is working properly the pump will run when the key goes to "On" (or "Run") and in turn it will fill the carb fuel bowl with gas. Once that happens the engine should start even if the pump stopped working because the carb is full of gas. Of course, it won't run for long.

When you turn the ignition switch to "Start" you will see the voltage at the pump go down considerably or, depending on how the relay is wired, go away completely. If you aren't seeing 12V at the pump with the ignition On ya need to figure that out first; a fuse, a relay, the OP safety, wiring, etc etc.

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Old 09-06-2013, 04:47 PM   #28
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Thanks Dan. I check the pump with 12 VDC and it is fine. I did find a break in continuity on the yellow/purple wire that goes from the starter to the fuel pump. The connection at the starter was good, but the copper wire is broken just after the terminal connector on the bolt. So, I think that has been the problem. I will fix that and try again. The starter was also loose, so it most likely shook and helped the weakened wire to break. Well, this is surely one way to learn about my boat...Never had to before, because I only had it in water for 4 1/2 years, so it never gave me any problems.
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Old 09-07-2013, 01:19 AM   #29
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The purple/yellow (as labled in wiring diagram) from the starter provides 12V to the fuel pump when starting. Once started (key returned to run position) the wire from the starter goes open and 12V is supplied from the oil pressure switch. The purple/yellow wires from the starter and OP switch are tied together to a shared purple/yellow wire to the pump.
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Old 09-07-2013, 08:02 PM   #30
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Ok, new water separating filter, checked the fuel pump with 12 volts from car and it worked. Now, I put it all back together and try to start the engine. It won't start, but I took off the fuel filter again and it is mostly filled with fuel, though not all the way so that it leaks out when I unscrewed it. I am guessing that the fuel pump must have pulled the fuel from the tank into the filter. Does it take some time trying to start the engine for the fuel to reach the carb after installing a new fuel filter? BTW, I am getting 7.8 VDC at the fuel pump while wife is attempting to start engine. Is that low? Is it enough to not allow the engine to start?
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