4.3 starting problem

[Mark R]

I have a 1997 4.3 Mercruiser with an electric fuel pump and standard 2bbl carb. I understand that it is set up with a pre lube system meaning that before the unit will start the crankcase has to have so much oil pressure. It appears I have a hot wire to pump and a wire to the oil pressure sensor on the back of the engine. I assume when the pressure is built the line from the oil sensor acts as a ground and then allows the fuel pump to work and force gas into the carb and there for "run" ..untill then I have no ground?
At present I hit the key and about 75% of the time it wire fire right and run about 4 seconds and then die, I then must dribble gas into the carb throat to keep running for about 45 seconds and then I am good to go ...the other 25% of the time it may start right up and keep running.
That being the case I ask this...can I cut into the wire running to the oil sensor and make that a "ground wire" and just by pass the sensor completely so when I hit the keep I will have completed the pos/neg circut and have the pump pushing gas into the carb..and not hurt anything?

 

[firehog6305]

Re: 4.3 starting problem

I wouldnt screw around with cuting the wires, they are on there for a reason, no matter how stupid it is to us, the designers put it there for a reason, you should always stay away from changing your vessels fuel systems, thats just my opition. I would read the thread from "Don S" its called testing your boats fuel system, its very helpful information

 

[thrasher]

Re: 4.3 starting problem

You could make the modification that you are suggesting, but it would be very dangerous. Your modification would mean that the fuel pump would be running all the time the ignition was turned on (regardless of wether the engine is running or not). This is considered an unsafe marine practice, which is why the pump is wired into the oil pressure switch.

Usually the oil pressure switch for the fuel pump switches positive not negative, and you then have a second wire coming from the starter motor which also provides a positive supply to the fuel pump whilst the engine is being started.
I suspect someone has added the fuel pump to your boat and tried to use the existing oil pressure warning switch (which switches ground not positive) to make the fuel pump safer (Of course I could be wrong).

The best thing would be to get the correct oil pressure switch and then correctly wire the pump into the pressure switch and starter motor.
You could also add a prime switch where you have a switch on the dash board that brings ground/negative to your pump so that you could run the pump before you start the engine. If you decide to do that, make sure you get a switch that is spring loaded so that it always returns to the off position, this means you have to physically hold it down to prime the pump and the minute you let go the switch turns the pump off, the pump is then dependant on the ground from the oil pressure switch. This would be much safer than just bypassing the oil pressure switch as you suggested, but it's more work to wire in an additional switch than just bypass the pressure switch.

Whatever you decide, do NOT just bypass the pressure switch and leave the pump running all the time that the ignition is turned on.

Gary

 

[getinmerry]

Re: 4.3 starting problem

It sounds like a simple choke problem to me. He hasn't said that he has actually checked for fuel flow when starting. Also, there should be enough fuel in the bowl to run the boat for at least 15 seconds before stalling again.

Chuck

 

[Don S]

Re: 4.3 starting problem

For one thing, your assumption of how and when the fuel pump runs is all wrong.
When the starter is engaged, the fuel pump IS or should be running. This will keep the fuel bowl full and allow you about 20 seconds before the carb runs out.
When the engine starts and the oil pressure builds to 5 to 7 psi it closes the oil pressure SWITCH, (located on the port side of the engine just above the oil pan) and puts power to the pump.
When you try to start the engine cold, do you pump the throttle a time or two to prime the engine, and as Chuck stated, verified the choke is actually working? Is this boat new to you? Lots of people new to carbs try to start them like a car with EFI, and it just doesn't work.

 

[Haut Medoc]

Re: 4.3 starting problem

Welcome to iboats, btw......​