Mercruiser 4.3 gen + no start, cranks but won't catch

[Mano26]

So I've been on a month long journey trying to get this motor started. Ran great last season, when I went to take the boat out it wouldn't start. Mechanic replaced oil pressure switch in the off-season and it fired up after 3-4 trys right before I put it in. Once in the water it started but as I drove to the slip it was stumbling and shut down. Couldn't start it again.

Mechanic came back out, said it was the electric fuel pump. I replaced it, still no start. I jumped the old and new pump to the battery to see if they spin (they DO)and if I could start it that way and nothing. It just cranks and I'll get a pop now and then like a backfire. Oil pressure is good, battery is brand new and putting out proper voltage, safety switch isn't tripped. Now the mechanic is "busy" so I'm stuck with a floating couch. Any ideas of what I could try? I'm not very experienced with engines but pretty handy. I've replaced the alternator, thermostat, fuel pump, etc in the 3 years I've had the boat.

 

[nola mike]

Spark, fuel, compression,timing. Which are you missing? If you're popping it sounds like a fuel problem. I'd check spark first. Verify that, move on to fuel. Pour a bit of gas down the carb throat and see if it catches. If so we can move on to troubleshooting the fuel system.

 

[Rick Stephens]

Might think about a new mechanic too. One that stops by, says buy *this part* and the results doin't change anything is not a mechanic.

 

[Mano26]

Rick, I'm way ahead of you. Replacing random parts is a good way to line his pockets, not fix my issue.

 

[achris]

Get a small length of electrical wire. Pull the connectors off the oil pressure switch and short them with the wire. Turn the key to ON. You should hear the pump running. Once the carb fills, try starting. If it starts and runs, you need a new switch or the wires connectors need cleaning up.

Chris....

 

[Mano26]

Get a small length of electrical wire. Pull the connectors off the oil pressure switch and short them with the wire. Turn the key to ON. You should hear the pump running. Once the carb fills, try starting. If it starts and runs, you need a new switch or the wires connectors need cleaning up.

Chris....

I jumped the switch(which was new) and pump doesn't run. I hot-wired the coil and it started with ease. Key switch has 12v in run and start. Purple wire at coil does NOT have 12v in run position....under 1v. Any ideas what to check next?

 

[79Invader]

So you put a jumper wire on the + side of the coil and it fired up? Sounds like you have a bad connection or a faulty key switch. No power to the coil in run position means no spark. Test key switch and if it turns other things on (like gauges) then it’s most likely fine, look for a corroded connection or a broken wire.

 

[Mano26]

So you put a jumper wire on the + side of the coil and it fired up? Sounds like you have a bad connection or a faulty key switch. No power to the coil in run position means no spark. Test key switch and if it turns other things on (like gauges) then it’s most likely fine, look for a corroded connection or a broken wire.

Yes, fired right up. key switch seems fine, I get voltage and gauges turn on.

 

[tank1949]

I jumped the switch(which was new) and pump doesn't run. I hot-wired the coil and it started with ease. Key switch has 12v in run and start. Purple wire at coil does NOT have 12v in run position....under 1v. Any ideas what to check next?

Sounds like you are not getting at least 12 volts from ignigtion switch back to coil? My MC motor helm switches each had an independent breaker that got corrosion inside and would not provide 12 volts back to coil. Go to Harbor Freight and buy a cheap multimeter to test voltage(s). Maybe mechanic "bumped" the wrong wire?

 

[achris]

Also. Look for a 'Deadman' or kill switch... If that's connected up, the lanyard may have been pulled out...

Chris...

 

[Mano26]

Any chance a bad shift interrupter could cause this?

 

[79Invader]

Any chance a bad shift interrupter could cause this?

Do you have an Alpha drive? If so no because your shift interrupt switch connects to the white and green wire on the back on the sensor in the Dizzy. Not the power wire to the coil.

 

[Mano26]

I think I've narrowed it to engine side of the 10-pin purple wire. At space 5 (under number 6) it looked like it was plugged up with rubber or whatever and I can't feel the female connector at all. It may be buried deep down. The boat side is 12v with key on run, engine side was 0. Is it ok to clip that purple wire on each end and put in a permanent jumper or do I need to wire up a whole new 10-pin?

 

[Mano26]

Jumped the purple wire and all is good!