Only runs when cranking

[Capt dave 258]

Have a Mercruiser 4.3 ltr, 1995. Recently purchased, clean boat. Carbureted, electronic ignition. Had it on the lake and was running great. Trailered it home during a severe rainstorm and got lots of rain and moisture in the engine compartment. Went to the launch a few days later and it'll only run in the crank position. The moment you let the key go, it dies out. I believe it's an ignition problem but not sure. Getting gas during initial cranking. I thought I remember that once you let go of the key, the electronic fuel pump stops its initial primer phase and goes into a low pressure phase that may run through a diode or transistor that's in line with the wiring?? My mechanic says that it should still run for a few seconds off of the gas that's in the bowl. He had me check the purple wire from the positive side of the coil when it's unattached and see what the voltage is. I did so and it's .05 volts, which I assume is nowhere close to the 12 volts it should be?? I'm waiting to hear back from my mechanic but thought in the meantime I'd get help/ideas from this post. Thanks for any input.

 

[nola mike]

Have a Mercruiser 4.3 ltr, 1995. Recently purchased, clean boat. Carbureted, electronic ignition. Had it on the lake and was running great. Trailered it home during a severe rainstorm and got lots of rain and moisture in the engine compartment. Went to the launch a few days later and it'll only run in the crank position. The moment you let the key go, it dies out. I believe it's an ignition problem but not sure. Getting gas during initial cranking. I thought I remember that once you let go of the key, the electronic fuel pump stops its initial primer phase and goes into a low pressure phase that may run through a diode or transistor that's in line with the wiring?? My mechanic says that it should still run for a few seconds off of the gas that's in the bowl. He had me check the purple wire from the positive side of the coil when it's unattached and see what the voltage is. I did so and it's .05 volts, which I assume is nowhere close to the 12 volts it should be?? I'm waiting to hear back from my mechanic but thought in the meantime I'd get help/ideas from this post. Thanks for any input.

Yes, your mechanic is correct on all counts. Ignition on should be 12v to the purple wire. Do your gauges work? If so check your dead man switch. If not check ignition switch. Otherwise you'll need to find where you lose power along the purple wire.

 

[bbook83]

I once trailered in a heavy rainstorm, got to the lake late, parked the boat on the trailer next to the cabin, and awoke to the sound of the engine trying to start on its own, and running only when the starter was turning. Turns out that overnight water had gotten into the ignition switch and was activating the starter. I replaced the switch and all was well.

 

[Scott06]

Yes, your mechanic is correct on all counts. Ignition on should be 12v to the purple wire. Do your gauges work? If so check your dead man switch. If not check ignition switch. Otherwise you'll need to find where you lose power along the purple wire.

What he said… either key or kill switch is cutting off power when starter disengages

 

[Capt dave 258]

Here
s where I'm at. The ignition switch seems to be ok. It's a three prong switch. I have constant power to the red terminal on the back of the switch, power to the purple wire when the switch is in the on position and power to the yellow wire when in the crank position. All gauges work when the switch is in the on position. I checked the deadman switch and have power through it when the key is in the on position. I do have 12volts to the purple wire on the solenoid when key is in the on position. I initially thought I did not, but didn't realize there are two purple wires on the positive side of the coil, one with power coming in and the other taking power out to the distributor. After doing a lot of checking yesterday, the boat happened to start when I tried it. I thought that maybe something that had gotten wet had dried out, I attempted to let it run for a bit with the muffs on but it ran for maybe 20 seconds and then died out. Attempted to start it again and same results, it'll stay running as long as the key is in the crank position. So, once again I'm not sure what's causing the problem. I also replaced the cap and rotor with new. Could it be something electronic in the distributor or an intermittent faulty ignition switch? I'm going to spray some WD40 in the ignition switch and let it sit awhile. I was thinking of running another separate fused wire directly from the purple wire on the back of the ignition, directly to the purple wires on the coil to see if that would correct the problem, leaving the original one on there as well so the gauges will operate. Any thoughts on that? Thanks for any constructive input.

 

[Rick Stephens]

Stick a voltmeter or a test light on purple. Start at the disty during all your tests. And work your way up stream until you get to the ignition switch.

 

[Capt dave 258]

Well the boat is running and after a few days of searching for the problem, I am still at a loss as to exactly what it was. The purple wire from the ignition was definitely losing power but not quite sure where. As I mentioned, the boat was trailered through a terrible rainstorm without the cover on and I believe something got wet that took a few days to dry out. My thanks to those that took the time to send me their thoughts and recommendations.

 

[airshot]

If not found and fixed it will probably happen again, when least expected...