Merc 175 cuts out randomly, able to restart instantly

[Gohawks688]

Howdy all,
I've got a new to me 1999 mercury 175 carb saltwater series that I am having issues with. It likes to suddenly die on me ( like you turned the key off , not a sputter sputter then die) after this I can crank it and it will immediately fire back up no problem. It does this on muffs and on the water. It seems to only happen at idle ( both in gear and out of gear). No problems at part or wot. very sporadic maybe twice an outing. Here's what I've done
Rebuilt carbs
Rebuilt fuel pump
Checked timing at idle within spec
Played with idle mixture screws (currently set at 1.5 turns)
I haven't dug into spark testing and I'm not really sure how to since the problem seems VERY intermittent ( once you go to restart the problem is gone)
I also have a hard time imagining that one dropped coil would instantly kill the motor. It's like it's losing all spark instantly enough to kill the whole deal.
I don't think it's heat related since it will instantly restart.
Any one have any ideas? Could one of the electrical boxes be shorting out? Are there any sensors on this motor that have the ability to cut spark (oil? )? Maybe the idle stabilizer?
Thanks for the help.

[Gohawks688]

I forgot to add that when I cleaned the carbs I pulled the intake and checked the reeds. they looked ok to me, no obvious damage. I am new to two stroke and outboards however.

could this be a stator issue? maybe the windings are occasionally shorting out? Is there a resistance test that can be done?

I checked spark with a timing light on all cylinders. thinking ,maybe I need to go back and use my inline spark tester to check spark quality. though suspect that it looks good until it doesn't and the engine cuts out.

[JOE54]

could be dout it though,yes there is a test, you could start it up run it without key harness connected to eliminate boat wiring , becareful youll have no way to kill it

[leonreno]

Also check the canon connector on the motor, if it’s loose or the pins inside have lost connection that can cause it to cut off. Sounds electrical to me as you seem to believe.

[Gohawks688]

Also check the canon connector on the motor, if it’s loose or the pins inside have lost connection that can cause it to cut off. Sounds electrical to me as you seem to believe.

thanks guys i will look into that. i will check the main plug on the motor and clean for corrosion/good connection.
As far as the boat harness killing the motor I have a few thoughts on what it could be. question is how to test them.

could be ignition switch intermittently failing to provide B+ to the ignition/engine. i guess i could wait for the engine to die, leave the key in the on position and then go measure for B+ at the ignition module. I know its theoretically possible, any one ever experience this before?

what about the kill switch? i guess it could be killing the engine too. seems strange that it would kill it, then allow restart with no manipulation.

same with the neutral safety switch. Also if I remember correctly from my other boats it stops the engine from cranking not from having an ignition source.

any other switches with that I am forgetting?

[EuropeanAM]

See Joe's comment above. YOUR engine would continue to run despite losing connection to key switched power (not a good thing but it would still run).

Your problem is likely ignition or fuel related.

If you would, please take a moment to review the Announcements at the top of the Forum, and be sure to provide the Required Info (serial number) when posting. Thanks!

[Gohawks688]

See Joe's comment above. YOUR engine would continue to run despite losing connection to key switched power (not a good thing but it would still run).

Your problem is likely ignition or fuel related.

If you would, please take a moment to review the Announcements at the top of the Forum, and be sure to provide the Required Info (serial number) when posting. Thanks!

Sorry my serial number is OG804616.

I guess I didn't put two and two together with regards to Joe's comment. How does the engine run with out signal from the key to the ignition? And how does it shut off if loosing b+ from the ingition does not stop it?
I'm new to outboards I'm a car engine guy.

[JOE54]

carbed engine needs no 12v from ign harness to run, ADI ign systems make their on voltage from stator, EFI mtrs must have 12v on purple wire to power fuel inj ststem carbs dont you can actually rope start it without harness plugged in

[Gohawks688]

carbed engine needs no 12v from ign harness to run, ADI ign systems make their on voltage from stator, EFI mtrs must have 12v on purple wire to power fuel inj ststem carbs dont you can actually rope start it without harness plugged in

Wow I guess I'm defaulting to old carbed car engines that need voltage to the coil/points.
So does the key switch ground out the coils/if ignition to kill the engine? If that was the case could the key be randomly supplying a path to ground?

[JOE54]

kinda sorta grnd path internal to switchboxes /yes could / thats what we are eliminating and possible wiring prob in boat its all a process of elimination

[SilverFox579]

And how does it shut off if loosing b+ from the ingition does not stop it?

LOL, haven't heard B+ for years. My USAF career was in communications-electronics so that made me smile.

[EuropeanAM]

The theory on the kill (stop) circuit (which is what causes discharge of ignition output to ground) comes from back in the Magneto days (often found on aircraft and marine applications). Many terminal-type output ignition switches actually label the terminals "M" (for Magneto).

In this case, the engine is "killed" by the switch completing a circuit between the Black w/Yellow Stripe (stop or kill circuit) to Ground (black). The Lanyard Switch is parallel wired to the same exact circuit.

[Gohawks688]

Ok so I'm on the process of testing the ignition components per the manual. I got a dva adapter. I am having trouble measuring the voltage input to the coil. It is jumping all over the place. I thought the dva adapter was to measure the peak voltage so there would be no jumping around. This is while the engine is idling by the way. Is this normal or should it be steady?

On a side not I ran the boat for an hour on the muffs with the harness unplugged and it didn't die, however this problem is so intermittent that I don't know if that tells me much. Plan on getting back out on the water this weekend to see if it happens again.

[EuropeanAM]

Should be relatively steady at a steady RPM. Simple method to determine if it is the Meter or DVA: Test all coil inputs. Should all be very close at a steady RPM (if one is bouncing all over and the others are steady, you've found a problem).

If they're all acting nuts- problem is in the Meter, DVA or cables.

Keep in mind that the DVA has a battery. Turn your meter to DC Volts... it should be approximately 3VDC with the DVA attached. If not, the battery may be in need of replacement.