2016 Merc 150hp outboard intermittent rough running issue

[nasdaqsam]

As stated, every once in a while, the motor will run like it is in guardian mode. All I have to do is bring it back to idle and start out again and it stops for a while. It doesn't matter if I am just barely moving or on plane it happens either way.

I don't think it is water in the gas because it will do it until I idle it down and go back up regardless of how long that is. I only use ethanol free gas and add ethanol treatment.

The tach does on occasion show overrevving not when the motor is acting up but occasionally. I just have to tap it and it goes back to whatever RPM I am running. My mechanic feels the tach could be the issue, but I thought the tach just showed the motor rpm not the motor relying on it to run properly. ???

I read on here it could be the coils it's a known issue. Is there any way to test this? I don't want to just throw parts at it.

Thank you in advance for any assistance you may offer. I do not get online a lot so might be a little bit before I can answer questions or get back to anyone.

 

[alldodge]

My mechanic feels the tach could be the issue, but I thought the tach just showed the motor rpm not the motor relying on it to run properly. ???

Your Mech could be right. The Tach gets it's info from the ignition system and if it starts shorting out or drawing more current this will effect the firing of the plugs.

Could disconnect the gray wire from the coil to see if it problem stops

 

[nasdaqsam]

I can't unhook just the Gray wire its a whole plug in connection. I disconnected it the connector and will try it to see if that's it. Might take a bit as like I said it's intermittent.

Thank you

 

[nasdaqsam]

Update.

I unhooked the tach. I've been out 4 times with no issues. So it seems this might be the issue. With that said. My question is. Is this definitely a bad tach? Or the tach getting to much voltage from the engine? I guess I don't understand how the tach that produces no power of its own can cause the engine to have issues. My understanding is it simply reports what is sent to it from the engine. How can the tach cause the issue? Does the engine rely on the tach to keep it from over revving?
Last question. Can this tach be fixed or rebuilt?
Thanks

 

[alldodge]

A Tach can be rebuilt but most times it cost more then a new one, just have to check

Keeping it simple, the Tach counts pulses. The pulses come from the coil firing. To fire the coil the Neg side gets switched to ground real quick, then opens back up.

The key is opening back up.

So if the Tach doesn't allow the "opening back up" the spark is lost and motor dies

 

[nasdaqsam]

A Tach can be rebuilt but most times it cost more then a new one, just have to check

Keeping it simple, the Tach counts pulses. The pulses come from the coil firing. To fire the coil the Neg side gets switched to ground real quick, then opens back up.

The key is opening back up.

So if the Tach doesn't allow the "opening back up" the spark is lost and motor dies

Thank you for explaining this. Makes sense now. I did check on repair price. They want $125. New is around $185. I'd go new fir $60 more.