2004 Mercury 115 four stroke tach issue?

[Alan Dykes]

I have a pair of these 2004 model 115HP four strokes on my boat. They have always run the same rpm with the throttles at the same position. Lately, one of the tachometers started showing about 5-600 rpm high. By the sound, I can tell that the engine wasn't turning any more than the other. I didn't think much about it. Sunday, I noticed the tach was correct. Now the problem is that that engine would drop 400 rpm for about 5 seconds, and then pick back up for 30 seconds. This continued for the 20 minutes or so I ran it. On the way back the tach was incorrect, 600 high, and they ran fine the whole way back. Any ideas?

Thanks,
Alan

 

[Alan Dykes]

Re: 2004 Mercury 115 four stroke tach issue?

Bump. Any ideas of where to start?

 

[Timestep]

Re: 2004 Mercury 115 four stroke tach issue?

Plug it into the computer diagnostic to see ??

Dave

 

[Alan Dykes]

Re: 2004 Mercury 115 four stroke tach issue?

That is a great idea......if I had one, Dave. That is a last resort. I am hoping someone had a similar problem and can tell me what caused it. I am wondering if low voltage could do something like that. I found a good bit of rust on that engines battery cable. Somebody put a plain steel cable end on a saltwater battery cable. I'm gonna replace that.
Alan

 

[Alan Dykes]

Re: 2004 Mercury 115 four stroke tach issue?

That is a great idea......if I had one, Dave. That is a last resort. I am hoping someone had a similar problem and can tell me what caused it. I am wondering if low voltage could do something like that. I found a good bit of rust on that engines battery cable. Somebody put a plain steel cable end on a saltwater battery cable. I'm gonna replace that.
Alan