RPM's erratic at times

[Kenny Guess]

Any idea why my rpm's 'go crazy' at times? Every now and then I look at my tach and I am crusing about 30mph and my tach is at 7000.....then it falls back to 4000. Z6 / 115 Opti

 

[joseph currier]

sounds like your prop is slipping.

you could be under propped,insert could be broken on prop shaft.motor could be getting over trimed, tachaometer could be broken.

check that tach first and go from there.

 

[Kenny Guess]

Joe, Thanks for that info....question about prop shaft insert-is that the rubber insert you are talking about? When I took the prop off, it looked at little beat,up/worn,. I assume those can be purchased new without buying a new prop. It is a laser II stainless. Would you be able to hear the slipping? Never noticed any change in sound. And how would you go about checking the tach? thanks again.....kenny

 

[joseph currier]

kenny.

the tach is easy, running the motor shut down at idle after you turn the key off the tach should read the rpms at idle, next turn the the key to the on position then quickly to off if the tach reads zero it is most likely good.

yes you can buy new inserts at any merc dealer.

do you get any warning alarms from the motor when this anomily happens?

as long as your not over trimming and no alarms my guess would be the tach is bad this motor would yell at you if you truely hit 7000 rpms

 

[Kenny Guess]

No alarms, no noticeable changes or surges at all , just tach needle bouncing up & down.sitting in garage tach reads 1000 rpm's. When I turn key to beep mode, tach jumps.

 

[Robert Jordan]

Kenny, I had the exact same problem with my boat. My tach was bad. When my old tach began to go bad it would happen just every once and awhile. It only got worse as time passed. Hope this helps.



ROB

 

[byron cooley]

Kenny -

It sounds like a wiring or tach issue. If the motor sounds the same it did not amd can not hit 7,000 rpm. At 5850 rpm the 1.5L's PCM will pull away power untill you back out of the throttle. Crawl under the dash with a light. Check that all connections at the rear of the tach are clean and tight, especialy the ground. Check both the power and grounds at both ends of the wires. If your tach has a selector switch on the rear - note the position and switch it back and forth a couple times then back too the original position. If you still have issues - replace the tach.

 

[Michael Meier]

Type of engine? Merc V6 engines have two power packs. Each one has a gray wire that feeds the tach but only one is used. Sometimes the feed for the tach starts to fail and you can switch the wire to the other pack. Keep in mind this is temp. as the power pack itself is going to fail soon. Best to replace both and keep the one that is not suspect as a spare. If you swap and the tach is still acting wierd then it's the tach. Depending on the tach you can send them in for repair.

 

[byron cooley]

He said it's a 115 Opti - 3 cylinder - no power pack.

 

[Kenny Guess]

Byron, I think the selector switch ( what ever it does...?) did the trick. I did like you said and noticed when I turned it back and forth the needle finally fell back to 0 and when I turn the key on and off, it does not jump up to 1000 rpm's anymore. The true test will be when I take it out for a run but I have a good feeling that was it. Thanks so much for your input Byron. I will let you know....kenny

 

[Jim in VA]

Kenny - Bryon beat me to the suggestion I had. Sounds like you have a Teleflex tach. A "film" can build up over time under the selector switch. Doing what Bryon said breaks up the film and restores connectivity on the switch. Happened to me too on a 150hp yamaha vxmax. Doing what Bryon suggested did the trick. Been fine going on two years now.

 

[Kenny Guess]

Yea Jim, when I turned it with a small screw driver, it was not a smooth turn, kind of 'gritty' and I think it will be fine now. I will let u know as soon as I take it out within the next few days.

 

[Kenny Guess]

Well, I took my boat out this week and Jim and Byron were correct, moving the selector switch back and forth did the trick.....tach working fine. thanks so much for the advice. kenny