fluctuating tach

[torcano]

At idle speed I noticed my port engine 4.3L Merccruiser MPI is fluctuating between 600 and 1000 RPM. My starboard engine stays at 600 RPM with no movement of the needle. Any ideas what might cause this? I also noticied the throttle levers are not exactly even at the same RPM settings.

 

[chiefalen]

Re: fluctuating tach

Try cleaning the connections on the one thats fluttering, maybe a wire is loose under the dashboard, or on the motor.

Tachs are never 100 percent accurate on boats. You would need a shop tach, on both motors.

Try to see if a wire is the problem i bet one is loose.

Was on the water weekend before this one got a 31 inch blue almost took me over the side of the boat. 20 inch blue also. 13 year old son got skunked nada.

Buoy 19.

 

[torcano]

Re: fluctuating tach

Thanks, I'll check out the wires. I recently had the dash off to put in a new dimmer switch. I probable tugged on the plug connector that goes into the tach.
There is nothing more fun to catch than bluefish. Fluke fishing is too boring for me.
It's been very windy on the bay lately. Looks like rain this weekend and this will be my last time in the water for the season. The nice thing about trailoring a boat is that you can get in the water whenever a nice day comes along not to mention the cost savings by not paying for a slip at $150/foot. I might downsize when I get my next boat.

 

[eriefeelin]

Re: fluctuating tach

I had the same problem with mine. On the back of the tach there is a selector switch for the number of cylinders your engine has. Move that selector switch back and forth several times and then reset it to the proper position. The contacts on this switch get a buildup on them and this worked for me. I got this info right from the Teleflex site and should apply to any brand gauge you may have.

 

[mkast]

Re: fluctuating tach

Switch the tachs for testing.

 

[Don S]

Re: fluctuating tach

Is the engine rpm actually changing, or is it just the tach fluctuating. If the engine is actually fluctuating in rpm, no use trying to fix the tach.

 

[chiefalen]

Re: fluctuating tach

This weekend no good to fish. Next week half day school so my 13 year old and i going out.

Bow season opened been busy with the sixteen year old bow hunter, but he don't like fishing to mush not exciting enough he says.

 

[Don S]

Re: fluctuating tach

This weekend no good to fish. Next week half day school so my 13 year old and i going out.

Bow season opened been busy with the sixteen year old bow hunter, but he don't like fishing to mush not exciting enough he says.

And just what the heck does any of that have to do with a fluctuating tach?

 

[chiefalen]

Re: fluctuating tach

Easy master nothing what so ever. o!o

He's got to get to the boat and post what he finds sir.

Local guy near me.

Have a drink on me send me the bill.

 

[torcano]

Re: fluctuating tach

I don't think the engine RPM is changing at least not to my ear. I recently had the dash out to replace a bad dimmer switch. I think I might have tugged on the plug-in connector that goes to the tach. I'll snug it up and see what happens. When I had the boat surveyed the surveyor hooked the engines up to a computer and was able to read RPMs right off the engine. I'd rather avoid that expense unless the situation gets worse.
Thanks for everyone's help.

 

[Don S]

Re: fluctuating tach

I don't think the engine RPM is changing at least not to my ear.

You sould be able to hear a 400 rpm change. IF you aren't sure, open the engine hatch where you can hear it better. Is this a rapid fluctuation or slow change?

 

[bolo78910]

Re: fluctuating tach

And just what the heck does any of that have to do with a fluctuating tach?

lol ha wow dons i got to give to you . i was saying the same thing.

 

[chiefalen]

Re: fluctuating tach

With a 400 rpm difference in engines you would feel it surge also, boat would want to turn.

None of that description was conveyed in original post.

Hence my deduction that it was a loose wire.

Then in a latter post we discovered that the dash was pulled for another reason. Maybe pulled on the tach wire.

Thus logic would suggest my original deduction was correct.

All we are awaiting is confirmation that the adage kiss may apply.

Not that i am suggesting your are the last s Don S.

 

[torcano]

Re: fluctuating tach

The tach would slowy go up almost as if it were floating. It then slowy dropped down. It has been cold and windy here in the northeast so I haven't been out in a few days. The weather should better tomorrow so that I will try to listen more carefully. I'll only turn on the port engine. When I first noticed this I had both engines running so that it was hard to hear any RPM change with the other engine running at the same time.
Thanks again.

 

[Don S]

Re: fluctuating tach

The weather should better tomorrow so that I will try to listen more carefully.

Good idea. The slower rate sounds a lot more like an engine/EFI problem than a tach problem. If it jumped erratically, I would suspect a tach, slowly, engine.

 

[torcano]

Re: fluctuating tach

When you say EFI as in replace the injectors? I have recently been adding some gas additive to treat the ethanol treated gas. It was supposed to clean the fuel injectors as well. I did this because my port engine stalled a few times. It would only stall at idle speed usually as I just would pull out of the slip. The port engine is the engine is question here as well. Any thoughts about using a straight fuel injector cleaner product?

 

[Don S]

Re: fluctuating tach

I never said you have an injector problem and need to replace them, there are a lot of things in a EFI engine that could cause your problem. The first thing you have to do is find out if the engine is actually changing rpm or if it's just a tach problem. IF you can't figure that out, by just listening to the engine, then you don't stand a snowballs chance of finding the cause of your problem. It doesn't take a rocket scientest to start the engine and hear a 400 rpm change in the engine while watching the tachometer.

 

[achris]

Re: fluctuating tach

If you're tone deaf and can't hear 400rpm change, then just swap the sender wires at the back of the tachos... fault stays with the same engine... Engine problem... Fault moves with the tacho...Tacho problem.... You guys with twin engines have life so easy... You have a ready source of working spares...use them!

Does your boat have a fly bridge, with a second station with instruments? If so, how's the tacho up there?

... I also noticed the throttle levers are not exactly even at the same RPM settings.

Not unusual.

Chris......

 

[torcano]

Re: fluctuating tach

I took my boat out today to test out the fluctuating tach problem. It was perfect. It didn't move a bit when at idle or any other RPM I was running. I guess it just had a bad day the other day.

 

[chiefalen]

Re: fluctuating tach

Thats why i didn't want to alarm you.

Could be alot of things, but did you fiddle with the wire a little?

Could have been the tach acting up cause of the cold.

Maybe a little bit of bad fuel cleaning out of the tank, with the gas additive.

Alls good with the world. If the boat is running good.

Ha!