RPM gauge not reading right?

[Sinko]

It seems that my RPM gauge is reading 1000 RPM over actual most of the time. When I idle, seems OK (a lttle high occasionally), but when I run at higher speeds it seems to be showing 1000 RPM higher then it should be. Motor sounds normal and even at WOT, it goes to 5600RPM but motor sounds like normal WOT. What might cause this problem? I did add fual stabalizer at the beginning of the season and it began shortly after that (same day). Coincidence? It's a 1997 4.3 L Mercruiser LXH 205hp. When I kill the engine, the guage typically stays up near a grand.

 

[QC]

Re: RPM gauge not reading right?

Well the first thing I would look at is the little switch in the back of the tach for 4, 6 or 8 Cyl. Does not explain why it started to work erratically, but it is easy to check and has been the problem on many a tach issue.

Next I would make sure all of the connections to the tach are clean and tight. And then I would make sure the gray wire coming off the coil was connected properly.

If all that is good, then I would think about getting a new tach . . .

 

[bruceb58]

Re: RPM gauge not reading right?

Its the fuel stabilizer for sure...you need to suck all the fuel out of the tank and start over!

 

[minnow44]

Re: RPM gauge not reading right?

I have fuel stabilizer in my tank,,,it has nothing to do with RPM reading of my tach. Your Tach measures pulses per minute from the Coils NEGATIVE terminal. It does not know if you have Additives in your fuel or not. I would say that you have defective tach. Buy (or borrow from a friend) a Precision Tach meter/Multimeter from a local auto parts store ( Checker/Shucks, NAPA, Pepboys, etc). Measure RPM at your coil (or spark plug) per instruction that come with the meter. Compare your Precision RPM reading with your dash board tach. Your Tach 'ZERO" may be adjustable..if not your tach is replacable. The previous reply regarding 4,6,8, cylinder is a "Fair guess"
however ZERO should be ZERO for any number of cylinders. Make sure you have good wire connections (NO rust/corrosion etc).

DON'T forget to HOSE UP your water inlet with those ear muffs, or make certain that your boat is in the water to prevent destruction of your water pump! PS > I Know You Know That!

Minnow44

 

[QC]

Re: RPM gauge not reading right?

bruce58 was kidding ^^^^^^^^^^ Welcome aboard Minnow

 

[bruceb58]

Re: RPM gauge not reading right?

bruce58 was kidding ^^^^^^^^^^ Welcome aboard Minnow

Sorry...I couldn't resist

 

[jtybt]

Re: RPM gauge not reading right?

There should be an adjustment other than the engine cylinder configuration. May be a little brass screw.

 

[bds85466]

Re: RPM gauge not reading right?

Many times when I turn my boat key to off, the tach doesn't go all the down to zero...residual hang up in the gauge probably.

Usually when I put the key into the on position, every gauge zero's however (with the exception of fuel, which hits its own level).

If it is typically 1000 over in all instances, I would guess that this is a bias error in your tach for one reason or another -- probably out of tare.

Would one think that if it was set up for the wrong amount of cylinders that you would see a proportional deviation as RPM's increased, not a straight bias.

 

[QC]

Re: RPM gauge not reading right?

Would one think that if it was set up for the wrong amount of cylinders that you would see a proportional deviation as RPM's increased, not a straight bias.

Agree, but it is so easy to check . . .

 

[Sinko]

Re: RPM gauge not reading right?

Good info. Thanks. It typically reads correctly (atleast I think so) at idle or just into gear. Occasionally it reads a bit high say around 1000rpm. I really notice it when I take off and cruise where I normally would be 3000 rpm, it reads 4 grand. Full out I go up to 5600 which freaks you out but I don't think the motor could hit that it it wanted to and it sounds like it's doing normal WOT.