Erratic tach

[HOOKED]

I have twin 1993 "Marine Power" Sport-Pac 350's.
The PORT tach started reading erratic today.
Aprox 500 RPM high at an idle, and goes haywire after that.
It also seems to go back to the rest position slowly when the ignition is turned off. Anyone heard of this problem, or know of a possible fix?

Sure would hate to miss FALL BASS IN THE RARITAN BAY.

[BOAT BULLY]

Check to ensure you do not have a faulty spark plug wire. The tach may be right and showing the misfire. My 1995 had similar issue last month.

[CrazyIslandMoFo]

Mine has done it...

[Keepin' It Reel]

I have a 1995 32' Convertible, the engines are 1999 Marine Power 454 MPI, I had a problem in the port engine, the engine would search up & down at low idol, changed the idol air control, huge, huge difference, much, much better, but still searches a little bit every now and again, does anyone else have any other suggestions?

[Esquire]

From easy to hard I would look at:Check to see if you have a vacum leak somewhere, check your PVC valve to make sure it is operating properly, check your sparkplugs to make sure they are not fouled or improperly gapped, check engine compression, check and or clean your fuel injectors. This is all assuming that your new IAC valve is working properly and that is what will directly affect your idle quality. I have MPI crusaders and had to replace the stb IAC valve, I have the engine shop manuals and there are steps to take to be sure that a new IAC works properly, I do not know if Marine Power is the same. If you had a mechanic replace it ask him/her if there are and if they were followed. You also might want to check your fuel pressure at the fuel rail, MPI engines are very sensitive to fuel pressure. My experience with the same age engines as yours is that the injectors get clogged up as a result of the quality of gas and the down time most every boat has. I have routinely used Seafoam to run thru to clean the injectors and have had the injectors pulled from one of the engines to have them bench cleaned. Depending on the hours on your engines you may want to consider that.

[Esquire]

I have twin 1993 "Marine Power" Sport-Pac 350's.
The PORT tach started reading erratic today.
Aprox 500 RPM high at an idle, and goes haywire after that.
It also seems to go back to the rest position slowly when the ignition is turned off. Anyone heard of this problem, or know of a possible fix?

Sure would hate to miss FALL BASS IN THE RARITAN BAY.

I had the same issue several seasons ago it turned out to be a loose connection at the sender on the engine, had nothing to do with any real engine problems just a wacky gauge. Suggest you check your connections at the Tach and on your engine.

[CaptnJack]

Mine just started doing tha , while cold the tach reads 500 rpms high but drops back once warm............?????

[Douglas Fairchild]

I do not see anything in your post that there's an engine problem. Only a tach problem. Swap connections with your other tach. If problem is cured you have a bad tach. If problem remains problem is in wiring between engine and tach. Check connection at engine and all connectors in between. Find any green your problem is most likely there.

[CaptnJack]

Did you see that mail I sent you

[Jim Shear]

Hi

This may not help, but I had weird tach operation on a 31 foot Sea Ray with GM 350 engines. After trying many things it turned out to be caused by a "floating ground". It seems a high resistance groung connection caused the tach (and other gauges) to seek a path to ground anywhere it could find it. I don't know which connection or connections caused the problem, but after cleaning all connections on the ground buss the problem disappeared. Other intermittant engine problems also cleared up. Good luck. Jim Shear