1999 5 Liter Mercruiser EFI skipping with wild tachometer

[Finchman63]

I have a 1999 210 bow rider with a 5.0L Mercruiser EFI. Just recently I have had some trouble starting it, and, more of a problem, once it has been running a while the tachometer goes crazy, bouncing between zero and 5000 rpm while the engine skips and pops. It sounds like the coil to me, but was wondering if it could be the ethanol that I cannot seem to avoid in Virginia. I changed the water/fuel separator at the beginning of the season, and stored it with blue Sta-Bil over the winter. My thoughts are:
1. Coil
2. Electronic Ignition
3. Ethanol.

Thoughts?

 

[Redhook98]

Start with a FULL tune-up. (cap, rotor, plugs, wires). Then start looking at your distributer pickup and coil.

As for changing fuel filters, a couple times a year is normal with ethanol gas.

 

[Dick Whetstone]

If all that fails, disconnect the tach and see how the enging runs. I have seen more than one "bad" tach cause ignition problems.

 

[Sman0624]

Sounds like the exact same symptoms that I had on my 1997 - 5.7L EFI. Mine would start and run ok when cold but after 10 minutes I couldn't give it any more than 3300 RPM without backfiring and a total lack of power. The tach would jump from 2500 to 5000 when only running at approx 3000 RPM. It was all over the place. On one instance it would not restart without a little playing around with the shifter.

With this problem I went through the basics; fuel filter, plugs, cap, rotor, wires, ran on an aux tank, new fuel pump, ignition control module, coil, tach gage...I know I'm missing something. After all these replaced components it still would backfire and had a jumping tach.

Finally after way too much money spent and the issue still there I took the boat out again with a buddy, as the backfire and jumping tach would occur we disconnected/bypassed the shift interupter switch. The tach settled right down and I had full RPM without any backfire.
The switch was changing states and momentarily killing the engine. We replaced the switch, approx $38 and the problem has never reoccurred.

I wish you luck and hope this helps.