Mercruiser 260 Won't start.

MSteinmann · Aug 18, 2016

I will apologize up front as this will be long, but I need a little help from the experts. 1986 Mercruiser 260's with thunderbolt IV. I noticed the starboard engine running just a little rough. It then shut down (no alarms, temp normal) I changed all plugs ( not excessively oily but black, no white) No start. I borrowed coil off port engine, it started up but still rough. Next day I bought new coil, cap and rotor and installed. Engine started up ran for approx. 1 minute then shut off. I pulled coil wire off and turned motor over, no spark at the coil wire. So I looked at the sensor in the distributor. It was rusted so I replaced it with the new version. Still no start ( engine turns and has oil pressure) I took the Thunderbolt IV and coil from the other engine and still no fire. I took the tach wire off the coil, no start.Fuel is not a problem the carb is pumping fuel. Can any of you think of anything else to try. Thank you!

clueless75 · Aug 18, 2016

I'm not very familiar with Mercruisers, but I suspect the ICM if you have one. My father's boat with a 2003 250hp Mercruiser had a faulty ICM, sounds sorta like what you're describing. He accidentally hooked the battery wires up wrong one year, didn't run the motor til the next year, ran great until it crapped out once out of nowhere, started it up a few minutes later, ran okay, then crapped out again for the final time. After lots of diagnosing, I took a gamble on a brand new $900 ICM and it fired right up. I read somewhere that if the battery is hooked up oppositely for more than 30 seconds, the ICM can get damaged but not totally break. It will just overheat on multiple occasions until it finally dies, and you would have no spark at all anywhere. If you have another ICM or if you can find one cheap, give it a try?