Mercruiser 496 won't stay running

I wouldn't jump to the ECM yet. The fact that it starts sometimes means you have to see if the ECM is getting 12 volts. On most boats, the starter circuit is independent of the ECM so it will crank until the battery dies. It is frustrating to chase a problem like this but replacing parts just adds to the chaos. A DVM and some patience is required.
 
Aguyindallas said:
Interesting turn....

Went to the boat tonight, one by one I unplugged sensors. No results.

This time, even stone cold, it wouldn't start. Plenty of crank, no fire. Gauges and Smart Craft all operating as normal (still have the 5V DC LO error). Disconnected a spark plug and tried to test Spark......no Spark.

Any ideas?


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Seeing spark can be tricky sometimes as it can jump straight to ground and not present across the plug gap. Get some carb cleaner (the mildly flammable kind) and spray it into the flame arrestor. try starting the boat. Do that 2 times or so to prove there is no spark now that there is combustible vapor available. Do not use starting fluid.
 
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