Mercruiser MCM 5.7L V8/350 Thunderbolt IV Killing Ignition Coils!!!!

[flightrn]

Ok this boat is driving me crazy!!!!! 1991 Larson 20ft ski/fish SEI with a Mercruiser MCM 5.7L V8/350 Thunderbolt IV /Alpha one. 2 months ago gave it a tuneup (oil,filter, gas filter, plugs, wires, rotor).... Tested it on the lake, ran great, Give it full throttle when pulling a skier...IT STALLS!!!! Never started again, Diagnosed a bad coil replaced worked fine....Last week, same thing, went 1/2 to full throttle and stalled...Never started again, Diagnosed a bad coil replaced worked fine....It runs fine, starts all the time untill I give it a quick full throtle and it don't matter if its running hot or cold...Now i'm not taken it on the lake until I figured this crap out..HELP!!!!!

 

[Raystownboater]

Re: Mercruiser MCM 5.7L V8/350 Thunderbolt IV Killing Ignition Coils!!!!

Check the voltage, at the coil, while you go to 1/2 throttle. Id also unhook the tach (grey wire) from the coil and see if that helps.

 

[flightrn]

Re: Mercruiser MCM 5.7L V8/350 Thunderbolt IV Killing Ignition Coils!!!!

Should there be no greater than 12volts at the coil and remove the grey wire permantly??? thanks..

 

[Raystownboater]

Re: Mercruiser MCM 5.7L V8/350 Thunderbolt IV Killing Ignition Coils!!!!

Its not more, its less that you are looking for. Start by checking the voltage with the key on, then while the engine is cranking and finally while the engine is running. All three should be close to 12v. If they are, run the engine, with the voltmeter hooked up to the coil and go to 1/2 throttle then read the voltage.

They grey wire goes to your tach and could be malfunctioning. I'm thinking you might want to buy a couple cheap coils until you figure this one out.

 

[Saqbcq]

Re: Mercruiser MCM 5.7L V8/350 Thunderbolt IV Killing Ignition Coils!!!!

I had similar problems, make sure the coil is OEM or cross-reference equal. Some coils are not compatible (ohm values) with thunderbolt ignitions. Service manual is how I found out.