Mercruiser 350 (300hp) Difficult to start when warm?

I did as you suggested with changing the Temp sensor. It did not work for me. I still have the same problem..
 
sorry to hear that - there are two sensors. On mine, the one that goes to the gauge has one wire and the one to the ECM has a two pronged plug. they are both on the thermostat housing. looking on line at diagrams and what I could find when i was doing research, showed some used the single wire for the ECM (not sure how that works or if it's just an error). I did end up ordering the one on Amazon as the Advance Auto one worked great hot, but had to unplug the sensor to start cold (after a day of not running). I haven't had any more problems, hot or cold since putting in the Amazon ordered sensor.

You may also want to double check corrosion on the connections.
 
Was having all the same problems as described, over 2 years changed countless parts, turned out to be the crank position sensor that gave the wrong reading when hot, had to take engine mechanic out with me, engine computer in hand to create problem for him to diagnose as it would not show itself in the workshop.
 
I have a 2003 6.2 MPI (377) which shares parts with the 5.7 (350). It gets vapor locked when using it for tubing if I don’t run the blower. The boat isn’t moving fast enough for good ventilation.

if you are stuck on the water some guys use the sprayer to cool down the cool fuel module on the side of the motor and open the throttle a 1/4 and it will start.

I have 2 pickups now, thru hull and the leg. No more vapor lock
 
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