What is the normal operating temp for a 350 MAG MPI? Been having some problems lately after a long run, (1 hour at cruise rpm) the motor starts missing and sputtering then stalls. Start it back up and it runs like the choke (like in a carb motor) is stuck on -runs really rough and black smoke from the exhaust. Turn the battery switch off for ten seconds and it starts up and runs fine. The thermostat in it is a 140, but lately the temp only reached 120. Someone told that the running temp needs to be around 160-170 for the correct fuel mixture in a EFI and that could be the problem. I hope all I need is a new thermostat, but what temperature do I buy?

Water temp is very critical in an EFI system ... sounds like yours is stuck open. For an EFI system before installig a new one it is best to check it for opening temp since their tolerances are quite broad. Often in cars as well as boats sand can jam them up. They are also designed to malfunction open so it wont cause over heating.

Normal operating temp on my boat (7.4 MPI) is usually 170. My old boat (350 carb) always ran 150. I would replace thermostat with whatever is in there now (pull it out, temp is stamped on it).
Be careful with your boat "running-on". Last weekend a buddy hydrolocked ( sucked water into cylinders) his 350 after it "dieseled " a few times (his fast idle was stuck). He busted a starter before he realized what was wrong.
And yes, operating temp controls many functions of your EFI.

Merc # 807252T2 is 160 degrees, proper one for a 350 mag mpi

P.S. don man,, if you do change to a carb from efi this winter something else you should change is the thermo from the 160 to the 143, The efi's are set to run hotter.

i have twin 350Mags one runs a constant 160 the other creeps up to 175-180?? should i replace the impeller or is this in an acceptable range? No alarms have gone off and its performing great!!

minimum true temp should be 165. it really make no dif what the thermostat says it is, it only matters what the true water temp is. a scanner has the ability to read the water temp sensor and is a good way to check the gage.

if the water is really at 140 it is running rich as hell. change the oil and fix it quick quick

The twin 350's with two different temperatures are kind of answering your own question. The two engines should be running about 160-165 with 160 thermostats normally, so the one running 175-180 could have water pump issues or such. Swap the wires at the guages and see if the temps move as this would indicate a problem with that engines cooling system and not an out of calibration dash gauge.

Best Regards,
Ray @ Raylar

You may have two problems. First what caused you motor to only run in the 120 degree range versus the 140-160? I would track that problem down.

Second, IF the motor is actually only running at 120 degrees that will not cause it to sputter and run rich as you have described. It may cause it to run rich, but not excessively so. Keep in mind that the temp is way cooler than 120 at first start of a cold engine and it runs just fine (albeit in a slightly richer mode).

You might have an ignition/coil problem or perhaps an injector is getting stuck open causing an overly rich condition.