I'm not sure if I have a problem or not. Engine is a stock 2000 merc. 502, drive has a low water pick up. I am only seeing about 110-120 on the temp gauge. Boat is obviously running in "enrich" mode as evidenced by a black transom. I replaced the thermostat with a new 160 with no change. I'm assuming the low temp is due to low lake water temp - 40 degrees- but others have suggested that I may have a problem and that the boat should warm up to 160 regardless. I'm used to blower motors so I don't have any reference and I bought a dealer trade in and can't locate the prior owner to ask him questions. I did hook it up to a scan tool and run it on the hose, it was throwing a cold temp code and the temp stayed below 100 the entire time we ran it. Could it be a bad sender? Any comments or suggestions would be appreciated.

The stat is brand new and i suspect the one I replaced was good as well. I can't imagine two being bad. I didn't fel the hoses and stat housing when running, I'll have to check that out. What will cause the cold temps? Incorrect plumbing?

I've had the same problem on twin engine boat - 2000 502 efi Both impellers on both motors went at the same time ever since then I've had problems with small pieces getting lodged in the thermostat where it seals when its shut. They look like old pieces of impeller. I've back-flushed several times and replaced or cleaned three thermostats. If you have your old thermostat, you can see if its stuck open slightly that's all it takes to make it run cold.

I know you said stock but do you have crossovers or a recirculating pump?

Nope bone stock, as it came out of the box from merc.

Don't know if this will help but it happened to me. I had low water temp (as per guage) of about 100-110F. I checked thermostat and that was ok. I checked the sensor and that was ok. I checked the guage and that was ok. When all said and done, it was the thermostat housing. It did not have a good enough ground which made the gauge read low. I added a ground and the gauge reads 140ish. This may or may not help you but may help someone. My app. is a 93 454/bravo1.
Ski

Thanks ski - somethig else for me to check - I appreciate it.

Low water pick-up sucking sand, small particles stuck in stat.
Since you changed it, should have solved it. Had this problem on a hp500 with bravo 1 performance drive. You in shallow water? if so that drive blows or sucks.