150 Merc heating up

[Destroyer]

Check your thermostats to make sure they are opening. A lot of times they jam shut with salt or just old age. A quick test is to take them out, put them in a saucepan on the stove under water, turn on the stove and watch them... if they don't open by the time the water boils it's time for new ones. If they do open, then do the same test again, but this time put a thermometer in the water and check the temp that they open at. It should be the same or very close to what the engine specs call for.

If they opened, then check all water passages for salt or sand or something that got ingested by the intake and passed up into your engine.

Also, you say that the horn is going off but you don't say anythng about the actual engine temp. Is there a water stream coming out of the engine? (is it pissing?) Is it really hot or is just that the alarm is sounding? What is your temp guage reading? I have a 200 Merc of the same year as yours that had a problem, and it turned out to be a bad heat sensor on the port side of the engine.

If the problem is your t-stats DO NOT be tempted to just run the engine with the t-stats removed. They are there for a reason. Your engine needs a certain temp to help burn off carbon.. you can ruin an engine by running it for a long time with the stats not installed. It's ok to do in order to get back to port, but install new ones at your earliest opportunity.