Stuck Thermostat?
 

I took the family on a nice "midnight cruise" tonight and noticed my temperature gauge never went over about 110°. After a decent distance (where I KNEW it would have normally been up to the proper temp) I opened the hatch. The starboard side manifold felt pretty warm, but the port side was quite cool.

I'm guessing this is a malfunctioning thermostat. Anyone had experience with this? Is this a MerCruiser specific part, or would Pep Boys or Auto Zone have what's needed? I'd like to get on the boat tomorrow, but I don't want to hurt anything. We planned to ski and/or tube so it'll be harder driving than just cruising.

Thanks guys!

You can't use a car themostat, they're typically set to open 25° to 30° higher than the marine version.

I ended up just going to West Marine and buying a Quicksilver thermostat. It's rated at 160° and says it's for my engine.

Problem is, I went to put it in and can't find the darn thing! I only looked for about 10 minutes since it was cutting into quality party time in cocktail cove, but I think it's mounted somewhere on the rear of the engine. This isn't going to be as easy as I initially thought!

Also, you can't used the manifolds as a real indicator of a faulty thermostat. One side will always be hotter than the other based on the water flow.

You can get a replacement thermostat at any Merc dealer, replace the gasket too just like you would on a car and see what happens.

Lake and ocean water is on average MUCH colder and much more extreme that what a car sees and how quick a car heats up. Therefore marine thermostats are set to more like 180 or so compared to 145 or 160 on a car.

I found the thermostat (right up in front, inside the housing where 4 hoses meet). I replaced it with the proper part, and started the engine. After about 2 minutes of running about 1500 RPM, nothing! Gauge still showed about 110°, and the starboard side manifold was fairly warm, while the port side was still cool (I understand this isn't the best method to tell, but I wanted to make sure it wasn't just the gauge).

I think it would have warmed up more in that amount of time and am worried something else is wrong. Anyone have ideas? It may not have been run long enough, but the rain was coming and I had to get it covered and off the slips.

Next you should look at replacing the temperature sensor that is usually screwed into the thermostat housing with one wire coming off of it. This is the sending unit for the guage (and depending on motor also the EFI) that is most likely gone bad.