I doubt it's a thermostat although it is easy enough to replace. So, might as well replace it. While you have it out, fill the coolant through the hole for the thermostat. Once you have coolant flowing out of the upper rad hose, reinstall everything. Start it up and make sure the coolant reservoir is at the max level. Hold it at 2000 RPM to allow the car to heat up and circulate coolant. I personally leave the heat set to high at this point. By the time the temp gauge reaches the middle, you should have heat coming out of the vents. Shut down the vehicle and let it cool. Refill the reservoir as coolant will be sucked into the engine as the engine cools.
As far as LIM (lower intake manifold) gaskets are concerned, the can leak and cause coolant to enter the cylinders and cause a mysterious coolant leak. In severe cases, it has caused coolant to get into the lower end of the engine, which will destroy the bearings. I have not seen it cause overheating but it will cause a loss of coolant which would result in overheating over time.
So basically, what he said, but it took me forever to type. lol
