With a coolant temperature fault, the computer will default to a set constant instead of actual temperature- usually 180*F. When warm, it shouldn't impact much.

Has it rained in your area lately? A muffler that is full of water will blow steam until it all boils off... if you have a normal baffled muffler, that is.

Did you tune up the engine before installing it? The smoke doesn't have that tell-tale sweet smell, does it?