my Jimmy using excess fuel

My 91 Jimmy is getting horrible mileage (11 miles per gallon) -- vehicle starts and runs just fine. I've changed plugs, spark wires, pickup coil, ignition control module. At idle speed, the throttle body injectors spray pattern looks OK (maybe the pattern is a little bit coarse). Other than poor mileage, the only thing that appears out of normal is the engine temperature -- whenever slowing down or coming to a stop, the engine temperature drops down considerably (is this maybe due to excess fuel cooling the engine?). I'm stuck -- any ideas would be appreciated

problem can be couse by stuck open thermostat or coolent temp sensor reading low

I thought I was the only one with LOW mileage in my '91... My mileage really sucks! I had a vacuum leak on the intake and just recently changed the gaskets. During this adventure I saw a lot of black soot in the manifold. I am going to take out the EGR (located between the TBI and distributor) and check/clean the valve. The egr is supposed to open up while the engine is at a steady speed like on the freeway. I can't imagine letting any change in vacuum of the intake allowing spent gasses being good for economy. I think this is my problem,. I will let youknow in a couple days.