Hello, my 1988 K3500 with a 7.4 and a th400 trans smells rich at idle and the plugs are black and sooty. My super old NAPA mind reader scan tool says that it goes to open loop whenever I let the truck idle. It doesn't care what the engine temp is it always goes into open loop, is this normal. Best I can tell all of the feedback from the coolant temps are normal, I installed a 180 degree thermostat a few months back could this be the issue. It has a recent tune up which included a new O2 sensor as well as a new MAP sensor.