How do you install a thermostat in a 91 Cadillac DeVille?

Answer 1

The thermostat housing is located on top of the motor, underneath the air intake assembly. The easiest way to remove it, is just take off the air cleaner assembly. Unscrew the plastic nut on top of the air cleaner (above the throttle body), and then remove the 4 screws that hold the top onto the air filter box. It will start to lift off now, but there's several hoses and vacuum lines attached. The rubber hoses should have plastic clips holding them in place, these can be CAREFULLY separated with a screw driver, and then just pull the hoses off. The vacuum lines are underneath it, CAREFULLY pull these off, or pry them off with a screw driver. Make sure you remember which ones go where! With this off, the housing is easy to see, the upper radiator hose on the driver side of the car goes right into it. Remove this hose, be prepared to spill some coolant. Have towels ready to catch/mop up what you spill, and have some extra coolant handy to replace what is lost when you're done. There should be a couple bolts holding the housing on, remove these. There might be a sensor in this as well, I don't remember, unplug it if it's in the way. Pull the housing cover off, and the thermostat should be right underneath. It just lifts out, make sure you put the new one in the same way the old one came out, in other words, don't put it in upside down, it won't work. Also, make sure you have a new gasket for the housing, you should never reuse the old one. If needed, clean of any old gasket material or sealant, try not to get any into the coolant. It's not required, but it never hurts to use some RTV sealant when you put the housing back on. Reinstall everything, add coolant if needed, and don't forget to recheck the coolant level after the engine heats up, it'll probably drop once the thermostat opens up.