Pontiac Repair: 2001 Grand Prix Engine runs cold, 160 degrees, coolant leaks, grand prix 3

Question
2001 Grand Prix 3.8 V6 128,000 miles. In the summer it had run about 180 degrees but now that the weather is colder it is 160-170 degrees and the heater does have heat. The antifreeze is a cloudy brown not the reddish color of new antifreeze.  I can change the thermostat but with cars in the past I have never found one to be bad.  I have a Jeep that ran at 140 degrees, but had no heat, so I changed the thermostat and no change. I then back flushed the heater core and then the engine temp went up to 192 and I got plenty of heat.  Is that what I will have to do with this?
Thanks, Robert.

Answer
Hi Robert:

Your on the right track. If the coolant is rusty looking it's bad get a product called presone super flush and follow the directions to the "T" then back flush the heater core and replace the thermostat after the flush. also make sure it doesn't have any coolant leaks. The upper inatke manifold on these car leak coolant uner the throttle body assembly If it's leaking replace the upper manifold with this many miles on it the plastic it distorted and it will never seal up again with a new gasket.