Cadillac Repair: engine runs hot, cadillac eldorado, degree thermostat

Question
I HAVE A 1989 CADILLAC ELDORADO 4.5 L WITH 97000 MILES. THE ENGINE HOT LIGHT COMES ON WHEN THE ENGINE COOLANT TEMP HITS 260 DEGREES. I THOUGHT IT WAS THE THERMOSTAT SO I PUT IN A NEW ONE (195 DEGREE). SAME THING HAPPENED. SO I PUT IN A 185 DEGREE THERMOSTAT AND THEN A 165 WITH THE SAME RESULTS. THEN I FLUSHED THE SYSTEM USING A PRESTONE KIT BOUGHT AT A LOCAL PARTS STORE. SAME RESULTS. ALL THIS TIME WHEN I PUT THE AUTO CLIMATE CONTROL ON 90 DEGREES IT BLOWS COOL. SO WE WIRED THE RADIATOR FAN TO RUN AS SOON AS THE KEY IS TURNED ON. SAME RESULTS. THEN I TOOK THE THERMOSTAT OUT AND THE CAR RAN FINE AT 150 TO 170 DEGREES FOR TWO MONTHS BUT STILL NO HEAT FROM THE HEATER. A MECHANIC TOLD ME TO USE A GENUINE AC DELCO THEMOSTAT SO I GOT ONE AT THE LOCAL CADILLAC DEALER AND PUT IT IN AND IT STARTED OVERHEATING AGAIN. ANY SUGGESTIONS? THERE IS NO ANTIFREEZE LOSS OR ANTIFREEZE IN THE OIL.

Answer
Hi Dan, As you are probably aware Cadillac uses seal tablets along with the anti freeze and if too many are put in and the system isn't flushed out every 2-3 years they build up inside the heater core and radiator and then no amount of flushing is going to clean them out and they will need to get replaced. Also temperature senders are known to fail and when they do they can show a hotter temp than is actually there. Getting an infra red thermometer from sears or such place and measuring the temps will tell if the sender is bad or the engine is really overheating. Or with the engine cold after sitting over night there are several temp senders that can be accessed  useing the on board diagnostics and they should be with-in 2 degrees of each other or they should be replaced.  Heres what I would do. Test the sensors to make sure they read correctly. Then reinstall the thermostat. Make sure the protection level is around -34degrees F. Replace the rad cap because for every psi that the cap can't hold it lowers the boiling point 4 degrees. Run the engine and make sure the cooing fans turn on by 221 degrees F. Diagnose them if that doesn't happen. Feel the 2 heater core hoses. If one feels hot and the other one cool then flush/backflush the core or replace it. If both hoses feel hot then look to the programmer as not being able to control the temps inside the car. Make sure that the rad and a/c condenser are clear of obstructions so air can flow through them. If that doesn't help then the water pump might be bad, the radiator still too plugged up inside and need to be replaced or you might have a head gasket problem. Hope that helps. Bill