Cadillac Repair: Cooling system, intake air temp, coolant temp

Question
Hi Bill,

After driving my 92 Seville in hot weather, the collant boils into the overflow reservoir and continues to boil for several minutes.  In trying to analyze the system, I see that the cooling fans come on at about 223 degrees F.  If I then let the car idle the temp drops to about 207 degrees and the fans turn off.  Once the fans are off the temp goes back up to 223 degrees when the fans come on again and the cycle repeats itself.  I am not getting any overheating messages and I'm reading the temps from the information showing on the dash.  The AC contributes to the problem.

I've changed the stat(92 degree), the water pump is about 6 months old, and the radiator cap is new.  I flushed the system last winter.  I also had to work on the heating system and discovered that a previous owner had used some sealant material in the cooling system which plugged the heating coil.  

Is my situation normal?  Should the fans turn on before 223 degrees?  What are the set points for the fans to turn on and off?  What's happening?  

Answer
Hi Dave, Sorry it took so long to return your email as things have been rather hectic around here. Now for the good news. According to what you have told me the system is operating as designed. Normal coolant temps are between 85C(185F) and 110C(230F). The fan is designed to turn on low speed around 106C and off at 98C. The fans switch to high at 108C and switch to low at 106C. With the a/c on low fan speed is engaged at 60C when read by the high side temp sender and switches to high speed above 70C. Should your engine temp reach 126C(259F) then the a/c will disengage and a code B450 will set. Coolant temp switches are known to go bad in the Eldo and Seville and the best way to check them is to turn the key on and check all the temp sensors i.e. coolant, intake air temp., low side a/c temp and high side a/c temp. after the car has been sitting overnight. They should all be with-in 2 degrees of each other and if one is off 5 or more degrees then I would suggest replaceing it. If you have access to an infrared thermometer such as is sold by Sears or Raytec then you can check the sensors running and know they are accurate. Sounds like they should be. Cadillac recommends useing seal tablets inside the cooling system and sometimes more then the recommended amounts get in the system if a good flush job isn't done and that will restrict the heater core or radiator from flowing the correct rate to allow the car inside to heat up or the radiator to cool the anti freeze. So it is good that you found it in there. And replaceing the core or rad might be required if backflushing won't allow good flow. Some reasons for boiling into the resivior would include a weak cap spring, consentration too high or low...-34F is the target, damaged rad neck where it seals to the cap, timming set too far advanced...10 degrees BTDC, radiator fins restricted with leaves, dirt etc. on the outside, thermostat stuck closed or opening partially. Hope that helps explain the system operation and if more help is needed I will try to respond quicker next time. Keep cool. Bill