Cadillac Repair: 1997 Cadillac Sedan Deville overheating, overheating

Question
Friend's '97 Cadillac Sedan DeVille Northstar V8 32 valve is overheating.  Engine Protection light comes on and a/c shuts down. Temperature reads 250/260 degrees.  Understand regular temp should be 215/220.  What could be causing this ?

Answer
Hi Gloria,

      The first thing that comes to mind and what I replace first is the thermostat. It might not be opening soon enough or just not be opening far enough.

      A weak radiator cap spring would allow the pressure not to hold. So for every psi of pressure that the radiator cap provides raises the boiling point 4 degrees. So if the cap is 4 or more years old I would replace it along with the thermostat.

      Along the pressure line of thought is that if there are hose clamps, a cracked radiator end tank or gaskets say at the water pump that are leaking then that will lower the boiling point. What I would do is a pressure test to see if the system can hold pressure. Sometimes leaks can be seen but a car owner doesn't see any anti-freeze on the ground.

      You have always heard the the anti-freeze needs to be to -34 degrees F. Well the flip side of that is if the solution were too weak then that lowers the boiling point. So if the anti-freeze is 4 or more years old or isn't between -34 to -60 I would flush the old out and install new.

      Maybe there is a problem with a coolant temperature sensor that is telling the computor that the engine is hotter than it really is. Useing an infra red thermometer to test the temperature is a good idea. Also that same sensor might not allow the cooling fans to run or the fan(s) themselves might be defective.

      The last item of concern is damaged threads in the engine block for the head bolts. The Northstar engine seems to have a problem with the headbolt threads pulling out and then the bolt can't clamp the head gasket propperly. When that happens the temp gauge usually will fluctuate. In other words it can warm up normally and then in a matter of 5-10 minuits running or driveing go from overheating to cold to normal and back to overheating again and continue that cycle.

      The thing to do is start with the simple and work your way/diagnosis to the head gasket. Hopefully I have explained this in easy to understand terms. Bill