Cadillac Repair: 1999 FLEETWOOD LIMITED OVERHEATING, head gasket repair, coolant temp

Question
Bill,

I have a 1999 Fleetwood Limited with the Northstar engine with 72,000 miles.  As of recently, my coolant temperature on my computer sometimes changes suddenly (decreasing or increasing). Just on this past Mother's day weekend, I took a trip approximately 100 miles each way. For the most part, the car drove pretty well.  However, everytime I stopped at a toll booth and started driving again, the car's temperature would suddenly rise, especially if I accelerated hard. Usually it would rise to about 248 and within a minute's time, begin to cool to about 212 degrees or lower.  At one point, the temperature reached 260!!  At this point I pulled over and let the car cool down and later added about one gallon of coolant.  Whenever I lose coolant, I lose it from the overflow tank only.  There are no leaks coming from the radiator or water pump.  I had the cooling system pressure tested to verify this.  Where do I go from here? I asked the technician at the radiator place if I could have head gasket issues, and he said that I shouldn't worry about it especially since there is no white smoke coming from my exhaust or antifreeze in my oil. Also, if I do need head gasket repair, how well do these additives work that supposedly seal worn gaskets?  Thanks for your help Bill

Answer
Hi Darryl, Unfortunately the Northstar engine is starting to get a history of head gasket bolts stripping from the block. First thing to do is check the basics like make sure the coolant level is good, make sure the engine oil level isn't getting higher, replace the rad cap and thermostat, have the system pressureized and leak checked and make sure the cooling fans turn on when they should and that the coolant temp sensor is reading correctly. If nothing shows up then drain the coolant down on a cold engine, remove the serpentine belt, remove the thermostat and fill the coolant back up so it is with-in 1/2" of the top. Then start and run the engine and watch for any bubbles at the stat opening. If you start to see a steady amount of bubbles then it might be head gasket time as well as redoing a bolt thread or two in the block. I had a Caddy 4.9 that had temp swings like that and although the oil color was normal it still had a bad head gasket and a stripped out bolt thread in the block.
Since I don't know the qualifications of the shop to diagnose your engine it might be best to get a second opinion possably at the Caddy dealer. That engine is supposed to have seal tabs installed from the factory and at every coolant flush. If not installed then leaking can occur. I have not heard of any long term fix for a gasket problem and most of the sealers plug up radiators and heater cores if too much is used. Maybe someone at a shop or parts house has had good success with something and you could try it to see if it helps. Otherwise get a second opinion on your temp swings and choose your shop wisely if you decide to replace the gaskets because there are special techniques that must be followed or the engine will be damaged. Bill