Pontiac Repair: Operating Temperature, head gasket, air bubbles

Question
Hi Todd
This will be my last attempt to fix this problem.Problem is that engine will stay at operating tempurature then go up and down. Mechanic did pressure test to find if there are any leaks. He said lower rad hose may be leaking, and change thermostat. I have changed lower rad hose and replaced thermostat.Still the same problem, but something I noticed tonight was a steady flow of air bubbles going into overflow tank.Where could this be coming from? Also did test if head gasket leaking,it is OK.----Frustrating problem
...................Happy New Year Thanks Mike

Answer
Hi Mike:

I still think it's a head gasket issue but without seeing it it's hard to say. the only way the system is going to get ait induced into it is by a headgasket leaking. have you had a scan tool on this and watched the engine temp for the engine computer to see if it's all over the place like the gauge is? This could also have a cracked head causing the same issue. If this engine overheated at one time bad enough and it was cooled down rapidly by adding cold water that is a possibility as well. If your sure you bled all the air out and it still getting bound up with air after running then that's the only logical answer i have for you.