Probably a couple of stupid questions, but I'm going to state the obvious anyway. Have you checked the thermostat and are your running a 50/50 blend of anti-freeze and distilled water?

 

Ok I replaced the cap with two wires on it and the temp heat sensor that it attaches to all with GM factory parts but I still have the same problem. Car starts out fine, nice and cold but as it heats up fan does not kick in. The heat gauge says just over the half way point and then the boiling over and steam starts.
Any more suggestions would be appreciated.
Thanks,

 

I haven't pulled the thermostat yet but will. I was running 50/50 however it has boiled over so many times that it is probably closer to 90 water/10 antifreeze right now.

 

So from cold start up how long did it take to start boiling water out this time?

 

15 to 18 minutes at idle from cold start, air temp about 80 degrees. hood was open which might make a small difference vs a closed hood for trapping heat.

 

More info which may or maynot help. I just ran it for 30 minutes at idle with AC on so fan was running. Did not overheat and heat gauge did not get to half way mark. So the problem is not the water pump but just the fan coming on at the proper time.