Is 245 a dangerous temperature for cadillac northstar engine?

Thermostat would probably be wide open around 85-90 degrees. Pressure cap limits boiling temp to about 105.

Not sure what the max temp would be. Info should be in owners manual. Or get online info free at your local library, they have access to online service manuals. Sounds like you may have a bad thermostat. When replacing the rad its a good idea to replace the t/stat at the same time. Large fluctuations in temp can be caused by air in the system. Many vehicles can have a very strange procedure to make sure all the air is out of the system. If yours has you need to follow it as described, otherwise air entrapment will occur and this is one of the results.

If the overflow tank doesn't lose fluid and there is no steam coming out of the tailpipe when running then the engine is probably okay after the short period of running hot.

225-230 deg F is what the Caddy forum says. 250 F can be tolerated briefly when towing, going up a mountain etc.

http://www.cadillacforums.com/forums/northstar-engines-system-technical-discussion/11308-coolant-temp.html

The normal coolant temp should be around 200-215 degrees.
The cooling fans will come on at around 225-228 degrees.

The most common overheating problem with the Northstar engine is a blocked purge line(small hose).The purge line is designed to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank (not the cap) ,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.
If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.
The hose goes in to the engine to a crossover and comes out the other side close to the thermostat housing,the blockage may be there.

The normal operating temperature for the Northstar engine is around 210 degrees.
The fans will not come on until the temp hits 225 degrees.

The thermostat in the northstar engine is located at the bottom radiator hose (unlike most conventional cooling systems).

The most common overheating problem with the Northstar engine is a blocked purge line(small hose).The purge line is designed to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank (not the cap) ,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.The hose goes in to the engine to a crossover and comes out the other side to near the thermostat housing,the blockage may be there.

If the other end of your purge line is not connected to anything then that is the problem......it must be reconnected otherwise the cooling system is not pressurized.

The most common overheating problem with the Northstar engine is a blocked purge line. It's job is to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.The hose goes in to the engine to a crossover and comes out the other side to near the thermostat housing,the blockage may be there.

The fans don't come on until the temp hits around 225 degrees, so relax, your temp range is normal for the Northstar engine.
Enjoy the car.
Cheers!

of 222 to 227 is normal, but u make have a leak in the system, check the all the water house for leaks, inspect the water pump

Your fans will come on at around 225 degrees,so your temp range is perfectly normal.

As for overheating,Check for coolant flow at the Purge line,this is a common problem on the Northstar.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.