My 98 deville had a cracked radiator. replaced it. all was fine till a couple weeks ago when it overheated due to coolant leak from purge tank. replaced it. no more leaks but still ran up to 140 w/ air on and runs 222-224 w/out air on , removed thermostat and no change. no coolant in oil or from exhaust and water pump belt looks fine. fans also working fine.Whats causing it torun so hot

There are a lot of coolant lines around the Northstar engine, but not one called a "purge line" that I am aware of. The coolant system is designed to be a closed system, and is never purged.

However, there is a rubber drain line on the pressurized overflow tank that will release coolant on excessive pressure sensed by the rad cap on the tank, or if the rad cap is faulty. If the system has a leak anywhere, it will not hold required pressure and that will always cause overheating. If you are losing coolant, that is the first place to look (bad rad cap, or cracked tank or line from the tank to the rad).

could be a air pockets in the coolant system that have top bled out
it is a simple exercise to have a compression test done to check for head gasket or cracked head

Hello, if you are losing coolant as you say than the most probable place is through your head and into your exhaust system, it is highly likely you have a cracked head or a major head gasket leak. The only other place it can go is if you have a automatic vehicle with the cooler on the bottom of the radiator tank (crack between two tanks), it will go straight into your transmission, you can tell if the transmission oil is overfull and watery (thin and whitish).

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.
Also note that the cooling fans don't come on until the coolant temperature hits around 225 degrees.
During normal highway driving the fans will very rarely come on(if at all) due to constant air flow through the radiator.They will only usually come on in heavy traffic conditions on a hot day.
But......if you have the Air Con switched on, the fans will run all of the time.

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.

hello,one of the major problem of this car is over-heating.kindly check for the engine cooler flow at the purge line,this is a common problem on the northstar.there is a small diameter hose coming from near 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,this simply means it is blocked and this will cause overheating.if the hose is clear,then kindly check for blockage at the nipple of the tank.i will also advice you to take your car to a service center because it's actually due for servicing,and this has to be attended to as soon as possible to prevent a crashing engine.
Thanks,you are highly appreciated.

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(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 further back until you locate the blockage.

Make sure that the A/C condenser in front of the radiator is unobstructed and that the fans are operating correctly.
I'm assuming that you have had the car tested for hydrocarbons in the radiator and that there is no steam coming from the tailpipe and you have no leaks? (head gaskets)

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.

The fans don't kick in until around 225 degrees.

As for the 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.

Regarding the misfiring, have you changed your spark plug cables?

VERY IMPORTANT....only use AC Delco spark plugs and wires.

Spark plugs ....AC Delco part number 41950

Plug wires....AC Delco part number 748Q

Ignition problems are common if you use anything other than the above OEM parts.