'99 Dak w/ 3.9 and 90,000 miles. Last week while driving tempreture started jumping up to 260 for a few seconds, then would drop back down to normal operating temp. I pulled over and saw coolant leaking from water pump. Topped off coolant and drove home with no problems but was still leaking coolant.
Replaced water pump. Now the temp rises consistently as I drive without fluctuating. I drive around the block for 5 minutes and it's up to 230, hotter than it ever runs. Also the heater only blows cold air now. I'm going to replace the thermostat tomorrow. Also there's no smoke or any indicators that I've got a bad head gasket. Any suggestions?

 

i would say the stat is stuck closed. they have the new failsafe stats that when they go bad they stay in the open mode not closed might want to look into those pretty cheap. under 10.00