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I got a 2004 z71 with the 5.3 vortec and whenever the outside temp is over about 70 my engine wants to jump around in temp. It usually runs just below 210 I'd guess about 195 or 200 but when it's 70 plus outside it will run up to about 220 which is pushin the safe zone and I'm not racin around and tearin it up. Just cruisin. On the highway whenever I head up a hill temp will rise then go back down to 210ish and in the city whenever I take off from a light or stop sign it will do the same. I just changed my oil today and there is no sign of anti freeze in it and nothing comes out of the exhaust and the coolant level has not changed... I've burped the cooling system to make sure all the air is out of it... Could it be just a thermostat or somethin more serious like a head problem or intake issue? Any help would be great. Thank you
 

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I got a 2004 z71 with the 5.3 vortec and whenever the outside temp is over about 70 my engine wants to jump around in temp. It usually runs just below 210 I'd guess about 195 or 200 but when it's 70 plus outside it will run up to about 220 which is pushin the safe zone and I'm not racin around and tearin it up. Just cruisin. On the highway whenever I head up a hill temp will rise then go back down to 210ish and in the city whenever I take off from a light or stop sign it will do the same. I just changed my oil today and there is no sign of anti freeze in it and nothing comes out of the exhaust and the coolant level has not changed... I've burped the cooling system to make sure all the air is out of it... Could it be just a thermostat or somethin more serious like a head problem or intake issue? Any help would be great. Thank you
More likely the radiator no longer moves enough coolant through the core, it's just crudded up with the stuff that collects in old cooling systems. It has Dexcool in it, this will be messy to fix. Probably the radiator will have to be replaced. All traces of Dexcool throughly flushed from the engine, heater core and all lines. Once clean you can go back to a mixture of Prestone and distilled or de-ionized water.

Bogie
 
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