I purchased a 03 DEVILLE WITH 117K ON IT. during HIGHWAY DRIVING the digital temp gage holds at the first line about half way and stays there. when I leave the highway and get into traffic the temp goes up and the overheat warning comes on. When I return to open road the temp goes down again. Help!

There is NOT good coolant flow....either because it hasn't been changed in three years....ever? you see the glycol degrades to it's constituent parts and starts reacting with the aluminum heads and clobbers the system up (if you've got aluminum heads) for steel will create rust blockages if plain hose water was used for a "panic" on the road fill...to keep you runnin' and was NEVER changed so now you've got a clobbered up cooling system...radiator kinda chokey and the thermosat is operating in a river of muck....get your coolant flushed a new t-stat installed and you might be okay on this one.....MAINTAIN the damn thing already~

Easy there Roy. I just got it! The water pump had just been replaced by a certified garage before I purchased it.so I hope that they used the proper coolant. Could there be an air pocket?

not really....it would disappear the minute you put the "defrost" on~ if there IS sufficient coolant flow one might think your heater core is perfectly okay...hot contaminated with muck~ the whole business of on temp-bake until you stop...then full bake to the red zone is a cause for alarm...glad you got a waterpump...no doubt they drained all the bad stuff out and topped you off with a fresh system FULL of the ethylene glycol...still radiators do get plugged...you could use a fifteen dollar thermal spot gun to tell where the hot spots are and if the temperature goin is much cooler than the return hose~

Thank you so very much for your help Roy! I will check this tomorrow and let you know. May you be blessed for the help that you unselfishly provide.

I would agree with judge_roy. Your problem is most likely a clogged (or semi-clogged) radiator and/or thermostat. At 117k either of these repairs could be expected. Especially when you may have an incomplete service history on the car.

Thanks Bobcat!

Thanks Bobkat~...*with a K~

if this one has an electric fan....the fan may not be coming on when it gets up to operating temperature...this here could be the culprit....not enough air flow~

https://www.google.com/#psj=1&q=03+DEVILLE+cooling+fan&tbm=shop

the tip off was when he came up to highway speed would cool down...now why that? answer NEED MORE AIRFLOW....the coolant is good like he says, gotta be the fan or thermostatic control of the fan ("wand" in the fins of the radiator) or block temp sensor...we know the sensor is good because the indicator light is doin' what it should~

there is a DOUBLE fan assembly as well...have to look at the hardware we are replacing...may be both fans on when the a/c is engaged...just one if it's "trimming" the operating temperature to the correct 195 degree coolant temp~

VVVLARRY, you have to check the operation of this/these electric fans to see if they're comin' on~

Thanks judge roy! I took it to a radiator shop and he said that the radiator was not clogged. The fans are working. When it would get hot idling in the shop he would give it just a little gas and the temp would start to fall and the heater would work again. He is thinking perhaps a faulty thermostat or pre thermostat air block which keeps water from contact and opening. We are going to get new GM stat with purge built in and install Monday. If that fails he suggests head gasket test. If that is ok he suggests that the new water pump may be faulty. Main issue is no water circulation at idle or when stopped. I just want to keep you apprised and appreciate your expert help. I appreciate all of your thoughts and shall keep you updated.

Judge roy he also let it run with the cap off and said there were no bubbles or lifting up of the fluid.

Check to see if your top hose id either soft or hard , hard is bad , head gasket most likely , soft keep searching , purge hose ??

Top hose soft. Will check purge.

Block test 3 times all say head gasket. Is this car worth repairing or is it better to sell for parts? Super nice and still runs great just heats little by little in heavy traffic. Run forever on open road with no issues.

Assuming that you bought the car to keep and got it at a really good deal - find a good an affordable mechanic and fix it. From your post, it appears that the car is otherwise in great shape. For example - if you got the car $2k less than you expected to pay and it cost $1k to fix it - sounds like you would still be ahead.

after all this you did not say that you had water loss if it was a head gasket you would have coolant loss

Judge. When is the cooling fan supposed to kick on after 245 degrees or less? I have same issues. I have spent enough cash already - thermostat, new radiator, water pump, even new O2 sensor.

Purge line is kinked over by the air filter box mine fid exactly what hw described