Recently purchased 2000 STS,120K. Ran well first few days on short trips. Heater worked great. Fell in love with the beautiful car.First trip of about 40 miles, temp guage rose and message to idle engine and turn a/c off. Immediately cooled down. Did this again and took to garage. Replaced thermostat, water pump and flushed and replaced coolant. Fans work, coolant not leaking, hoses cool but still states overheating and now heater has quit putting out heat. Has never went into limp mode. Help, please.

 

When engine is at operating temperature are both heater hoses hot?

 

Sorry to take so long to reply. My hubby only checked the front hose and had to wait til this morning to recheck because it was at the garage. Both hoses were cool, but good news. The mechanic finally found a mysterious piece of rubber in the thermostat housing that was impeding the coolant flow. Removed it, and car is running great. Drove it all day and temp gauge stayed on 12:00. Brother-in-law, who is a used car dealer, bought this car wholesale from a new car dealer and doesn't know the history, but this is probably why it was traded in. Anyway, I now have a beautiful car with a new water pump, new thermostat, flushed radiator, new coolant, a steady temp gauge, a new oil changeand my brother-in-law is footing the bill.Thanks so much for taking time to read my problem and being willing to help.

 

Makes one wonder where the piece of rubber is from ?
Is possible a bit of silicone or maybe even gasket material that had been hanging and come loose.

The fact you now have a new water pump is good news to you. As long as it is the one recommended by GM.
The original pump in your car would have failed eventually. Gm has since made/put out a remodeled one.
It was never a recall item because it lasted in "most" vehicles for an X amount of years. But the design has been re-done.
So's if you have the new one then you should be good for quite a while.

I am curious as to what coolant was put in ? As to what coolant was in before ? Were the tabs installed ??

You have to know that coolant care in these vehicles is very detrimental.

 

Sorry to be slow in responding. Had a death in the family. Had orange coolant in when we got the car. Replaced it with Dex-Cool and six tabs in lower radiator hose. Now operating very well. Thanks again for your interest.