Temp Guage Mistery

I just took my 2001 Sillohette into a shop and had the dreaded intake manifold gasket replaced. The temperature guage when going down the road reads at or below 1/4. If I let is set and idle for 5 to 10 minutes it goes up to almost 3/4. If i rev the engine it just stays there. If I take the vehicle for a quick drive it immediatly goes back down to below 1/4. It did this before the intake gasket was replaced and I thought that the guage would go back to normal operation after this repair. They said they checked the thermastat when they did the job. Shouldn't the temp guage stay realitively stable. It does on my other vehicles. Running below 1/4 does not concern me although it used to be at 1/2 under normal operation (freeway speeds). It is the fact that it goes up to 3/4 under idle and stays there that concerns me. If you get caught in very slow traffic it gives you the feeling your overheating. Should I be worried. Any help anyone can give would be appreciated.

Thanks.

you might try adding a little pure [not 50/50] antifreeze and it will run a little cooler. if you are full of antifreeze in radiator then drain 1/2 gallon out ,then top back off .also check to see if both fans are coming on.the second one should kick on if it gets if it gets toward 3/4. the first will kick on just a little over 1/2 and if the temp keeps going up the 2nd kicks on. with the a/c on both should kick on a little over 1/2.your 2nd fan might not be kicking on.

I'm thinking your radiator needs flushed out. There is a lot of gunk that builds up around those intake gaskets and much of it falls down the cooling passages when they clean it off for a new gasket. Look inside the overflow bottle and see if the coolant is clean or if it shows debris floating around in the water.

You can do this yourself when it gets warm enough outside by using a garden hose and taking of the inlet and outlet hoses from the radiator. Just flush the system out with the hose and see what gunk you can get out of there!

Unless your thermostat is bad, you have a plugged up radiator I think.
John