Recently my 2001 STS does not maintain a constant temperature. Lately it runs slightly above 1/2 position (1/2 is its normal in winter and summer). Anyway when not running the AC the gague will go to one full slash above normal when the car is sitting idle at a stop light, etc.

Now I have already had the radiator replaced about 2 weeks ago under warranty and my warrany expires in September.

Does anyone have any ideas what could be causing this so I can help the dealer help me???

It has never set off a code or the warning light yet. Becuase I usually start moving before it gets too bad.

Several easy to check DIY things. Is the water pump drive belt and belt tensioner in good condition?

Why was the radiator replaced?

You could also have a lazy or failing thermostat causing the coolant to run just a little hotter than usual.

The temperatures you are describing are not abnormal or alarming but you are correct to look into the situation before it gets ugly. On my '98, the first mark past 12 o'clock is approximately 233 degrees. The second mark is 250 degrees.

When you are using the A/C, your radiator fans are both running at low speed all the time. The PCM will kick both radiator fans to high speed at approximately 233 degrees and that is what you are seeing on the temp gauge at idle in traffic.

Since the radiator has been recently changed, the system may have an air pocket. Do this: when the engine is cold (like in the AM) remove the coolant reservoir cap. Start the engine. Let it idle until it reaches temperature. Work the throttle with your hand, doing some bursts, and some constant medium throttle. I normally do this for about two minutes. If the level in the reservoir has gone down, add some more fresh coolant, as you have just gotten rid of the air pocket. If the level does not go down, then an air pocket is not the issue.

Don

Don,

that procedure you just suggested should work on any car, correct? I just replaced a bad thermostat on my chrysler (the 2nd one!) because of severe overheating on Monday. The car now does it's normal 'fluctuations' but i'd like to make sure there isn't also air in the system. Thanks!

Not sure about Chrysler, but the Northstar will purge itself IF the purge line is clear and flowing.

That is a big if! Get a pipe cleaner and clear it out if you think you have a problem. Mine had a clog right as the purge line started.

Don,

that procedure you just suggested should work on any car, correct? I just replaced a bad thermostat on my chrysler (the 2nd one!) because of severe overheating on Monday. The car now does it's normal 'fluctuations' but i'd like to make sure there isn't also air in the system. Thanks!
It has worked on all of my cars. After draining and refilling the coolant, most engines will self-purge the air out of the system. But sometimes an air pocket can form. So working the throttle, on an open system (engine up to temp, thermostat open), that is not pressurized (coolant cap off), will get the waterpump to flow high quantities of fluid, and get rid of the air pocket. It is a no cost test, that will most likely pay off.

Don

A word of caution here, as the engine will begin to overheat and overflow if it is run too long with the cap off once it gets to normal operating tempuratue.

thanks for the caution and advice. I'll give it a try this weekend. I will plug in my scan tool also and monitor the engine temp while i watch the fluid flow to make sure it does not over heat.

A word of causion here, as the engine will begin to overheat and overflow if it is run too long with the cap off once it gets to normal operating tempuratue.
If everything is working properly, this statement is not true. Not at idle anyway. Now if you take it out on the highway, then it will overheat. As long as the flow is proper, and the thermostat is working, there will be no problem. You can idle it for hours.

Don

Don,
That has never been my expirience, though admittedly not with a Northstar. I would have to bet against you on this one. With the cap off it is not under pressure and will boil at just slighty higher than 212 degrees and it is bound to reach that tempurature even at idle. Have you tried it?

Water will boil at 212*. Coolant will boil at a higher temp (although I do not know what it is). That is one of the reasons for the coolant. In an unpressurized system, that is functioning properly, you should never be higher than a few degrees more than what the thermostat is rated at. This is all considering that there is no load on the engine. As soon as you try and put a constant load on the engine, then yes, you will require the pressure to help out. Cylinder temps rise significantly, and require the pressure to cool efficiently. At idle though (with a few short bursts now and then) doesn't need it. That has been my experience.

Don

Well, I know that a cap on radiator system will overflow once the cooant heats up due to expansion. Perhaps that is not a problem with a surge tank system (coolant does not expand enough to overflow).