I noticed while driving down to Houston today, that the temp gauge would creep up way past where it usually is, really only when idling, and only after I'd been driving for more than an hour (although that first hour was only highway driving, not idling...).

When at ~40+mph, the needle stayed dead center, which is where it'* always been when the engine is completely warmed up/operating (200°F?). Houston traffic being what it is, I had to turn on the heater at full blast when I was idling, so the temp would remain around its normal value. Otherwise, the gauge would go up to the next marking, and even a little past it. I pulled over immediately after I first noticed that, looked at the coolant...the level is fine, it'* just above the "full-hot" line on the reservoir.

I'd noticed some mild overheating like this (but not to the same extent) before, but it would occur sometimes even at highway speeds too. It turns out that I was about a quart low on oil, oops. So I added a quart and everything seemed to be great; temps remained dead on at the center of the gauge, needle straight up. Today was the first time I had this problem at idle though....what could be causing this? It'* even cooler than it'* been lately too...

The only thing I can think of is that the coolant looks kind of...dirty, or something. As far as I can tell it doesn't look like there are any other fluids in it, it just looks dirty. Would this overheating problem be something that a simple coolant flush would fix? (I've not done that since I bought the car in September, and I really have no idea what the interval for doing that should be...)

I should mention too that I think the "low coolant" sensor has been acting up for months...it'll trip the light in the DIC at random times, when the coolant is visibly NOT low, and then it'll go away after a minute or two. I'm assuming that'* something separate though, since it'* never been overheating at idle until today...

As always, any help is greatly appreciated.

what degrees themostat do you have? It is recommended to have a 180 degrees, with our cars, becuase GM realized later, after production, that the 195 degrees t-stat they put in can lead to overheating. I would swich to a 180 t-stat, quickly, with the temps you are seeing.

So, you know for sure the car is overheating, or is the needle just acting strange? If you got a coolant flush in September, you are fine.

Well, what kind of coolant are you running? The flush could have dislodged debris, and settled again, blocking a port. In TX, I'd switch to a drilled 180 year-round, but that won't do you any good in stop and go traffic unless you either leave your climate control on Auto (forces high speed fans on with AC comp), or do a fan override like I did.

Remember:
A cooler thermostat only benefits you IF you have air flowing through the radiator.

No need to turn your heat on full blast. Just turn the ECC to auto. I'd be tempted to do another flush for peace of mind, and have it checked. Drill two 1/16" holes in the rim of the new 180° stat, and consider a fan override switch. You may want to change your ECT sensor just to be sure it'* not a phantom. Very little investment.

Sorry if I was a little unclear....I meant that I hadn't yet done a coolant flush at all; I didn't do one in September either.

How would I go about replacing the thermostat? And what does it actually do? (i.e., why would the 180° one prevent overheating that the 195° one wouldn't?)

I'm also not sure what kind of coolant is in there...I'd just been adding Prestone 50/50 stuff as needed. Also, what is the ECT sensor?

Twister97, I'm pretty sure it'* not just the needle acting strangely because I just got the temp gauge sensor/sender fixed two weeks ago (the gauge hadn't been working for a few weeks before that), and I hadn't noticed these temps at idle until yesterday.

The thermostat is under the outlet that the upper radiator hose is attached to. Two 10mm bolts. You need the fiber gasket and the rubber seal as well. It opens 15° sooner than the 195, preventing initial heatsoak, and allowing it to cool to a lower temp with proper airflow from forward motion or fan activity.

What color is the coolant? What was it originally? Did a previous owner conver to dexcool, or was it still green when you got it? I'm curious about a possible mix of the two types.

Engine Coolant Temp sensor. Under your throttle body, in the end of the lower intake manifold. Brass, with a single wire going to it.

How far are you from Damemorder? He might be able to lend a hand personally.

About 300 miles from Damemorder, it seems. Not too bad.

The coolant is kind of a brownish color? Like I said...it looks pretty dirty, and I have no clue how old it is. It'* basically always looked the same. I could get a picture of it later if I head out to the car.

Change it.....change it NOW.
Brown is rust from your engine block caused by highly acidic condition.

The stock low speed fan comes on at about 195 (or if you turn on the A/C) and the high speed fan comes on at about 215.
Now remember that the 195 F stat spec is average. It actually begins to open at 188F and is fully open at 206F
The normal operating range for our cars...by the book...is 190 - 220 F.

Your coolant should protect you up to 265 F but if it'* old and dirty........

ooooo.....I see. I'll get on that right away (probably will have to be done on Tuesday, since I only have one class that day). I imagine it'* pretty inexpensive to have a shop flush the coolant.....?

(I'm not sure if this is something I could do myself, but more importantly it'* basically impossible to do anything in a crowded University parking lot, lol...)

Does this indicate a problem with the engine or something? :?