Engine getting hot at lower speeds/gears, what to check?

[blurredvision]

The temp gauge on my 2004 Pontiac Grand Prix is an arc that is divided into 4 equal segments. The 4th segment is labeled with the "warning" color/graphic. From what I've noticed, my car typically runs below the halfway mark, but I've never really paid attention. I've owned the car for 7 years now and it has around 133k miles.

Earlier tonight, my wife and I did some driving around town and then went to a concert. The entire time, the air conditioner was on (first time I've turned it on this year). We arrived a bit early for the concert so we sat in the parking garage for a couple of minutes with the A/C on. Right before I turned the car off, I noticed the temp needle slightly into the warning segment. Looking at the entire gauge arc as 100%, I'd say the needle was hitting about 80-85%.

This obviously concerned me. We enjoyed the concert and on the way home, I kept check on the temp guage. It seemed as when I was at lower speeds (55MPH and below) and lower RPM's (~1600 or less), my temp guage was reading upwards of 65% or so (getting close to the warning). When I hit the interstate going about 75MPH and 2100 RPM, the gauge dropped back down to a much respectable 30-40%. When getting off the interstate and slowing back down to around 50MPH, the temp went back up as described before. The temperature tonight is upper 60's.

Given that I've just discovered this tonight, I've not checked anything. I am due for an oil change. I will check the coolant in the morning as well, as I haven't looked at this in quite some time.

Any other ideas on what I can look for to resolve this issue? The temps I saw tonight definitely are not normal and I'd like to nip this in the bud before it causes any damage.

 

[Ronstang]

I would check the thermostat first. It is the easiest to check and the most likely culprit. I had a very similar problem last year and it happened in winter. It was the thermostat. Sometimes you can see physical damage or failure and sometimes you have to boil them to see what happens. The easiest thing is simply replace it since you have it out and see if the problem is gone.

 

[thedarkwolf]

After checking the coolant I'd check to make sure the cooling fan is coming on. When moving at decent speed the fan isn't needed to cool the engine.

 

[5150MyU]

I believe that has two cooling fans(at/on the radiator)-first thing is to make sure they both work(and work well).
They usually kick on right after the A/C is turned on to cool the condenser.

So if you start the car cold with no A/C or defroster on, the fans will not be running, but as soon as you turn on the A/C the fans should come on rather quickly.

If the car does have dual fans, one dead fan will cause what you are experiencing.

 

[5150MyU]

BTW I am not saying that is what is wrong -just something to check.
At your mileage it could be a number of other things.

 

[MaxFusion16]

Thermostat most likely, probably stuck partially shut, so engine not getting coolant circulation.

 

[Billb2]

1 - Low coolant
2 - fan(s) not operating

Let us know

 

[Lark888]

I had a similar problem in a 2003 Saab. I checked the thermostat by hooking up a test meter that reported coolant temperature and pulled off the hose leading from the thermostat. I had blown off a hose and was running pure water for this test. Bottom line - thermostat opened at the right temp. The single fan was running.

What I finally found out was the fan was a two speed fan and the resistor controller had failed. It never went to high speed. After we found the right power resistor, the fan switched to high speed and stopped overheating.

You may want to look up how your system is designed to work and observe the fan (s) operating first before draining coolant to replace the thermostat.

As a note, replacing the entire fan with a Saab part was some incredible price (like $800).

 

[DaTT]

More than anything it sounds fan related, especially with temp fluctuations like you are experiencing.

I had the exact same issue you are having (I mean exact) in my old Thunderbird Supercoupe and it turned out to be the fan was not turning on at all.

 

[blurredvision]

Thanks guys, I'll be checking it out later this morning.

 

[Rifter]

I would suspect the fans before the thermostat but check both to be sure.

 

[SkullWalker]

Maybe your radiator is clogged from the inside or dirty the outside and it isn't capable of normal cooling when cruising in lower speed.

You can easily check out the outside of the radiator by looking at it but to check check the inside will be a little bit tricky.

 

[SpatiallyAware]

133k miles; what do you know about the history of the cooling system?

If it hasn't been flushed/cleaned I would do that first, and replace the tstat at the same time.