Dear Car Talk:
I've been driving cars for far longer than I'd like to admit, and I've never had this happen: I drove a few miles to the market in my 2004 Jeep Grand Cherokee, and normally by the time I get there, the temperature gauge has climbed to where it should be. This time, it got only halfway there. It was in the same place by the time I got home. The next day, I drove about 70 miles, and the same thing happened. Only when I sit at idle for a few minutes (maybe four to five) will the needle start to climb a bit. But once I start driving again, down it goes. Now, I've had this car overheat a few times (I needed a new thermostat), but never have I had any car stay cold. What's the deal here? Is it OK to keep driving it? And how expensive do you think it'll be to get it back to normal?
You may need another new thermostat, Catherine.
Your old thermostat failed because it failed to open -- and therefore to allow coolant to flow through the radiator and cool off. Your current one may be opening too early, or may be stuck open and allowing too much cooling. That's the most likely explanation for a car that runs cold.
It's OK to keep driving it for now, but the reason thermostats exist is to regulate the engine temperature, because engines tend to run most efficiently at right around 200 degrees F.
So don't be surprised if your mileage drops. Although on a 2004 Grand Cherokee, you might not notice the drop from 12 mpg to 11.
Start by having your mechanic verify that your gauge is accurately representing the temperature of the engine. He'll do that by using his own pyrometer to check the engine temperature.
If the engine is actually running at a normal temperature while the gauge says it's cold, then you may need a new gauge or a new temperature-sending unit.
But if the gauge is accurate, and the engine really is running cold, then I'd change the thermostat and see if that fixes it. I'm guessing that's all it is, and that's cheap.