temperature gauge issue in '99 camry. (vehicle, auto, idle, engine)

[neonwattagelimit]

I have a quick question - largely just out of curiosity - about a problem I have been noticing with the temperature gauge on my '99 Camry (car has about 125k miles).

Basically, when I drive at normal or higher speeds, the gauge goes down to C. Frequently very far down. When I drive at low speed or idle, it goes back up to where it's supposed to be. I first noticed this a couple of months ago and have been assuming that it is simply a problem with the gauge, because:

1) Why would the car run cold when the engine is working harder?
2) I have not noticed any other problems with the car. It accelerates fine, idles fine, starts fine, drives smooth, and the engine has not been making any unusual noises. There has not been any change in the car whatsoever since I first noticed this temperature gauge issue.

Nevertheless, I periodically wonder if I am, in fact, grossly incorrect to assume this seeing as I know nothing about cars. So I am curious as to whether you think my assumption is reasonable, and what the problem could be, if anything.

[Steve-o]

Change your thermostat, its on its way out.

[neonwattagelimit]

Change your thermostat, its on its way out.

Thanks. That makes sense. How much would it cost to have a mechanic do that? I really don't trust myself to do it.

[iTsLiKeAnEgG]

It's a fairly simple replacement to do yourself. A similar thing happened on a friends 1999 Civic during a trip to Tahoe. The gauge temp kept falling and the vents were blowing cold air when it was freezing outside. We had to shut the vehicle off for a bit and it returned to normal. She replaced the thermostat when we got back to San Francisco and all was well.

[Nativechief]

Low coolant level can also cause this. Check the level first. If none is needed THEN go get a new thermostat

[M3 Mitch]

While changing the thermostat, it would make sense to change at least the upper radiator hose, I'd be inclined to replace both while you are in there, using something high-quality like Gates brand, beware the big chain auto parts stores and the cheap parts they peddle.

A complete drain and refill with the red Toyota brand coolant mixed 50-50 with distilled water would go a long way towards 86-ing gremlins for the next few years.

That is, assuming you don't have low coolant level as Nativechief notes - the real problem here is the temperature sender is getting uncovered, so while the gauge reads low there are parts of the engine that are not getting coolant...not good.

[trlhiker]

When was the last time you changed the coolant, hoses, and thermostat. if you never have, get them all changed.