Hello I have a 1998 Lexus es300 front wheel drive. Very good car. One problem however. I noticed recently that the engine seemed to be running cool on the highway and the temperature was going slightly above center. Not normal for a toyota/lexus. So I had the thermostat changed. But it still fluctuates a little. Much less granted. But at highway speeds it cools. It goes below center and then at idle it goes very slightly above center. So what the heck do I do?
If you don’t have drivability issues, I would not worry about it. Odds are the temperature sending unit is acting up.
Get a cheap laser temp gauge , about 20 bucks, measure the temp of your radiator in several places while the car is hot. If you have any fluctuations in certain spots more than 20-25 degrees then you have a blockage in your radiator.
But I doubt that’s it. Your water pump is driven by your timing belt. If you have never changed it and are above 150-200k in miles more than likely its worn and stretched and is slipping on the water pump at idle. When you’re at highway speeds even slipping it’s moving fast enough to turn the water pump more efficiently.
I’ve seen this same type behavior on serpentine belt water pumps. Also on alternators that wouldn’t charge at idle but did fine at driving speeds.
Now this is a possibility from what you have described, your car has an interference engine, which means if it breaks then the engine is toast. If you do have higher mileage and haven’t had the timing belt replaced you at least need to have it checked. There is an inspection cover and can be done quite easily,
