Since this past winter, my 89 260E (150,000 miles) engine/coolant temperature stays well below 80 when driving, and never overheats when idling, even in 90 degree weather. (In the winter, the gauge was at around 50-60 degrees when highway driving.)

In the past, I could let the car idle for a while in sub-freezing weather and the auxilliary fan would kick in. Now, there is almost impossible to get the engine that hot.

I am a big fan of Z-Max, have used it for more than a year, and never had this problem until last winter, when I added an extra amount. Since then, my engine is running a lot cooler than normal. (The stuff works a little too well, I am afraid.) The big problem is that my gas mileage is bad -- about 20-22 mpg whereas before I was getting 28 mpg. I assume this is because the engine is not hot enough.

I would think the effect would have worn off by now, but it has not. I am due for an oil change soon, and hope that will help.

But are there any other possible causes?