Engine Too Cold??

[EricSilver]

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?

[JimF]

From what I've seen, all MBs run at least 80C when warm. Also factory tests usually quote 80C or warmer as the 'warm' test point.

So either your temp gauge is not correct (most likely) or the coolant sensor that drives the gauge has changed value.

[rudy]

Eric,
change/replace your thermostat, ...possibility that it is stuck open. Your coolant temp sensor is saying that you need more fuel to reach proper operating temp, ..thus more gas comsumption.....

[joel]

agree with Jim, gauge maybe broke. 80 deg.
is normal driving temp. in fact, in the mornings
i warm up my car till 80 deg. before heading out.
i will take 22 mpg anytime. 28mpg is diesel
consumption. definitely something doesn't jibe.

[David C Klasse]

My engine did the same thing. The thermostat had stuck in the open position. Good luck!