So...
My coolant temp has been registering at around or below 40C when the car is warm so I tried changing the thermostat and the sender unit next to the #2 Injector.

Still reads 40C.

I noticed what looked like another sender unit just above the Thermonstat housing. Should I change this one?

Or maybe my guage is not working correctly.

Should I be concerned?

I read on a thread a while ago that on some of the latter models, if the temp is reading as low, the engine will stay in "warm-up" mode and it will run ineffeciently. I'm trying to impove my mpg.

Any thoughts? This is a '80 TD with an '83 turbo engine installed.

Thanks

__________________
1987 300TD
1985 300TD (sold)
1980 300TD (sold)
1972 280SE 4.5 (sold)

Christophe,

If you want to check the thermostat, put it in a pot with water and put it on the stove, with a thermometer and check when the thermostat opens, then let it cool and see when it closes. These thermostats have more than one moving part as they also perform a mixing function to prevent very cold water from being pumped into a warm engine so you have to watch both parts moving.

If the unit opens at the rated temperature and closes near there as it cools down, it is probably ok.

Also, the dash temperature indicator is not connected to the engine management or any other system. It only shows water temperature on the dash instrument. So if it is not working, it is not affecting your engine operating system, or fuel economy. It is not a good idea to run without engine temperature sensors working though as it is one of your most useful indicators of engine health. Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)