The engine temperature gauge on my 1998 c230 is very erratic. Also winter cabin heat, <0 deg, is poor. Changing thermostat the fix?

The thermostat is always a good starting point!
Does the temp guage jump around OR not go up to 80C when it is cold outside?

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Thanks for the response. Sometimes it warms up to 80+ immediately, other times it takes longer or does not warm up to operating temp at all. Has progressively gotten worse over the last year.

I have a '92 300SD and I recently changed the glo plugs, a monumental task for so simple a job. For some time my temperature gauge has not been working or worked sporadically. I figured it was just the connection with the sender.

While I was doing the glo plugs, I discovered that all the wiring in that harness was trash. All insulation gone brittle and useless. Further, the connection to the Temperature sender was hanging on with a single copper strand. So I had the proper wire to rebuild the harness for the glo plugs, but did not have the appropriate wire for the temperature system (coolant). I live in a very rural area and have only the one car, so I could not get the proper wire and used some light gauge wire less than household extension cord wire but probably heavier than bell wire.

Now the gauge works, but does not indicate the proper temperature. it says the engine is running hot, but it isn't. I measured it with a thermometer and it is not up to 212 immediately after turning off.

Since i know the system is based on resistance, I have two questions.

1. Does the wire gauge affect the system?
2. What is 0,75 wire? Is it .75 sq mm? That seems about 21 gauge wire which is just about bell wire.

Thanks, Allan