I recently changed the thermostat on my M10 engine e21. No particular reason apart from the fact it was an easy & cheap thing to do and I was changing the radiator fluid anyway (I don't think it had been done for a while). The brand of the new thermostat is Mahle.
As you are aware the temp gauge on the facelifted e21 has 5 marks (or lines) creating four 'zones'. Previously the car consistently ran a bit over the middle mark into the third zone. After the thermostat change it now consistently runs a bit UNDER the middle mark in the second zone.
Is this all ok and expected? Or is my thermostat not quite right?
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- My E21(s): '77 EU 316 (2.0 - manual/dogleg - lsd)
- Location: ATHENS-GREECE
When my car was all normal - and radiator working all right (normal meaning factory specs: thermostat 80 deg C, mechanical fan operating following engine turn) safe temp level was when the needle was staying from a bit before middle up to a bit over middle (max). When outside temp was normal and car was running on a free road with normal speeds, needle stayed a bit under the middle point ('dash'). At traffic (car idling for long) and/or very hot days and/or very spirited driving, or big loads, or driving uphill etc., the needle could climb a bit over the middle, but that would be all. When temp started climbing over than this it was time to inspect the system.
All the above, when after years I replaced factory specs thermostat by the 71 deg, started happening at half 'zone' less. I mean that normal temp was held a bit over the 2nd 'dash', and when things would get hotter, then the needle started climbing towards the middle.
Now with the custom made engine things are a bit more complicated, with combination of mechanical and electric fan etc. but I will not continue because it would be confusing.