Originally Posted by HighlandPete
The main thermostat is rated at an opening temperature of 88C, hence the 88 - 92C working range we see in typical use. The same thermostat, as used in the Rover BMW M47R 2.0d engine is rated at 88C +/- 5C tolerance. BMW don't seem to declare that data for the BMW models.
EGR thermostat has twin stage elements, first stage 70C opening temperature, with restricted flow, and full flow (second stage) at 90C, to follow the main thermostat opening temperature. Otherwise (as another post states) the faulty EGR thermostat flow by-passes the main thermostat control and chills the engine, and/or prevents efficient warm up and optimum ECT.
If an EGR thermostat opens pre main thermostat (88C), assuming the main thermostat is still in tolerance, the engine will not reach peak ECT in normal use. Although it may still exceed 90C during a regeneration, as there is typically much more heat in the engine.
Ahaaaa....didnt realise the EGR stat was two stage hence my confusion thanks Pete. So how do the gearbox stats interfere with all this....are they too set at 88 degrees hence if they fail it gives similar symptoms to a failed EGR stat? Just trying to figure out exactly what is wrong with mine. Now on second EGR stat and already replaced main stat - will give it a proper run tomorrow but early signs are again similar - rockets up to 50 degrees then struggles up to 75, then seems to go up and down from there but never getting past 80