this as an information alert for anyone who might be interested
I have discovered more or less by chance that the thermostat in my 2GM20
had failed in an half open position. I don't know how long ago it failed but it may have been like this for many years
as I had no external evidence of any problem with it. Being half open, the engine
has been getting enough cooling
so as not to trip the high temperature alarm
or so it seems.
I had removed the thermostat prior to flushing
to descale it as the RWC GM series can't be flushed with a shut (cold) thermostat. It was then I noticed it was half open and further investigation showed it was faulty i.e. wouldn't open or shut from this position.
Note, I am referring to the raw water
cooled engines, not freshwater cooled (F) models.
FWIW, I will checking the operation of the temperature alarm