My info is do not change the thermostat. The computer system is set up to optimize engine operation with the stock, 190 Tstat. Apparently if you install a lower stat you get other running problems.
I just spent a bag of money to get the heads redone, because of overheat.
While everyone talks about cleaning the airspace at the rad, nobody seems to replace the rad. My thinking is ,after 20 years, how clean can a rad be? My mechanic suggests flushing is a waste of time. Maybe a job for the spring.
Sure it's 20 years old or so but to clean it is important and not very easy. My suggestion would be to start there by cleaning properly even if it means removing it from the car and cleaning it.
I personally think removing it from the car is a good investment in time and money.....to make sure it's functioning as it should.
No a thermostat change should not be necessary -- replacement yes but change to a different temp - no, for the reasons you've stated.
It seems to me that for the system to work properly, at least in my C4, needs to be in tip top shape ... ONce age sets in and crud develops both in and out of the radiator and system, it cannot function well enough and will show signs of overheating and sometimes to the point of warping the heads, unfortunately as you have found out.
For me, a complete flush and cleaning both inside and out is in order and will be done next spring as well as a manual control for the fan(s) as an interim solution until that cleaning can come about.
Not sure how my thermostat was changed or if it was changed from a 190 to a 180 -- but if it has been changed, I'm not concerned about a 10 degree change; and nothing that can't be remedied if necessary.
Sorry to hear about your warped heads. Are you sure it was from overheating and did you see the excessive overheating, or are you assuming that that was the problem?
Thanks for sharing your story.
Colin