Just one other thing I will add to this. I bought one of the cheapy thermostats on eBay. Something like 13.99 or close to that. Which comes with the housing also,put it on and it worked fine for 2 weeks then suddenly it over heated! Took it to a garage who thought it was the heater matrix and tried flushing it with no luck.. So it played on my mind why some pipes were still cold, then decided to take the thermostat out to test it. Got the housing off and saw that the piston in the middle of the stat was not in its hole causing it not to open.
Problem solved. Don't buy cheap! I regret that now!
