i would be doing 3 checks first,
1. Check that the new thermostat has a bleed valve, as some cheaper ones do not and this will case air bleeding problems.
2. Check that radiator has not got a partial blockage.
3. Get a head check done, this is a simple chemical test for exhaust gasses in the cooling system, if so you will have a head problem, start here.
Sunday, June 6th, 2010 AT 6:45 PM