right after changing two senors on the thermosatat housing with new thermastat.when i started it up idie went to 1100 as not been started for 6 days anf i was buzzin thing it was fixed.looked at temp gauge in middle thinking fan should of kicked in by now,so took header tank cap of and put my finger in to test how hot the water was and it was cold so,felt the pipe coming from thermastat housing and it was hot so i pressed it to see if the system had any air in and as i was pressing in the pipe engine dipped to 850rpm but lumpy