too much pressure in cooling system
I have a 95 sable that lost heat about a year ago. We put in a new thermostat and that didn't fix it, so we took it in and it was full of sludge so we had it flushed out. The tech said the radiator and heater core looked fine and seemed to be operating fine. My heat came back and it was fine the rest of the winter. then summer came and you could smell that it was very hot so once agin we flushed out the sludge and it seemed to be fine. Now I lost heat again and the sludge has began to boil out the reservoir. So today my husband flushed it out again and the water coming out is crystal clear and we replaced the radiator cap. I have heat again but the pressure is still building to fast and to much. After running for 5 min it was so full of pressure that when he popped the rad cap, it bubbled out the neck. By the way, my husband has been a mechanic on and off for the last 15 years, so he does know about taking off a rad cap when it's hot. But, he's at a loss as he has never seen it boil in the reservoir. We went to a rad place, and all they want to do is replace the rad and core, but still can't explain what's causing the pressure to build up and boil in the reservoir. We also thought about a cracked head, but that doesn't add up either. If anyone has any thoughts, please share!!! The mystery is becoming a nusance. My husband has talked to other mechanics he has worked with and we've already done what they have thought to try. We're pretty sure that the sludge is not oil, but rust particles coming from the rad. Oh...It also runs in normal range for the temp gage and has not overheated.