Boiling coolant system

had cooling system back-flushed at repair shop, has new radiator, water pump, thermostat, antifreeze, and heater core. after shutting engine off it sounds like a coffee percolator and after a few weeks there is like brown powder around the cap any idea has new heads also, about 180k

It would not happen to be a GM car with Dexcool would it? I don’t know how to say it but dexcool may sludge due to air in the system. A flush and peak global would be my advice and find the source of air in the system. The percolator effect is probably due to low pressure in the system. A good mechanic should be able to cure your ills.

its a 93 explorer, where would air get in the system? there is no leaks

Has it passed a pressure test? Percolating is due to loss of coolant pressure, low coolant, a thermostat problem, or system problems such as fans not running, water pump failing, bad head gasket etc. The better the diagnosis the better the cure. You may be in the hands of a mechanic rather than guesses without further information.

What you describe usually is the result of “hot spots” forming inside the engine because of plugged up cooling passages. The areas around the exhaust valves are prone to deposit formation and flow restriction… When you shut off the engine, the trapped coolant boils…High-mileage engines that have not enjoyed regular cooling system maintenance are prone to this…In an aluminum engine, coolant changes are critical…

This can also be caused from a defective pressure cap. Be it on the radiator or the coolant reservior.