I have a 2004 ford fiesta flame and it is boiling the water in the coolant tank but is not overheating the engine, also a knocking noise between 1500 2500 revs then disappears. And also had a milky substance in the oil filler car but the car has sat for 5 weeks and it has not been on a good run so unsure weather its just moisture or head gassket going any help?
Air bubbles in the coolant reservoir are usually from a leaking head gasket. The clue is the coolant won't be hot or steaming. A leak can be verified with a "sniffer" test. Air is drawn from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.