Trouble diagnosing overheating M50
!994 525it overheated and I am in need of some help in diagnostic. My son overheated the car and said the needle did not go to red. He added coolant but was unable to bleed it properly. I got a look at it today. The oil is very clean. The fan clutch seems fine. I filled the expansion tank until coolant flowed from the bleed valve and started the car. It stared well and ran smooth. Fan started immediately. Temperature needle just past blue and top hose gets warm. Idle for 15 minutes and needle steady at 1/2 mark. Turn heat on full with blower and begin to bleed. Coolant with bubbles for a few minutes and then steam but not heat in cabin. Turn off engine, fill expansion until flow out of bleed valve and repeat with same result as before..bubble then steam. This is repeated for an hour with same result and needle steady at 1/2 mark. There is some white steam out of exhaust but here always has been. I began to raise the rpms to 2000 after the pure steam began coming out of the bleed valve and would get more coolant and bubbles. I disconnected the top radiator hose because it seemed soft and empty. I did get flow but the hose did not seem "full". I do not know what it should look like. Less exhaust vapor now but still some at idle. Not heat in cabin. Have been bleeding for at least 1 1/2 hours by now. Took car for a drive. The temp gage fluctuates slightly between 1/2 and 2/3 and always comes back to 1/2 at idle. Begin bleed procedure again. The expansion tank losses very little coolant each time but after a few minutes of coolant and bubbles (escaping fast then slower with a slight pulsation and then steam endlessly) I again disconnect top hose and manually fill with coolant, it takes a quart or more. I am still not getting cabin heat so I begin to disconnect the heater hoses and there is hot coolant. For the first time I am getting some heat in the cabin but only from the driver side vent. All three heater hoses are hot. There is a fourth which enters the fire wall next to the three and goes under the intake manifold and this one is not as hot. I loosen the clamp and see coolant flow. I am encouraged. I take the car for another drive and it seems to run fine. The temp gauge is steady at 1/2. There is some heat on the driver side and I notice that when I get the rpms up as high as I can between red lights that there is a surge of hot air as I decelerated to idle but it does not last that long. The needle is steady at 1/2 and is so steady that I am fearing that the gauge is broken. I return and there is no water vapor visible at exhaust. I cover the exhaust with a clothe and detect moisture. I start my truck and compare. My truck (F-150) blows some vapor but not as much as the BMW and stops sooner. I check exhaust with a clothe and there is also moisture but the truck exhaust feels hotter. The cabin heat is intermitten and never is warm on the passenger side. I bleed again and again but it never stops steaming out of the bleed valve. My son drives to the grocery store and I follow him, all seems well. He calls me after he gets home to say that the car began to overheat just before he got home. A 6 mile round trip. The car engine probably ran for at least 4 hours either at idle, at 2000 prm or on a few short drives before it finally overheated. THe radiator looks very good and I could spot no leaks. I do not know what is acceptable amount of water vapor. I have read a few threads on the subject with no real conclusion. I think I may need to get a pressure test. There is a noise from the upper front of the engine which sounds like valves tapping. I do not think it is new since the overheating and it may have been there all along. Any helpful suggestions would be appreciated. Is there a definitive test for the water pump? The thermostat appears to be opening but the flow may be low. I just do not know. It seems like the emphasis on bleeding air out of the system is so great that I would think that the top radiator hose would be solidly full of coolant but it feels soft and empty. I do not what is normal. Thanks for any help.