Only cold air from my heater on E 300 turbo diesel

there could be an air lock in either lines, you will have to loosen off a line at the rear heater and bleed the air out.

sound like the impelled in your water pump may have failed. Or your system is air bound. Try to open the rad cap when its cold and let it run and see if air bubbles come to the top

run the car with the radiator cap off or the expansion tank cap off, which is part of coolant system and is pressurized, not to be confused with an overflow bottle which is not pressurized, and the heater on high. drain some coolant to allow for expansion. look/listen for bubbles. air from the vents should get hot. give it about 20 minutes

air in the system. or dirty blocked radiator and heater cores. put a radiator flush in it. run it for 20 miles then drain it. refill with 50/50 mix of antifreeze .now bleed it. this is done by turn your heater on full.with the expansion bottle cap off .run the engine till water getting hot . with finger and thumb push in the hose to push air out of the radiator hose into the expansion bottle .then refit the cap.

thermostat is sticking open

If the coolant was hot when you noticed water coming out the radiator, that is normal from the heat expansion of the coolant. That is why there are coolant overflow tanks-also called coolant expansion tanks.
When refilling coolant, always top up the radiator, and fill the overflow tank to at least the cold mark level (about half-way up on the tank). With cap off, run engine till the thermostat opens- upper radiator hose will become hot-then shut off engine, add coolant to radiator as needed, replace cap. This method of filling will help to expel any trapped air in the system. Oh, and remember to set your heater controls on high when filling coolant, so the heater core and lines also circulate and expel air.
Check the seal on the radiator cap that it is still good. The cap seal is important to seal the coolant system, as it is a closed system under about 15 psi of pressure. The reason to pressurize the system is that the pressure will raise the boiling point of the coolant by about 10 degrees- important when you consider how hot the coolant has to be when running. OK, enough tutorial-I don't think you have a problem with the water pump! G'd luck, reeve.

Sounds like it might be blowing gases into the water jacket. Blown head gasket?
I would fill system up, start engine with rad cap closed. Rev up COLD engine and then check the upper rad hose for pressure. Squeeze it. If it seems to have pressure already, its likely head gasket. Put a towel over rad cap and take it off slowly, listen for pressure release while still cold engine. (Bad sign) Then wait for it to warm up and themostat to open, rev the engine and look for bubbles in the flow of coolant which burst with a little puff of steam or smoke. If neither of these happens, that is a good sign of not being head gasket. But it may be occuring only at high pressure, "under load". Dont know how much load that means..
Good luck
PS. I'm not really an expert here, (turbo diesels) just general knowlege.

FILL COOLANT SYSTEM UP BY POURING COOLANT IN THE EXPANSION TANK UNTIL COOLANT LEVEL STOP DROPPING IN THE EXPANSION.WHEN YOU POUR COOLANT IN EXPANSION TANK AIR BLEED OFF THROUGH THE SMALL COOLANT LINES GOING TO EXPANSION TANK.WITH PRESSURE CAP OFF RADIATOR COOLANT LEVEL IN EXPANSION IS AT FULL COLD MARK.WHAT YOU TO DO CRANK ENGINE UP LET IDLE WARM UP UNTIL COOLANT DROPS IN EXPANSION TANK KEEP ADDING COOLANT UNTIL STOP DROPPING STAY AT FULL COLD MARK.WHEN COOLANT LEVEL START RISING IN EXPANSION TANK.PUT PRESSURE CAP BACK ON RADIATOR.WHILE ENGINE IDLING WATCH TEMPERATURE GAUGE.IF START CLIMBING TURN OFF ENGINE LET IT SIT FOR LITTLE WHILE TAKE A LARGE RAG OPEN RADIATOR PRESSURE CAP A LITTLE AT A TIME TO KEEP FROM GETTING SCALDING TO RELEASE AIR IN COOLANT SYSTEM.WHEN PRESSURE IS RELEASED OPEN PRESSURE CAP ADD MORE COOLANT IN THE EXPANSION TANK.CRANK ENGINE AGAIN LET IDLE WHEN TEMPERATURE GAUGE START CLIMBING. TURN OFF ENGINE WAIT FOR WHILE BEFORE OPEN RADIATOR PRESSURE CAP.WHEN YOUR THERMOSTAT OPENS WHEN TOP RADIATOR HOSE IS HOT AND ENGINE COOLANT TEMPERATURE GAUGE STOP CLIMBING AND COOLANT LEVEL STOP DROPPING IN THE EXPANSION TANK.ALL AIR IS EXPELLED FROM COOLANT SYSTEM TO KEEP ENGINE FROM OVERHEATING.

look on top of the thermostat housing or anywhere in the system that sits up high as it has to be a high point to bleed off any trapped air. usually there is a small relief valve that you unscrew until fluid runs out. sometimes youmust do this several times to get the job done.

If car heats up quickly and overheats , the problem is the thermostat. The thermostat stays closed and traps coolant in the engine untill it reaches about 185 degrees , than the thermostat opens and allows coolant to flow to the radiator where it is cooled by airflow. ( airflow while driving is usually sufficient ) while idling , or in slow trafic , the fan keeps air moving through the rad. The heater core is strictly for heat in the winter inside the car. The thermostat is the most likely cause of the problem , and even if it was good before , the overheat / boil situation likely cooked it anyways. I am assuming the water pump is still getting turned by a belt. No water circulation by pump would cause same symptom as bad thermostat. Overflow and expansion bottles are designed several different ways on different cars , but the theory is that as the engine heats up and the coolant expands , it goes to the tank. Later , as engine cools down after being shut off , coolant contracts. Radiator is replenished by coolant returning from expansion bottle.