I have replaced the rad cap, water pump, thermostat and water pump heater on my 2000 Chevy Venture. The rad and heater core are good. The water pump heater was in for 2 days working great and now I'm getting no heat and it is gone again. I don't understand why the water pump heater went again?

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Is both heater hoses hot? Could need radiator and heater core flush. Water pump propellers could be detoriated. I have seen this on vehicles before. Just some ideas to look at.

did you bleed the system. you have to run the vehicle with the rad cap off ,controls set to hot and keep it revved up to 1500 rpm.it should girgle inside indicating movement of water. top up and overflow the rad and close the rad cap.once the heat comes thru. if no heat then go after the thermostat.

have you checked your rad. cap for pressure

Does the temp gauge read normal. You might have to backflush the heater core.

There is an air bleed on the top of the water pump outlet and another on the thermostat housing/upper rad hose neck. With the rad cap open and these screws loose, fill the cooling system until water comes out, then close the screws. Do not over tighten them.

If heat gauge reads normal, stays in the middle, then you do not need to mes up with the thermostat. I own a 2000 Chevy Venture and changing the thermostat was not fan.
It may be the Temperature Control Module, which you can buy for $45 and replace without problem, or may be an partially obstructed heating core. The GM Dex-Cool antifreeze, the read one, has additives, but if you do not flush the radiator every 2 or 3 years, can result in partially clogged heater core. I have done on my car twice till now, and replaced with green Prestone, on my own risk. Hope this will help. If need more information let me know. A have the Haynes manual for Chevy Venture 2000, and I am a car dealer.

Your water pump may be defective. The impellor may be loose. When the engine is cool, open the rad cap & start the engine. As it heats up & the thermostat opens, look in the rad and see if you have enough circulation of fluid. If not, change the water pump. Check to see if the rad fins are not loose & in good shape. If not, change the rad. Pressure test the rad cap in order to make sure it is holing the proper pressure. Hope this Helps you. Good luck!

could be waterpump?
i was going to say rad fan,but if so heater would be hot.
lack of coolant will make heater core to blow cold,if engine heats up with full coolant,usually means poor or no circulation.
water pump?
run engine with full coolant,at operating temp.with rad cap off and look in rad cap hole and see if there is movement of water inside rad.
[rad fan will not come on if water is not circulating through rad water pump?]

sounds like another plastic rad has bitten the dust,replace the rad or hose if its split and refill with antifreeze and see what happens ,if ok then good if not post here again for more advice