The heater in my 1990 olds wont heat up we checked the fluids they are fine and we put in a new thermostat. the heater just blows cold air

Sounds like you have water in the oil which means you could have a blown head gasket. Can you tell what got in the oil. Is it antifreeze or gas. If gas you probably have a injector sticking open.

clog heater core,thermostat open,water pump.

could be your heater core, um try checking the heater assembly in the dash, something may have broke so the heat isn't being switched over, and it is staying on cold. Regularly check the fluids in the motor and top up as needed

low fluid level or heater control valve may need replace..check hoses going to heater core...if cold...between heater control valve and heater core..replace control valve

did you buy the heater cores new heater core and radiator is two different things sounds like you thermostat is bad touch your bottom hose and your top are they both hot if ones hot and ones cold water is not circulating bad thermostat is the heater core put in right

Since you have a new thermostat installed and no heat and overheatingyou have one of two or a combination of problems. You either have a bad water pump or a clogged radiator. The radiator can be flow checked at a radiator shop, if it flow checks fine replace the water pump. The pump may not have play in it indicating it is bad, but the shaft inside cold be broken so that the fan turns and the pulley turns on the outside but not internally where it counts.

are you using a 190 degree thermostat..do you have a faulty sending unit,also check the heater control valve,make sure you are putting the thermostat in correctly,dont mean to make you sound bad,some of these thermostats are funny

yes,i would definately check the thermostat,its usually the problem..

did you check the fluid level by removing the rad. cap? Radiator should be full to top. If Chryslers are low on coolant, the heater will not blow warm air.

I had problems with a similar truck,a 1998 GMC diesel after checking to make sure the control cables were operating the blend door and blowing the mess of the heater core,I replace the thermostats.Still no heat turns out there was a small amount of debris collecting in the fitting for the inlet hose at the thermostat housing after removing it and blowing out the lines the heat was fine.