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Do you know if both coolant hoses to matrix into the bulkhead are both hot/warm?
What about radiator are both hoses warm?
What does the engine temp sit at when running or does it fluctuate?
Ok... could be air lock in the system maybe are you sure your hot/cold blend doors are working?
Normally the engine running cold like that you'd think it was stuck open thermostat..given that its been replaced aswell as the radiator..my asumption could be that you have air trapped or an air lock somewhere in the system.. you need to ensure the heater is set to hot setting..ideally if you know someone with a vacuum bleeding kit "you need access to compressed air" - try bleeding the system that way and see if that helps..
Well flushing doesnt get rid of air pockets..If you havent bled it properly..you might have air trapped in the matrix or somewhere in the system.the spike in temp suddenly could be a clue..id maybe try the vacuum method and see if that helps.unless your sender on the thermostat housing gone...do you have access to infrared thermometer? might be worth once the engines hot go around the system and see if their is any temp differances anywhere ie: cold spots that could be clue to an internal blockage of sorts and whether or not the temp gauge the cars sensing is correct or not..