1994 Chevy Silverado sometime the heat works

[quote:78b715e057="mactown478"]Heater problem
1994 Chevy Silverado Two Wheel Drive Automatic 372000 miles
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the problem iam have is coolant seems to be flowing thru the heater core sometime..pressure is building up and water comes out of the overflow tank... i changed the thermostat and the problem continue.do you have any suggestion..engine size is a 6.5 td.[/quote:78b715e057]

Hello !!

Well the "overflow" on the reservoir has to be with the radiator cap, replace that and be sure that is the same "temperature" as the original,

as for the heater core the pressure just return to the radiator (when you close the "hot" valve that allows the water to enter to the heater core)

but if the heater core its not "heating the air" then flush it and that should solve your problem.

Good Luck!