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Q: no heat on 1999 Buick Park Avenue

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Flushed the radiator completely and changed the thermostat and still have no heat in car.
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You may have an air pocket in your cooling system following your repairs. Let the engine run, then very carefully release the radiator cap. You can burn yourself on the scalding coolant, so use extreme caution. Opening the cap will let the air that has risen to the top out. Repeat this and then hopefully any air will be released from the system.

Also, check the inlet and outlet hoses of the heater core. Are they both hot? Neither hot? Hot in but not out? This can give you clues as to why you have no heat too.
Is your radiator coming warm after about a few (3-5) minutes of operation? If so and you do not have a warm output from the heater with it turned on, two possible explanations come to mind. Either the heater core inlet/outlet hose or one of the couplers at the engine is plugged (The core itself could also be plugged.) OR the controls aren't functioning to divert coolant through the core. The Park Avenue has a transversed (side) mounted 3800 V6 engine. At the right side of the engine (closest to the passenger compartment) you will see two black hoses leading to the firewall, this is the wall between the engine and the passengers. The hoses are connected to engine and firewall using hose clamps, often the "pinch" type. Take a pair of pliers, squeeze the clamp and pull/pry the hose from its fitting on the engine(either side) or using a screw/nutdriver to loosen the screw type of clamp. Once you remove the hose, stick a garden hose in the end of that hose and turn the water on to a slow level. You should see water coming out of the engine fitting that is open if there is no blockage. Make sure you put a drip pan under the engine to catch the fluid coming out of the engine and heater core. If water comes back out of the hose you are feeding with the garden hose, the core or one of the hoses is plugged. If the engine gets hot, but the radiator doesn't warm up, either the thermostat didn't open, allowing water to circulate or you potentially have a defective water pump.

Allow the engine to cool before you remove hoses or lift cap from radiator!
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