The heat is kinda warm in the city but, when you go down the highway it gets cold and stays cold. I tried head gasket sealer and it didn't work. Father-in-law thinks fan may be kicking in to early and keeping engine cool.
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Check fluid level in radiator first. If it is full try putting a piece of cardboard in front of radiator and see if it heats up. If that solves the problem most likely the thermostat. Follow the big upper radiator hose back to the engine and the thermostat should be located there under the thermostat housing.
With your front heat working the most likely cause for the rear heat not to work is that there is an obstruction in the line that is preventing the warm coolant to get to the rear heater core or the rear heater core itself is plugged and not circulating. Another possible cause could be low coolant level where the system has enough coolant to provide the front system with enought warm coolant but not the rear system. Hope this helps
Open the petcock on the bottom of the radiator and allow the old coolant to drain.
Close the petcock.
To refill you will first fill the degas container to the cold fill range, leave the cap off the canister. Turn all of the heater controls to warm and turn fan to full. Idle the engine until the thermostat opens. Top off the coolant and put the cap back on. Idle at 3000 RPM for several seconds and then idle again for a minute or so. Check the level and then repeat this 4 times, or until heat is coming from the front and rear heat units.
If you want to flush the system you will have to remove the thermostat and hook up your flush system to the radiator hose at the thermostat housing. Follow the instructions on your flush system then put the thermostat back in. Follow the instructions above to refill.
Try bleeding the air out of the cooling system. At the front of the engine, on top, should be a bleed valve. It's small, and looks similar to a grease fitting. This is the highest point in the cooling system and eventually all air in the system will accumulate here.
Make sure the coolant reservoir is serviced to the correct level. With the engine cold, but running, open the bleed valve until coolant starts coming out. Close it and wait for a few minutes. Keep opening it intermittently until no more air bubbles out -- until the engine is warm.
One other thing to look at is the vaccum actuator on the heater box, under the dash on pass. side, verify its not leaking vaccum and holding the internal door open for heat. You can normally grab the actuator shaft and operate it by hand to see if this gives you heat... also verify that both hoses to the heater core are hot. hope this helps.