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Re: 1996 GMC 1500 4X4- NO HEAT- NEW THERM / NO HEAT TO...
There is a vacuum line running to a vacuum switch on one of the hoses try taking the switch out and connecting the hoses together. Thats what turns off the hot water while the air con is on. If that works then its bad or the vacuum line going to it has a problem.
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You would have air pockets with a new thermostat and none if you leave it out. Are you sure you are burping the cooling system with the new thermostat installed ? Its also possible you have a leaking head gasket or other problem that is being amplified by having the thermostat installed. Something may be heating up with and not without.
All kinds of things to look at. Start with coolant level in radiator. Very often if the radiator is low, the heater will not work, or work well, because if there is air in the system, air will rise to the top, leaving the heater core with an air bubble. Whenever you replace any hose on the car, you must run the engine with the heater on at a medium high RPM (1500 RPM) for a while (5-10minutes) to "burp" the air from the system. You'll need to make sure the system is full of coolant after that. Then there could be a partly plugged radiator, a pinched or crimped heater hose, or the heater control might not be allowing the hot water to flow fast. Be sure to test the temperature of the radiator, to be certain the thermostat is keeping the radiator coolant at near 200 degrees. A thermometer in the top of the radiator will tell you. If you have a sealed system, you might VERY carefully feel all the hoses to determine the heat inside the radiator after the motor has been running for 10-15 minutes. Thermostats can get stuck in the "open" position, requiring replacement before you will get heater heat.
It looks like you changed everything involving the hot water. When the engine is warm, you should be getting hot water going through the heater core. By holding each hose from the heater core you should feel heat from both hoses. I did not see a new thermostat listed.
You need to also have the ductwork under the dash moving correctly. If you find that the water lines from the heater core are both hot, then you need the (doors) or flaps in the ductwork to route the heat inside. I do not see that anything was looked at under the dash.
I am going to assume that you checked to make sure the cooling system is full. The next thing to do is flush out the heater core. The easy way to do this is to clamp off the heater hoses with visegrips and remove them from the heater core. You will only loose the coolant that is in the core this way. Take a garden hose and try to backflush the core out until the water is clear. Being that you said you don't feel the inlet hose getting warm the chances are the core is completely blocked so it may take some working back and forth between inlet and outlet to dislodge anything in it. I have also had to use an air compressor to do it but you need to be very careful not to use too much pressure or you can rupture the heater core. Worst case is you will have to replace the core but I would try flushing it first. Hope this helps.
feel the hoses when engine is hot to see if the water is circulating,before you do anything else.If one is hot and the other cool then its not circulating,and heater core need a good flushing with some some caustic soda
I have a '96 Eddie Bauer Limited and it has the same problem. They told me that there is a small butterfly valvu located in the vac lines to select between hot or cold air. There is a tension spring connected to the valve, to an acutator, and to your ac contorl system. The spring on mine broke, so the valve opens and closes due to suction only. It is supposed to be about a $700.00 fix (due to the valves location by the firewall) but I have found a way around that. Try to aggressively apply the brakes making the vehicle slow rapidly, and at the same time turning on the blower fans with full bast heat selected. If this does not work, try doing the previous while going around a right hand corner. The corner will help the momentum of the valve turn in the right direction. Remember you have to switch the fan on full blast when you try either. If it works you should hear a thud as the valve flips shut and the heat should start coming out of the vents.