Hello my name is XXXXX XXXXX I will be more than happy to assist you.
There are only a few things left that can cause your problem.
Heater Control Valve, Blend Door, Blend Door Actuator, or The HVAC Controller.
To start off warm your vehicle up to operating temp 175 - 210 deg...turn the heater on full blast.
Take your hand and feel the heater hoses that go to the heater core at the fire wall.
Does Blue Line A and Blue Line B Feel the same temp?
Does Red Line A and Red Line B Feel the same temp?
For what you stated it seems like the heater hoses are backwards. Swap the two heater hoses around, retest, and let me know what happens.
We havn't solved your issue yet so I don't feel it would be right for you to pay me yet.
When you added coolant did you add it to the overflow or in the radiator when the engine was running?
Every thing going on has to do with the thermostat. Coolant is bypassing and not allowing the engine to warm up and not suppling hot enough coolant to the heater core.
I know you replace the thermostat twice, but would still pull it out and do a boil test on it.
stick it in boil water with a thermometer in the water, get the water up to 130 and stick the thermostat in it and watch when it opens. if it opens up below 175 deg then it is bad. If it opens open after 185 deg then it is good.
If the t-stat is good then coolant has to be by passing the seal, bad seal, the seal is not thick enough, or the housing is warped. Since you had a new intake put on there is a chance there was a change in design and the thermostat seal is to thin.
Do you still have the paper work when the intake was replaced. If so look for the part # XXXXX the intake and tell me what it is.
If you find any thing out this weekend reply back and let me know, If I am not online the website will send a mesage to my cell phone/PDA and I can log in on my cell or my computer and reply back.
I feel like i am going in circles. I checked the thermostat and seal, both are good. I believe my temp sensor is off as the hoses are getting hot after it warms up. It has been very cold the last few months and so I wasn't getting the engine up to temp when checking the heater hoses. they are both getting hot and are close to the same temp. So i went inside the vehicle and checked a few things. If I have it in the mode that runs air out the vents, and I put on the recyle button, I see the motor in front of the blower turn and the air gets plenty hot. however if I turn it to defrost which i need to do to keep the windows from fogging the motor in front of the blower opens up and the air is just lluke warm which keeps the windows clear but doesn't heat the car. Is the motor in front of the fan working correctly? With outside air of below 0 comming across the heater core, it cools it off prenty quickly and the air gets pretty cool. If i turn the vents back on with the recycle button pushed it gets hot immediately. I am confused. I would think when the temp is turned up the ait should not becomming from the outside or it cools it down. What am i missing??
Yes and No, when you have it on vents and recycle it will blow the hottest, when you have it on defrost it will blow cooler because the air is coming from the out side and also the ac compressor is kicked on also.
So either it is normal for how cold it is or you have a blend door sticking and the out side cold air so blowing across the heater core and AC evaporator core cooling the air down even more.
For as the blend door being stuck you would normally have the same problem on vent also for a few minutes after you go from defrost and vents
If you change the setting to vents with out hitting the recycle button. Does the vent blow the same temp as the defrost?
Comparing the 96 Tahoe to your 2004 Silverado won't do you much good. GM did a change in design so the difference my be normal.
Before you start tearing into the dash, Let do one simple test to make sure there is a problem and it is not normal.
On the AC compressor there is a connector by the pulley, disconnect it and see if the air gets hotter (see pic below)
I have to look at a 1996 Tahoe tonight after work, I will check out and see how hot the air gets on the defrost and vents.
I did a check on another 96 Tahoe temp out side 15 deg, I had a 15 to 20 deg difference from the defrost to vents. I did not expect that much of a drop.
With you being around -15 deg the will make a sizable difference in how hot your vehicle will blow. If I was in weather that cold I would be blocking off 1/4 to 1/2 of the radiator to keep the air flow down on the radiator so the vehicle get heated up more.
Check the radiator fan with the engine off see if it turns easily with one finger. If it is hard to turn then the fan clutch is bad and that would explain your why you don't get up to temp in the summer also.
If you have to really pull or push on it then yes, I would just block off part of the radiator that should help you get up to a hotter temp giving you more heat.