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Discussion Starter · #1 ·
On my 1999 Suburban, (5.7), the heater won't heat. I tried starting it up now to see if the heat would come on, and the engine temp. gauge shot up.

I think that its the thermostat, but could that also cause the heater not to work?

btw: I installed a new heater core in November of last year.
 

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A thermostat that is stuck open will prevent the engine from warming up, and, thus, you won't get hot air through the heater. So, yes, a bad thermostat can cause a no/low heat condition. Of course there are other things as well, such as low coolant level, a bad heater control valve (or whatever GM calls the valve that regulates coolant flow to the heater cores), clogged heater hoses, stuck blend door, could also cause this condition.

If you want to start with the thermostat. Is the engine warming up at all according to the dash gauge? How hot are the radiator hoses?
 

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Discussion Starter · #4 ·
Engine coolant level s fine. Heater core and heater hoses were replaced by me in November of 2007. What is the blend door and where is it located?
 

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The blend door is the gizmo that controls how much air goes through the heater core and how much bypasses the heater core, and is controlled by your temp knob. I'm not sure where it is on these, but it will likely be in the dash somewhere near the heater core.


Are you heater hoses getting hot? Are both heater hoses getting hot all the way to the heater core? How about the hoses going back to the rear heat (if installed)? If both hoses are equally hot, but you are getting cold air, then I'd look at blend door issues. If the hoses aren't getting hot, then there's something wrong on the coolant flow side.
 

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Discussion Starter · #6 ·
Yeah, I did discover a leak in the radiator, ordered a new one and received it today. quick question:

This vehicle has an oil and tranny cooler. The lines that go into the radiator have nuts and also plactic rings where it attaches to the radiator. Do I remove the nuts or the plastic rings?
 
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