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2005 Pontiac Grand Prix: the heat is not blowing hot ... getting hot

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I have a 2005 Pontiac...
I have a 2005 Pontiac Grand Prix. the heat is not blowing hot. I took it to my mechanic and he told me the engine isn't getting hot enough and the thermostat needed replaced. It felt hot when i picked it up friday but quickly cooled off. I notice the guage on the dash is not as high as it usually runs. Could there be something he and I missed? A sensor or something else?
Submitted: 8 years ago.Category: Pontiac
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Answered in 28 minutes by:
11/23/2009
Pontiac Mechanic: Steve, ASE L1 Master Technician replied 8 years ago
Steve
Steve, ASE L1 Master Technician
Category: Pontiac
Satisfied Customers: 5,494
Experience: 25+ Years experience as a professional working automotive technician; ASE L1 master technician
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Hello!
The thermostat is the only component in the engine that can cause it to run too cold. There are no sensors or other components that can cause this to happen; your mechanic's diagnosis is correct.

Whenever your engien is running it is constantly producing heat. The heat is absorbed by the liquid engien coolant, which then flows through the radiator to get rid of the heat it contains. The thermostat is a valve that shuts off the flow of the coolant through the radiator to allow it to warm up to normal operating temperature (195 degrees). Once the coolant reaches 195 degrees, the thermostat then allows it to start circulating through the radiator to maintain the temperature at 195 degrees; if it drops below this it is supposed to close to shut off the flow through the radiator and stop the heat loss.
If you have replaced the thermostat and the engien is still not reaching 195 degrees the thermostat is either not functioning correctly, or may be the wrong one for the engine. If the thermostat was from a discount auto parts store, you may want to try getting one from a Gm dealership parts department; they cost around the same (around $10). However, the ones sold at the dealers meet GM specifications; ones from discount parts stores are made overseas by the cheapest suppliers that they can find so they can compete with each otehr on price alone, and the quality is often questionable. You will find very few professional mechanics buying thermostats at discount parts stores for this reason, as they are often out of calibration.

Edited by Steve7654 on 11/23/2009 at 12:42 AM EST
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Customer reply replied 8 years ago

I am confused, is there any other reason that the heat would not work in the cabin. It blows sporatically. I know the heater coil is good, there is plenty of coolant, could there be anything other than the thermostat that could be amiss? I will call the shop that replaced it and see where it was purchased.

Pontiac Mechanic: Steve, ASE L1 Master Technician replied 8 years ago
You stated in your question that the engine temperature gauge is not getting up to normal either. If the engine is not getting up to 195 degrees that problem needs to be addressed before looking at anything else; the engine coolant has to get hot so that it can heat the heater core.
If the engine was getting hot and the heater was the only thing not working, that would indicate some other problem. If the engine itself is running too cold, you need to get that corrected first, and the ONLY thing that can cause an engine to not get hot enough is a thermostat problem...
If you had a repair garage replace the thermostat, did they offer you any explaination why the car was still not operating correctly when you picked it up?
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Pontiac Mechanic: Steve, ASE L1 Master Technician replied 8 years ago
have I been helpful, or do you still need help with understanding how the heater system works? I'm here to help you with anything you may not be fully clear on...
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Customer reply replied 8 years ago
I understand how it works. The garage said that it was running at the right temp when I picked it up. Can a computer scan to see if the thermostat is bad? I would like to have good information before I go back to the shop. My largest concern is that the heat is not working. How can I tell if the engine is getting hot enough? If the thermostat is good what would I need to check next for the heat? Should I flush the coolant and start fresh; this has never been done in this car.
Pontiac Mechanic: Steve, ASE L1 Master Technician replied 8 years ago

Well, the best way to find out if the engine is at the correct operating temperature is to either place a thermometerin the radiator neck to take a direct engine coolant temperature measurement, or use a non-contact thermometer (IR temp gun) to take an engine temperature measurement at the thermostat housing. You can plug a scan tool into the car's data connector and read engine temperature on the scan tool as reported by the coolant temperature sensor, although sometimes this is not totally accurate as coolant temp sensors are not quite as accurate.

My concern here is that you stated the engine temperature gauge is not reading as high as it used to/ where it was normally before the problems started. This needs to be investigated to find out if the engine temperature actually IS below normal. If the engine temperature is at a normal 195 degrees then there is a problem affecting only the heater operation, such as a restricted heater core, air trapped in the heater core, or an internal temperature blend door problem inside the heater case.

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