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Discussion Starter #1
2005 Envoy replaced water pump ,fan clutch- radiator -and thermosat and it's still overheating going out of my mind any ideas
 

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2004 chevy trailblazer_ls_ext
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Did you replace the radiator cap? It's possible that the system is not pressurizing, and that is letting the engine overheat. If not that, then the first thing that comes to mind is a blockage in the cooling passages in the engine. Have you had the system flushed? It's also possible that the PCM is not telling the fan clutch to engage. Does the fan kick on when the AC is turned on?
 

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2004 chevy trailblazer_lt
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How are you determining that the engine is overheating? Are you going by the gauge on the instrument cluster? If so, it could be that the engine coolant sensor is defective.
 

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Discussion Starter #4
the radiator water is blowing back into the resevior filling it to the brim with scolding hot fluid and the temp gauage is pegged on 260
 

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Short answer yes, the new thermostat could be bad.

If you have the 4.2L engine, the thermostat and housing comes together as an integrated unit. If you have the 5.3L engine, you can buy them individually or as an integrated unit. IF you have the 5.3L engine AND you only replaced the thermostat, you might have installed the thermostat backwards.

I included the extra information since you do not indicate which engine you have.
 

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I would replace the new thermostat & housing unit with the old one and see if that makes a difference. If it solves the problem, return the new thermostat & housing unit as defective and purchase either an ACDelco or Stant unit.
 

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2004 chevy trailblazer_lt
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Did you replace the radiator cap? It's possible that the system is not pressurizing, and that is letting the engine overheat. If not that, then the first thing that comes to mind is a blockage in the cooling passages in the engine. Have you had the system flushed? It's also possible that the PCM is not telling the fan clutch to engage. Does the fan kick on when the AC is turned on?
I have 3 vehicles: 2004 TB w/4.2, 2007 Mercury Grand Marquis, and 1995 F150 w/5.8 (total of 475,000 miles). The F-150 and Mercury were bought new and still look that way. In my opinion the cap has NOTHING to do with the temperature. My vehicles never go over 210 which is well below boiling point of antifreeze. The pressure cap is used to keep it from boiling so I decded I didn't need the pressure. I disabled the pressure feature of the caps on all of my vehicles over 10 years ago. I NEVER have any leaks and certainly no blown hoses. Can someone convince me that I'm wrong?
 

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the radiator water is blowing back into the resevior filling it to the brim with scolding hot fluid and the temp gauage is pegged on 260

Change the fuel cap it can cause issue the fluid must never flow back into the reservoir it it only meant to take the water from the reservoir it works on the vacuum system
 

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the fluid must never flow back into the reservoir it it only meant to take the water from the reservoir
Totally incorrect. That tank is a catch tank for when the coolant expands (as it gets hot) and contracts (as it cools down) so you're not having the system blow a small amount of coolant out every drive cycle as the coolant gets hot and having to refill it when it's cool for the same reasons. The radiator cap has a spring that is designed to "open" when the pressure gets above a "set pressure point" to release coolant into the reserve tank, and another that opens when the pressure goes into a vaccuum as the coolant cools to draw it back in the system. This system was added to autos and trucks in the 1960's to fix the issue of people running their vehicles without coolant in the radiators. Before this system was added, there were many fried engines from lack of coolant!
 

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2003 gmc envoy_sle
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So, I take that as a "No."
Totally agree with TigerMike1
Can explain it to you, can't understand it for you.
In racing we use high pressure caps in order to not lose coolant at temps. like 250°F to 260°F. You aren't leaving yourself any room for for error, that's for certain.
 
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