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My 2005 envoy XL 4.2 started idling irragular, the moment the heater was turned on. The engine appeared to be overheating too. Warnings of low oil pressure and engine overheating warning light made it look as if the engine was about to cease. The lower coolant hose attached to the radiator stayed cold. So it looked like the thermostat wasn't opening up. So there was no circulation? This could mean a bad water pump, blocked coolant flow or leaking head gasket.
To save you lots of money in parts and time, check this first: are all vacuum lines and connections in good shape? The rubber parts dry out and loose their sealing function. Therefore the heater control valve stops working, affecting propper flow and circulation. The other big influence on sensors etc. is a clean and functioning air throttle unit. This unit should be kept clean and free from sticky oil deposits that causes the valve not to close and open in the correct positions therefore the amount of needed air is not correctly regulated and that causes also erratic idling. Before taking it out, remove positive battery connection.
So take the unit out and clean it away from the engine on your work bench with plenty of rags and a toothbrush.
Once clean install everything back, and reconnect the battery. Reset the computer by turning ignition key 3x on and off (don't start the engine).
Then start engine and it should behave without irregular idling.
If so you just took care of the most common causes without spending money on parts you might not need.
If problems still persists, move on to checking your coolant circulation. There are plenty of posts that discribe how to.
Good luck!
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