I have a 2001 GMC Sonoma 4x4 with a 4.3L engine. When the truck is under load the heating system will change from heat,defrost, and vent
Thats normal, you are taking vacume from the heater system when you are under a load,but it returns when you let back up on pedal,if you wanted to alter this affect you might try installing a vacume ball to the heater vacume hose,
You have a vaccum leak. first place to check believe it or not is under the hood a small vaccum leak when under load will do this to the heat ( i know i have the same truck and had the same problem my leak was at the back of the engine compartment)
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