I have a 96 chevy s10. when the heater is on the truck will want and sometimes die a stop lights and only idle at about 6 rpms. when I turn the defrost on it will not want to die and will idle around 12 rpms. what’s up with this? Thanks
On many cars, turning on the defroster activates the air conditioner. The AC cools the air to drop out the water, then the heater reheats the air before blowing it on the windshield. This does two good things - it defrosts your windshield more effectively because the air is both warm and dry, and it lubes your AC clutch bearing periodically during the winter when it would not otherwise get lubed.
It sounds like your S10 uses this design.
There is a solenoid on your throttle that kicks the idle up a few hundred RPM when the AC activates so that the engine can pull the extra load of the AC without stalling at idle. That would explain why your idle is faster when you turn on the idle.
Now, the reason why it wants to stall when the defroster is off could be a lot of things. Check for air leaks between the air filter and the throttle. Look around the throttle and see if there is a simple screw adjustment that the throttle stops on that can be adjusted to increase the minimum idle speed a bit.
One more thought - When you take off the air tubes to check them for cracks, take some carb cleaner and clean around the throttle butterfly. If it is gunked up, that might be the problem.