GM Repair Problems? Ask GM Repair Expert Now.
What year model is the truck? Some of the late model Chevy trucks have temperature sensors that adjust the timing electronically based on the engine temp. If one of those sensors is faulty it could cause the condition you are describing. When you say the timing is off, have you checked it with a timing light, or scope? How many degrees off is it? Advanced or retarded?
Or are you referring to the idle when you say timing?
The timing mark on the crank moved to the left of the indicator? That's usually retarding the timing. I would think there is something wrong in the distributor of it isn't affected by engine temperature. There are weights and springs in the distributor that advance the timing as centripetal force increases. Those springs are probably worn out. Any other symptoms? Noise, smoking, etc.
I think that's your safest bet. It's not the cheapest, but there is very little else that affects the timing on this model.