I have a 2000 Blazer 4.3 that has been going into a very fast engine idle when cold starting. Placing the truck in gear will accellerate it to about 25 mph without thoching the throttle. When warm the truck operates OK. I live in Wisconsin with cold temperatures of about 20 degrees.
What can I do to fix the problem?

 

What rpm is the fast idle? Also does it do it every time you start or only sometimes?

 

If it's below 1200 rpm, i wouldn't worry about it as long as it drops down once it's warm.

 

This morning my wife said that the engine was turning about 1500 RPM until warm. She had to hold her foot on the brake to stop the truck from accelerating out of the driveway.

 

I wouldn't worry about 1500 RPM when its 20 degrees out. That's not too far off from being normal at 'normal' temps.

 

i have a 1995 blazer and when its cold out it goes to about 1200-1300 rpm its just the sensors tellin the motor it needs to warm up...whether its warm or cold i still have to hold my brake in otherwise it will take off not very fast at all though

 

Does it drop back down to 500-700RPM once it is warmed up?

It should not require holding the brake once you put it into gear though... Have you cleaned the throttle body, specifically the IAC (idle air control) valve and the bore it fits into?