1997 Blazer 4.3, replaced crank & timing chain. When it's in park the idle goes up to about 3rpms then goes back down below 1, up & down. Please anyone know what would cause this???

 

I assume you mean 3000RPM & 1000RPM. It sounds like you either have a rather large vacuum leak or a problem with the IAC valve in the throttle body. I would start by inspecting all vacuum lines and then clean the throttle body completely.

 

Thank you, sounds like i have a vac leak in back right of the motor but I can't find anything! Would the mapping sensor make a loud leaking sound? I have replaced all of the hoses.

 

The MAP sensor is located on the intake manifold to the left and slightly behind the throttle body looking at the motor from the front of the truck. If you can hear a vacuum leak, then it shouldn't be that difficult to locate the source.

 

thanks been looking 2 days straight can't find it, frustrating