Refer to your owner's manual for specific instructions of the removal procedure for your Van's year.
There are some typical drawings at
http://astrosafari.proboards.com/index.cgi?board=interior&action=display&thread=28&page=1
On a 2004 Chevy Astro, the camshaft sensor is located in the front of the engine. It is on the left side of the timing cover behind the camshaft sensor.
I the rear of the engine under the "doghouse" or cover behind the glove compartment.
It should be at the bottom front of the engine by the timing cover.
The 1997 Chevrolet Astro EGR valve is located on the top of the engine valve cover. The EGR valve can be access from under the hood. You will not need to remove any engine cover.
Yes it would be easer to get at it with the dog house removed.
The crank position sensor On your astro is located on front of the engine on timing cover, left side. Behind the crankshaft pulley. with a connector of 3 wires.
You will have to unbolt and remove the engine access cover inside the passenger compartment. Then you can easily get to the air cleaner.
On top of the RH Side of the Engine. You will need to remove the engine cover from the inside of the van to get to it. Just trace the coil wire from the distributor.
Take it off
module is under distributer cap.and coil bolts on passenger side intake . all can be gotten by removing engine cover inside van.hope this helps.
The PCV valve on a 1999 Astro should be located on top of the valve cover. It plugs in and can be easily replaced.
pop the cover off that is between the front seats