Hi. I understand I need to replace my EGR valve on a Pontiac Montana, 2000. (3.4 L) Any tips or guides on just how one gets to this valve? (behind air intake assembly)

Sounds like you have a clogged egr port with carbon and build up. The egr valve needs to be pulled, checked and visually inspect the port. Be certain to get a new gasket before taking it apart.

Hi Bruce,here's a AutoZone repair guide showing you your canister purge valve. Repair Guides Component Locations Component Locations 1 AutoZone com

Could be a plugged EGR pipe.

Betty, Check the attached EGR links,instruction and guides, Good luck
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Exhaust Gas Recirculation Valve EGR Checking
Faulty EGR Valve Symptoms and EGR Valve Cleaning
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Installing New EGR Valve

Hi Roneal,here's a EGR valve.Here's a second opinion.

Take the EGR valve off the engine, then use some fuel injector cleaner (like you pour into the fuel tank) to clean the pintle (shiny steele rod inside the egr valve) and the seat (the valve that opens and closes off the exhaust flow). Once the pintle and valve seat are clean, check for smooth operation by pressing the diaphram with finger tips and releasing. It should feel like it is moving smoothly both ways.

Sounds like EGR Valve (Exhuast Gas Recirculation Valve) is sticking in the open position allowing exhuast gas into the intake manifold to soon which at idle will cause the engine to either run real rough or to stall out.Some EGR valves can be taken off and clean the carbon out of it or you can replace it.

Its located between the throttle body and the firewall. It bolts onto the upper intake.

You would need to remove it and look into the passage that is covered in black soot. there is usually a little poppet that moves up and down to allow the exhaust gas to pass thru it, some are vacuum operated and some are electric. you can push the poppet open and scrape out the carbon with a little screw driver, generally they dont get all that pluged up on newer cars. I looked at a picture of yours at auto zone.com It appears yours is electrically actuated. If either the electric actuator or the position feedback in the valve are bad you would need to replace it. You could attempt to clean it and move the poppet up and down it could just be stuck

Most GM it takes about 100 start cycle to clear any DTC codes that do not reoccur. As for the lack of power when accelerating could be a dirty fuel filter and needs to be changed.
As for the EGR Valve code it may be getting carboned up and need to be replaced or cleaned soon with carb cleaner. First try changing the fuel filter for your lack of power.
If you are ever going to have you Montana Rescanned and don't mind spending a few extra dollars, Have a Dirve Cycle DTC test done. http://myweb.accessus.net/~090/dtcobd2n.html
Good luck and hope this helps. Note: The program on the transmission on the Montana thanks to GM and there wisdom was made to slip when the transmission is cold to warm up the drivetrain quicker, which premature failure is common around 150,000 KM or 100,000 Miles just after the warranty. Regular transmission fluid and filter change will help exten the life of the transmission on the Montana.