very carefully
Which plenum-intake or. . . .
It is on the underside of the intake manifold
yes it is
To replace the intake plenum on a 2.5-liter Mercury Mystique, first disconnect the negative battery cable and remove any components obstructing access, such as the air intake and throttle body. Carefully detach the intake plenum by unbolting it from the engine and disconnecting any vacuum hoses, electrical connectors, and sensors. Clean the mating surfaces, install a new gasket, and then position the new plenum, securing it with the bolts. Finally, reattach all previously removed components, reconnect the battery, and start the engine to check for leaks.
no, it is very easy, it just takes time, plan on aprox. 6 hours so you do not feel rushed.
Not sure what engine you are working on but if you are using a new gasket there is no need for silicone unless the directions included with the new gasket require it.
You have to remove the intake plenum air chamber to get to the 3 plugs against the firewall. It requires to buy intake plenum gaskets and go ahead and replace the PCV hose on the back of the plenum. Need to get from the dealer. Gaskets also from dealer.
The manifold gasket is the same as the plenum gasket. The manifold gasket is a barrier between the manifold and the engine block.
P0300 : Multiple cylinder misfire. see related questions below. Look up TSB-09-05-00. The intake plenum gasket may have failed, causing the intake to suck oil from the upper engine into the cylinders. remove throttle body and look down intyo the intake and check for oil. if there is oil, replace the plenum gasket. the TSB has the procedure. This is very specific to the 98-99 durango.
remove upper intake plenum
No.
First of all you can get to the first 3 plugs very easily. The other three you have to take off the plenum (be sure to replace the old plenum gasket)