How do I replace intake manifold?

Doesn't mean it's intake manifold , could be burnt intake valve . Have a cylinder compression test to find weak cylinder , then do cylinder leak down test ! If you are unfamiliar with this , I suggest you take it to a ASE certified repair shop ! IF it is a burnt intake valve that cylinder hear will have to be taken off an taken to a automotive machine shop !

Depends on the model. On the 2.0 it is easy. Intake manifold is in two pieces. Take of the fly by wire connection on the throttle body and the connecting hose that goes to the air cleaner. Then look for the Allen bolts on the upper intake taken them out and make sure you put them in a good place so you do not lose them. Then you need to do the same with the lower manifold and taken the Allen bolt out as well.and it will gave you access for injectors and spark plugs. If you have the VR6 like a GLI or GLX then the hole front of the car need to come off! Not too bad it takes 30 minutes to take a part. YouTube has plenty of videos that show how to take it off. Leave the AC connections in place this way there is no need to recharge the AC system. Then when you bend down you can see the bolts on the bottom of the manifold. These are a one time use and need replacing when taken off. The intake gasket needs to be replaced as well. After replacing my valve cover gasket I spent $800 plus in parts and these were my jobber prices as well. VR engines in general are expensive to.work on. On the 1.8 turbos it is the same procedure you have to take the front of the car.

Its probably an intake manifold leak or a dry belt. check the dry belt by physically looking at the belt at look for cracks or and wobbling pulley. Try adding a lubricant to the belt to see if the noise goes away. If it does get the accessory belt or pully replaced. also check the leak by spraying brake cleaner (or another aerosol cleaner) around the manifold edge (where the intake manifold meets the block and where the gasket is) to see if the noise goes away. If it does then you need a new intake manifold gasket which should be taken to the shop to be replaced.

engine running lean. Usually caused by vacuum leak. If you have a V engine, the failure is usually intake manifold gaskets. If you have inline (4-cylinder), the cause may be intake gaskets or bad or unhooked vacuum hoses attached to the intake manifold. Check all vacuum hoses for cracks or loose connections. Replace any suspect hose. To find manifold gasket leaks, spray ether or carb cleaner on the gaskets with the engine running. If the engine revs at all, you have a leaky gasket and must remove the manifold to replace. Please let me know if you have any more questions.

It's hard but possible to do that without removing intake manifold , if you don't have experience and right tools , probably removing the intake is easiest way.
If you want to try hard way , you need to clear some room for your hands by taking vacuum lines out, that go to throttle body and throttle cable with bracket (two 10mm heads bolt) , on the other side of manifold take out pcv hose (just pull it out from intake manifold and turn it out of your way.don't worry about clamp , it will come out and go back the same way when you push it in)
The worst part of this job without removing intake manifold is disconnecting harness plugs from coils and braking loose spark plugs.(Practice first on front harness connectors.)
Good luck.

1. Take loose the intake tube from the intake and breather box.
2. Remove the wiring harness from the intake mahifold there should be 4 injector plugs 3 plgs on the back of the manifold .
3. You must take loose the fuel lines both the supply line on the left side of the fuel rail and the return line on the fuel pressure regulator
4. Throtle cable must be taken off of the throtle body(and also dentent or shift cable if it is an automatic trans.)
5. There is going to be a bracket on the underside of the manifold that more than likely will have to be gotten from the bottom should be a 14mm take loose from the manifold not the block
6. There will be 8 to 10 12mm bolts that hold the intake manifold onto the head all of which are hard to get with the head still on the car but it can be done for a couple of them u will need a wrench and possibly even a swivel socket
7. Remove the intake and pay attention to the hose that runs from the intake to the oil breather on the back of the motor its a bit tricky to get back on if u dnt have samll hands
8. Make sure the gasket area of the intake and head are clean before installing the new gasket
9. Reverse the instructions to replace the intake and make sure you torque the intake stud nuts to 16 ft/lbs

It's located deep in the valley! Under the "LOWER intake manifold". copy and paste this link in your browser, or try clicking on it. It may be helpful to you.

http://x.nissanhelp.com/forums/pathfinder/7890-3-3l-knock-sensor-relocation.html

I didn't find this link until after I had already disassembled my truck. This is the hardest job that I've taken on thus far, hope I can put it all back together properly. I changed the gaskets on the valve covers since I had everything off.
To answer your question: It's under your lower intake manifold, so it's a hard job!

Intake manifold-to-cylinder head bolts for 5.0L V8:
Step 1 = 96 in-lbs Step 2 = 16  " Step 3 = 23 to 25  "
Numbers taken from an after market manual, so use at your own risk.

http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c1528008effd
this should help