yes it is
In the plenum tube that goes from the air filter box to the intake manifold. It is the only thing plugged into the large rubber tube ontop of the engine.
I just had my intake gasket changed on my 99 Monte Carlo for the same thing. My cousin is a mechanic and has changed the same gasket on several cars with the 3.1 engine. The gasket is about $60 - $75 plus labor.
It is the black plastic looking thing on the top center of the engine. They give alot of problems by wearing through and leaking water into the engine. <--- on a 3.8 but a 2001 century does not have this engine in it!
Pull of the plenum and all hoses connecting too and mark all hoses your not sure about like vacuum hoses. Now unbolt the intake off the engine and u will see a sending unit looking thing in the base of the engine leading down toward the engine. Unscrew the old knock sensor out and replace with a new one. Put everything back on reverse order. be sure to install a new intake gasket when your in the engine.
First of all, it'll take most of the day to do. Start by taking all the attachments to the plenum on both sides of the engine. Remember where everything was attached. There is a fuel line on the rear of the plenum on the passenger side that is bolted to the plenum. Then unscrew the bolts on top of the plenum and lift the plenum up from the engine. There are two more vacuum hoses that are in the bottom rear of the plenum that needs to be disconnected before the plenum can be completely removed from the engine. They will be hard to reconnect. The main thing is to remember where all the vac hoses goes and where the wire harnesses are attached. Remember the firing order of the spark plug wires. Don't buy the gaskets from the dealer.....too expensive. Try Pep Boys or Advanced Auto. They may have to order them, but they'll be in the store the next day. You will need the complete valve cover gasket kit with the goumets for the screws, and a plenum gasket. Once you place the gaskets on and reassemble the parts, make sure you torque the valve covers and plenum bolts by the manufacturer's specifications. It's a time consuming job. I did mine in about 8 to 9 hours. You may want consider purchasing the parts and have a garage put them on for you. It's not like in the old days where it was easy to get to the valve covers. Good luck. The valve cover gasket kit was $59.00 at Pep Boys and the Plenum gasket was $6.00 at Advance Auto. Most places want over $100.00 for the valve cover gaskets. The dealer wanted $139.00 for the $6.00 gasket that I got from Advanced Auto.
The best thing to do is buy a Chilton's or Haynes manual. There are two gaskets associated with the intake manifold. The upper plenum and the lower plenum. A lot of times it's just the upper that is leaking. It's fairly simple to unbolt and change. The lower involves removing the upper plenum, injection system, and other components. Buy the book.
== == First, you have to find out where it's leaking FROM. It might be blowby with a lot of oil mixed in it, picked up by the PCV valve and dumping into the intake manifold. It might be transmission fluid from a bad vacuum modulator. Find out where it's comming from first, then figure out what's wrong and what needs to be repaired. oil should not be leaking into the intake manifold. If you are burning oil then the first thing i would check are the heads and pistons Answer that is a very common problem on those trucks. there is an inspection cover on the bottom side of the intake manifold called the plenum. the plenum gasket blows out all the time on those trucks. it causes the intake to suck oil out of the cam valley. there is a replacement gasket that is now metal instead of silicone, as well as aftermarket inspection covers.
Try checking behind the air intake plenum. It should be a round thing.
First thing is disconnect the negative battery cable and the relieve the fuel pressure to the fuel rail. Then remove the upper plenum to expose the fuel rail and remove the vacuum line from the fuel pressure regulator and the 2 bolts attaching it to the lower plenum. You then gently pry the fuel rail up with an easy rocking motion and your injectors will come out of the intake manifold.
Valve cover gasket holds oil from the valves from leaking off the top of the head . Head gasket is lower and holds cylinder pressure between the cylinder head and the engine block.
It can leak oil out from the rear of the intake manifold gasket. The oil sending unit is right behind the distributor and it can leak oil too. There is also a gasket that fits on the bottom of the distributor where it goes into the intake that can leak. And the worse thing it could be is the rear main gasket leaking oil and you would have to remove the transmission to repair that.
back side center of the intake, right up against the firewall, under the big silver intake plenum. Small hands and arms are a good thing. It has a hose running to it, but should slide out easily enough if you can get your hands on it.