The leak can either be to the exterior (not so bad) or into the engine
oil (very bad!).

Anyone with one of these engines is well advised to have a used engine
oil analysis done at least once per year to catch contamination before
it gets bad enough to completely trash the motor.  Many places offer
used oil analysis services for around $20.  The one I use is:

www.blackstone-labs.com

They caught the problem on my '02 Olds at about 40,000 miles.  Luckily
it was still under GM's extended warranty at the time.   I keep checking
it every other oil change to see if it comes back.

John

How do you explain what?
If enough antifreeze leaks out then gravity will eventually pull it to the ground.

Ernie,
Mine is doing the same thing, and it sure sounds like a leaking intake
manifold gasket. The manifold to cylinder head joint is directly above
where the coolant is pooling up on the transmission in my case.
Personally, I would be carefull with the pressure test as you don't
want to force coolant into the engine.
Howard

Wild a.s guess, Ernie...When our 1998 Buick ate the plastic plenum, the
dealership suggested about $700-800.  An independent mechanic (of
impeccable reputation) offered to do it for $250...

In the long run, the Buick dealership offered $350 with coolant change,
etc.  We accepted, since we would have documentation in case GM
ever decided to recognize their bastardization problems.

Now, the plenum is different from what you are facing.  Yours may well
be a little less.  But you can extrapolate the probable region of cost.

By the time we are done doing the job (we charge 98 dollars
an hour), it's usually a 750-800 dollar job.  The labour
charge is about 6.0 hrs, but a good tech can easily do it
in 2.0 hrs.  I wrote up a detailed explanation of how to do
this job a while back.  I'll paste it below and you can do with
it what you want.  It should be pretty close to what you will
encounter with your vehicle.  It was written for a van, but the
procedure is very similiar for your car. There will be some
difference at the throttle body area and the fuel lines for the
injector rail go way down under the exhaust.  Don't bother
trying to disconnect any fuel lines, simply remove the nut
that holds the fuel lines against the rear head and then position
the injector rain and lines up out of the way and bunjee cord
it up to the hood.  It's too much work to try and disconnect the
fuel lines and then snake them out under the exhaust crossover
pipe.  You will see what I mean when you get there.

Ian

Ok, I'll go over the basic procedure for the van.

First step would be to drain the coolant.  This year, you
need a 1/4 inch square drive ratchet and preferably a short
extension.  Use the 1/4" ratchet to turn the drain cock
counter clockwise about an 1/8 of turn, then you have to
pull the drain cock out while turning it further.

Next...pull the rubber hood seal off at the front of the
cowl area...then you can pull off that plastic piece that
covers the wiper motor area (passenger side of the
engine compartment.

Pull the air filter housing and snorkel..usually, I find it
easier to remove the rubber section first, and then you
have more room to remove the air filter cover and air
filter.  There are a couple of electrical connections on
the snorkel and the MAF sensor, unplug them...and then
follow that wire harness back and unplug everything else
on it, except where it plugs into the coil pack ( oh yeah,
I'm going to give you the "faster" version).  So you should
have a couple more sensors at the throttle body, a red
connector at the evap solenoid and then a harness clip
at the coil pack.  You can position this harness back over
the coil pack.  Remove the front three spark plug wires at
the coil pack.  Unhook all the clips that hold those wires
to the engine, but don't bother to take the wires off the front
plugs, just lay the wires over the front of the rad support.
Remove the vacuum booster hose at the upper plenum.

Remove the two bolts that secure the coil pack bracket
to the upper intake plenum (10 mm head).  Now you want
to remove the MAP sensor which sits bolted to a small
bracket on the upper plenum right in front of the coil
pack.  You will need a 7 mm socket or wrench to undo
the two long screws that hold it in place.  Then unhook
the vacuum lines and the electrical connector and lay
the sensor and vacuum lines somewhere.  Now you
can remove the upper plenum bolts.  At the passenger
rear side of the upper plenum, there is a small brace that
goes from the alternator to one of the bolts in the upper
plenum.  Get that brace off and then you can take that
one plenum bolt out.

Now go to the back of the engine, you need to find
the EGR pipe that comes up to the upper plenum and
EGR valve.  The temptation is to remove the valve, but
don't bother. Just remove the bolt that holds the flexible
pipe to the upper plenum.  There is also a small nut that
holds the Transmission filler tube to a stud on the upper
plenum.  You need to remove that and push the filler
tube off that stud.  There is also one nut at the throttle
body that holds a coolant pipe in place, remove that and
disconnect the two small throttle body coolant hoses at
the back of the engine there.  At this point you should be
able to remove the upper plenum.  Do "not" bend the
coil pack bracket up to give yourself clearance.  This makes
it very difficult to start those bolts properly later on.  Just
slide the upper plenum out from underneath it.  You may
notice that I said nothing about the cables to the throttle body.
I do not disconnect them, just leave them on the plenum and
flip the plenum upside down and off to the side.  It will just
sit there.

Next, you need to remove the fuel rail and injectors.  There are
two 10mm head bolts that hold the rail to the lower intake, and
one that holds the fuel lines to a bracket at the rear of the rear
head.  Remove all three, carefully pry/pull the fuel injectors and
rail out of the holes.  Take a close look at the injectors once
you have them out, any o-rings that have come off need to
be retrieved from the intake manifold area.  Sometimes they
will just be loose, other times, they are stuck in the injector
holes. Again, I don't disconnect the fuel lines, just position the
rail and lines off to the side somewhere.

There is a heater pipe that goes into the lower intake at the
rear of the engine. You need to simply loosen the nut that
holds it to a stud at the rear of the rear head, and then
you can pull the pipe out of the intake.  Note: replace
that o-ring seal.

There is also a heater pipe and bypass pipe assembly
that runs along the front of the front valve cover.  That
is the same pipe that had the two small throttle body hoses
connected to it.  Remove it, and either move it off to the
side, or disconnect the "quick connect" fitting (sometimes
they aren't all the "quick" and so I just leave them).

Now you can remove the front valve cover, if you need
room to get at the one bolt, remove the left engine strut
and you can get a run at the bolt.  Then loosen the two
rear valve cover bolts that you can see.  One will be
under the alternator, the other is under the coil pack.
In both cases, there is just enough room to completely
loosen the two bolts.

Now you have to deal with the power steering pump.
Remove the small plastic shield that sits right above
the water pump pulley (this just give you a little more
clearance for the ps pump pulley).  Now, remove
the serpentine belt.  Then remove the three bolts
that hold the pump to the engine, you can access
them thru the bolts in the pulley.  They are 13 mm
bolts.  Now, you must loosen the ps pressure hose
fitting at the pump.  Use a 5/8 wrench and just
loosen it until the hose will be able to move.  In other
words, don't completely remove the hose.  Now, reach
your hand down the two ps hoses and you will find some
plastic clips that the hoses are in.  Pop the Pressure
hose (left one as you look down at the two hoses) out
of the plastic clip.  Now, you should be able to pull
the pump up and lay it down between the front of
the engine and windshield washer bottle.  You may
need to fiddle with the rubber return hose, as there
is a bracket in it's way, you can move the hose around
the bracket.

At this point, remove the lower intake bolts (8 of them)
and the upper rad hose from the lower intake.  The intake
should now slide out from under the rear valve cover.  Trust
me, it comes out.  Now you have to loosen the rockers
arms enough so that you can get the push rods out.  The rear
rocker arm bolts can be reached, even though the rear valve
cover is in place.  Just loosen each rocker arm just enough
to be able to remove the push rods.  Keep them in order,
as it's important that you don't mix them up.

Clean up your gasket surfaces..etc.  Then install
your intake gaskets to the head first, then the push rods.
Short pushrods to the intake valves, long to the exhaust
valves.  Be meticulous about cleaning the front and
rear surfaces of the block and the intake manifold, as
this is where the RTV sealer goes.  Also clean the joint
between the rear valve cover and the cylinder head and
where the intake manifold will meet the two of them.  Do
not mess with the rear valve cover gasket, just make sure
it stays up in it's groove.  Before you install the intake, make
sure you have a nice bead of RTV on the front and rear mating
surfaces, and put a dab of RTV in that corner where the rear
v/c and head and intake will meet.    From there, it's pretty
well just the reverse of the removal procedure.  Don't bother
trying to tighten those two rear valve cover bolts until you have
torqued down the intake manifold.  Then they will just slide
right in.  Also, when you install the upper plenum, you are
going to slide it back under the coil pack, and then engage
the EGR pipe and the throttle body coolant hoses. Take your
time here, and make sure everything is where it should be
before tightening down the intake bolts.

Hope this helps, and all goes well.  The procedure for the
Century will be almost identical, other then the Trans dipstick
tube.

Ian

My bet is that all will eventually fail, but that the time and mileage
too failure is highly variable.  Do an oil analysis from time to time to
find it before more serious internal damage happens.

GM should have issued a recall on this years ago.

John

The mileage was 95,500.