The single biggest issue I see with a 97-03 W-Body here at the shop. The most common cause of water in the passenger floorboard is caused by the "Plenum Water Deflector" (GM Part #10388810) which becomes dislodged and allows water to run down the windshield, under the cowl and directly into the blower fan intake. This deflector is nothing more than a rubber bulb seal that slips onto the metal edge directly below the bottom edge of the windshield. The plastic cowling seals to this rubber piece and is held in place tightly against it with the large plastic retainer with the slot in the top that you can see on the passenger side of your cowling. This deflector is notorious for falling off and allowing the water to rush on in. Also, the loss of the large plastic retainer visible on the top side of the cowl can also cause the cowl not to seat tightly against the deflector also causing the possiblility of a water leak.

Fixing this problem is simple.

Remove your windshield wipers. You may only need to remove the passenger side wiper if you have the two piece cowl design (Mine is this design, so that's what the pictures show). Some W-Bodys have a single piece cowl design that is not split in the center which would require the removal of both wipers. Each wiper is held in place with a single 13mm nut under the plastic cap at the base of the arm. After you remove the nuts, the easiest way to remove the wiper from the post is to push down with a good amount of pressure on the wiper arm joint just up from the post and wiggle a flat screwdriver under the arm just below the nut. This will usually free them from the post, however, sometimes a wiper puller is necessary if they are heavily corroded. Disconnect the wiper hose from each wiper near the base and set the wipers out of harm's way.

Next, while your under the hood, take a small flat screwdriver and pry out the center section of the three retainers (GM Part #10053182) that are located along the cowl. There is one at each end and one in the center. These have heads on them that accept a phillips screwdriver. Sometimes it is possible to unscrew the center section of these retainers out, but most often they just turn in the holes. Prying the center out will not hurt them in any way. Once the center is pulled out, you can now pull the rest of the retainer out of the holes. These retainers work by spreading out when the center part of them is pushed in. Prying them out without first pulling out the center can damage them.

Now remove the large retainer from the cowl about a third of the way over from the passenger side end. It is the one with the slot in it. Most of the time, you can simple unscrew this retainer by hand without the use of tools.

Now that all the retainers are out, pull your wiper hose out of the clips that are molded into the edge of the cowl under the hood. You more than likey will need to pull off the hood seal across the back of the engine compartment to do this and it will make removing the cowl easier if it is off as well.

The plastic cowling is now ready to come off. If it is a two piece cowl, remove the pieces one at a time out from under the hood (hood open) towards the front of the car. A one piece cowl will come out the same way, but you will need to flex it a bit in the center to get the ends out from under the corners of the hood.

Now that the cowl is out of the way, you can clearly see the air intake into the blower fan on the passenger side. The Plenum Water Deflector is the piece of rubber that is now either partially pulled off the painted metal flange directly above the intake box or it is usually laying across the top of the box completely off of the flange altogether. This rubber deflector needs to be placed back onto the metal flange so tht both ends extend past the ends of the intake box directly below it. This is what makes the water coming down from the windshield go around the intake opening rather than in it. I strongly suggest cleaning the rubber, cleaning the flange it presses onto, and applying a small bead of RTV sealer into the groove of the rubber deflector before putting it back on. This will prevent it from falling off again in the future. Now is also a good time to install a new cabin air filter if you want one since it goes into the opening of the air intake here as well. If the filter isn't there, don't panic, they didn't come with one from the factory.

Cabin air filter info if you desire:
Bosch part #P3720 - Autozone
O'Reilly/Microgard part #3011 - O'Reilly Auto Parts
Wix part #24780 - O'Reilly Auto Parts
Fram part #CF8392 - O'Reilly Auto Parts

Now reassemble everything in reverse order. All of the cowl retainers can simply be pushed straight back in and it is not necessary to screw them back in. Just remember on the three retainers that hold down the cowl (One in the center and two on each end) you must put the outer piece of the retainer in the hole first and then push in the center piece. Push the larger one with the slot on the head that goes on the passenger side back in firmly so that the cowl is held tightly against your newly corrected deflector below it.

That's it. Your water in the passenger floorboard problem should now cease to exist. Make sure to check this out as soon as you notice any water on the passenger floor. If you let it go too long, you could be looking at having your blower motor quit and having to spend your mod money on a new one.

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