In January, I purchased a used "certified" 2011 Silverado 1500 regular cab work truck with 66,000 miles on it from a GM dealer. It has a vinyl flooring. I noticed at the time of purchase that the door weatherstripping at the sill plate was cracked, and stained with rust. I didn't think this was an indication of any water infiltration issues. About a month later I took the truck through a car wash. A couple of days later, I entered the cab and felt ice crystals "crunching" under the flooring. I discovered that the padding under the vinyl flooring was completely saturated and had frozen on both the driver and passenger side floorboard. Water had apparently wicked up under the seats, though the padding was not as wet the further toward the rear. I removed the door sill plates to access the padding and begin drying out the padding. The amount of rust and obvious water marks along the plastic receivers that hold the sill plates in place indicates that the water infiltration has been a problem for a while. This is particularly troubling because wiring runs through these receivers. Fortunately, there does not appear to be any significant rust on the floor pan itself. With the vinyl mats pulled back as far as possible (without removing the seats, I dried the padding over the course of a couple of days. I called the local GM dealer (not the one I bought the truck from) and asked what I should do and whether this may be covered by the remaining warranty on the truck. The service department manager told me that it was doubtful if the problem would be covered by GM and that I should first take a look at the seal on the third brake light over the rear of the cab and see if the gasket was "wavy" and improperly seated. An improperly seated third brake light gasket was, she explained, the most common cause of water infiltration they see at their dealership. She also indicated that water leaks are hard to diagnose and that they typically charge about an hour's labor to test a vehicle in their wash-bay, which seemed reasonable enough. On further conversation and after reading some forums, I removed the third brake light assembly and found it to be slightly bowed. It also appeared from that the and that the gasket may not have been fully compressed whereas there were two spots on it that appeared brown with dust and pollen where water may have been able to pass through, perhaps under the pressure of a car wash. So, right or wrong, I applied a generous double-bead of a good exterior grade 3M silicone sealant to the gasket and re-installed the third brake light assembly. Then, after a day or two to let the sealant set up, and with the flooring and vinyl pulled back for inspection, I had a friend run a water hose continuously on the front of the truck starting low on the front fender at the door hinge area and very very slowly "climb" to the level of the cowling at the base of the windshield while I checked for water. We repeated the process on both sides. This did not produce a leak. Then I had him move up on the windshield in the same manner to no effect. Finally we moved to the third brake light. After applying a significant amount of water to all of these areas, there were still no leaks. The padding, sill area, and firewall remained totally dry. No leaks or water observed. I thought the problem was resolved with the application of sealant to the third brake light gasket.
The following week a very heavy but brief rain occurred. the truck was parked on a level driveway. I did not use the truck until a couple of days later. I opened the driver side door and found that the plastic receiver tracks to be very wet, though the padding was not. As I was moving around to the opposite side of the truck, I shut the driver side door. When I shut the door, water came pouring out from the underside of the driver's- side fender near the door hinge. When I looked under the truck in this area, I saw water exiting between what I think is the firewall and the fender. I suspect water is pooling somewhere in between the two. I saw in a forum where water can collect in the opening in the sidewall next to the emergency brake, wiring harness is mounted here behind the vinyl flooring. This may indicate a seam that is improperly sealed in the firewall door framing. I intend to contact the dealer tomorrow and discuss it with them. It would appear that the fender needs to be removed and the seams investigated and repaired.
