RESOLVED: AC drip pan water leaking onto passenger floor
I'm re posting this with all the info I've written on this problem so it will be in one place and hopefully easily found by anyone with the same issue.
This has been a long, frustrating process, but I believe I have finally licked this problem! It's a cautionary tale for all B-body owners because it is possible to have this problem and have no idea it is there while it silently corrodes your floorboard.
I believe I have had a substantial water leak from the AC drip pan since buying my 96 RMW about three and a half years ago. I did not realize it for a long time because the drip was coming from the front of the box (firewall side, towards the front of the car), on the right side, below where the drain pipe is, dripping off the foam padding that is applied to the firewall that the drip pan presses up against. The result was that water drained down the firewall, under the carpet and down to the passenger floorboard soaking the insulation. There was no dampness that could be felt on the top of the carpet and the majority of the water still drained out of the box in the normal way, so there was a normal condensation puddle under the car when parked. The only indicator that there was a problem was a mild, vaguely moldy smell that was noticeable when the car was parked for very long with the windows closed. The smell wasn't bad enough to make me think the car had a problem, I just figured (foolishly) that it was the natural odor of the car as I'd never owned a Roadmaster with cloth seats before. I happened to notice the damp insulation when digging into the dash for an unrelated AC problem. Fortunately, the rust on the floor was not too bad. Just a few small areas and nothing very deep or widespread.
I started trying to figure out this problem by checking the common causes people talk about on b-body forums:
- The drain elbow was present and accounted for and fit tightly on the drain pipe. In fact, I had put a hose extension on the elbow shortly after buying the car because I don't like the way water drains onto the frame and randomly seeps to the ground.
- The water was not coolant (from a bad heater core).
- It was not rain water.
- My first theory was that the seal between the drain pan and the evaporator/heater core housing was bad, the tricky process for dropping the pan I outline at the bottom of this post. After resealing and replacing the pan, the problem was still there. It is possible that the pan was leaking some because it sure looked like there was water coming off the right side of the seal and trickling along the seam to the front of the pan and then down. Also there was some mold all around the old seal in that area. But, if there was a leak there, it was incidental to the main leak.
- My next theory was that it was condensation dripping off the outside of the box. This would also explain the water that looked like a seal leak and in hot, humid Houston, there seemed like an excessive amount of condensation sometimes. I insulated the box with adhesive backed foam padding and foam tape, both of which were medical items I happen to have access to in my job. [see photos at bottom of post] This also did not fix the problem, but I am going to leave it in place because it still seems like it has value to keep the area as dry as possible.
- I thought maybe the AC was too cold, causing excessive condensation off the evaporator and making the pan overflow on the front (firewall) side where it is not sealed. The pan is only sealed to the housing on 3 and a half sides. The side on the front on the evaporator portion of the housing is not sealed. I have no idea why it was designed this way, but I assume there was a reason. So, I adjusted the high pressure cut off switch to where the AC was cutting off quite a bit. This made no difference in the leak.
- Next step was to drop the pan again (I am getting pretty good at that procedure) and try to observe how the water was dripping off the evaporator. I set the pan below the housing and tilted to the right so that all the water ran into another container. I was able to actually measure the amount of water the system puts out, which was about 1.5 quarts every 30 min of driving with temperatures in the low 80's and high humidity. The pan holds about 16 oz of water and drains it out the drain pipe in about 20 seconds. This ruled out the overflow theory. I also noted that there was no sign of a crack or leak in the pan itself and there wasn't any leakage around the elbow where it is attached to the drain pipe.
- With the pan sealed in place, the water looked like it was coming between the front (firewall) portion of the pan and the foam padding attached to the firewall that goes around the hole in the firewall that the drain pipe goes through. When I had the pan dropped, the water inside the housing appeared to be dripping down the inside of the housing near the right front corner. This water would drip down the foam padding exactly like it appears to do when everything is in place. Could this be what is happening? Is it somehow dripping forward of the front, unsealed edge of the pan? I went to seal the pan back in place, but this time I focused on having PLENTY of RTV sealant in the channel on the right front corner and put a generous quantity of silicone caulk on the front unsealed part of the pan between the pan and the foam padding on the firewall. I did this by putting the caulk on the padding near the right corner so that corner would have plenty of caulk and on the rest of it I put the caulk across the whole unsealed section on the pan itself as I put the pan in position. I left it to dry for about 36 hours.
After driving for a few 30 minute plus trips in both the morning (high humidity) and afternoon (near 100 deg temp, lower humidity), there is no sign of water at all. After spending months watching a tiny stream of water run down the firewall no matter what I did, it was really exciting to see everything dry and no puddle on the floorboard. I put everything back in place including the passenger seat, which had been hanging out on top of the lowered rear seat and sliding off on hard stops. I did leave the trim off the carpet edge so I can check under the carpet for a while, because I am not 100% confident.
I'm not completely confident because I never found anything that was obviously "wrong" with the car to account for the improper water drainage. It's hard to feel like you fixed something when you didn't find something broken. My only idea is that the pan on the front edge where it doesn't seal has a very slight inward bow, but it doesn't look severe enough to alter how it seals with the padding. Besides, it doesn't make sense that it would need a water tight seal with the padding as designed because water shouldn't be getting in there in the first place. For some reason, it is. I'm not sure if the evaporator has been replaced before I owned the car, possibly reinstalled with the position being slightly off. This is the cautionary part of the tale, because if my car, which is in generally excellent condition, could have this problem then any car could. I'm providing all this detail for anybody researching this topic may hopefully find this useful. I posted a few pictures at the bottom of the post.
Tips for removing and replacing AC drip pan:
Removing the AC drip pan is not simple, as I am sure any of y'all that have done this can attest! I read the FSM procedure, which was really no use as they say basically to remove the entire dash. That would make it easier, but definitely not going to happen! I did do a few things which made it a little easier. I removed the passenger seat, which really improved the access from totally aggravating to just a little annoying. I removed the glovebox, which made it easier to visualize the cover and also allowed me to remove a bolt on the right side of the support that the glovebox hinge mounts on. This allowed me to bend that support out a little bit to be able to pull the box out. 5 of the 7 screws that hold the cover in place came out pretty easy. The last 2 required a trip to Sears to purchase a 5.5mm box end wrench (actually a kit of several small box ends, since they won't sell just one in that size) and a 1/4 inch drive universal joint. The boxend was for the screw along the firewall on the left side, and the universal joint worked along with a long socket for the one along the firewall in the middle. [see photo of tools at bottom of the post]
Remove the old seal entirely and replace it with RTV silicone gasket maker. Put the new silicone in the channel roughly level with the edge. If you put too much in, it will just overflow the sides a little and won't hurt anything. Better too much than too little. Make especially sure you put plenty in the right front corner (firewall side). Let it sit for 10-15 minutes, just long enough for the silicone to be dry to touch. Replacing the pan is of course basically the opposite of removing it, but I did a dry run putting the pan in place without any silicone just the make sure I have my moves down and can do it without touching the soft new seal. I recommend once you have the pan in place not pressing it in place very tightly. Let the tightening of the screws press it in place all the way. This way the seal will compress automatically the exact amount it will need in its permanent position. If you press it tighter than it will be after you let it go, the seal may not seal as well.
Last edited by jon7190; 08-16-2016 at 08:07 PM.