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Wet Driver's Carpet From Leaking Heater Box Joint


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#1 Riverfred

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Posted 14 October 2009 - 04:34 PM

I recently discovered the driver's side floor was very wet in my 98 GS 300 with 108K miles.

I first had this problem 5 years ago after the evaporator core was replaced to fix an a/c problem (a very extensive and costly repair). The a/c radiator assembly was also damaged in the course of that repair and had to be replaced. Water under the carpets (both sides) shortly thereafter was due to the AC drain not being properly reseated into the external tube during that repair/replacement.

Now I have water under the driver's side carpet again. All the drains have been checked and are clear. The mechanic says the translucent AC drain is clear and does drain when the car runs. Leaning up under the dash, he thinks the leak is a slow drip he saw which appears to come from the heater box joint. The approximate $1600 cost of tearing out the dash to get to the heater box to fix the joint is excessive given the value of the car.

My questions -- is this a common problem? It seems an unusual coincidence to have both these issues with the heater box/evap core. I don't see many postings on the topic. Did the prior replacement of the evaporator core require splitting open the heater box? Is it likely or possible that the heater box joint/seal could have been damaged during the replacement of the evaporator core? What is the nature of the material sealing the heater box joint in this particular model? The mechanic tried sticking some butyl tape on the small part of the joint he could reach (without removing the dash) but that didn't stop the leak.

What should I do? Are there any other possible causes or fixes for this problem? Has anyone else had this experience?

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#2 AG-GS300

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Posted 11 December 2009 - 04:02 PM

I recently discovered the driver's side floor was very wet in my 98 GS 300 with 108K miles.

I first had this problem 5 years ago after the evaporator core was replaced to fix an a/c problem (a very extensive and costly repair). The a/c radiator assembly was also damaged in the course of that repair and had to be replaced. Water under the carpets (both sides) shortly thereafter was due to the AC drain not being properly reseated into the external tube during that repair/replacement.

Now I have water under the driver's side carpet again. All the drains have been checked and are clear. The mechanic says the translucent AC drain is clear and does drain when the car runs. Leaning up under the dash, he thinks the leak is a slow drip he saw which appears to come from the heater box joint. The approximate $1600 cost of tearing out the dash to get to the heater box to fix the joint is excessive given the value of the car.

My questions -- is this a common problem? It seems an unusual coincidence to have both these issues with the heater box/evap core. I don't see many postings on the topic. Did the prior replacement of the evaporator core require splitting open the heater box? Is it likely or possible that the heater box joint/seal could have been damaged during the replacement of the evaporator core? What is the nature of the material sealing the heater box joint in this particular model? The mechanic tried sticking some butyl tape on the small part of the joint he could reach (without removing the dash) but that didn't stop the leak.

What should I do? Are there any other possible causes or fixes for this problem? Has anyone else had this experience?


I had the same problem on my 2000 GS300. Took it several times to the Lexus dealership and at first they could not find the problem as the car would not leak while in their garage. once they finally found the problem they wanted over 2K for the repair. Apparently the seal will wear out and the only thing they can do is replace the entire unit.

I finally took it to another mechanic (who only works on Lexus and other imports) and had them pull it out and re-seal it. It cost me $600 for the entire job. It's now been 3 years since they fixed it and it is still working perfectly.

Good luck!

#3 thunter

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Posted 03 June 2010 - 09:37 PM

I have a 98 GS300 with 117,000 mi. Just noticed today I have the exact same problem? My AC is draining fine, but
when I lifted my floormat on the drivers side, I found it was soaking wet.

Used a wet vac, then pulled up the carpet and found standing water and the 2" padding was soaked. Dryed the floorboard
with a towel and ran a box fan for about an hour.

Next, ran the car in the garage with the AC on. After about 15 min, noticed water coming from behind/under the black rubber mat in the corner by the gas pedal.

That was great, considering I just got in back today from having the ball joints replaced.

I've loved my Lexus, but not impressed by these two failures.


I recently discovered the driver's side floor was very wet in my 98 GS 300 with 108K miles.

I first had this problem 5 years ago after the evaporator core was replaced to fix an a/c problem (a very extensive and costly repair). The a/c radiator assembly was also damaged in the course of that repair and had to be replaced. Water under the carpets (both sides) shortly thereafter was due to the AC drain not being properly reseated into the external tube during that repair/replacement.

Now I have water under the driver's side carpet again. All the drains have been checked and are clear. The mechanic says the translucent AC drain is clear and does drain when the car runs. Leaning up under the dash, he thinks the leak is a slow drip he saw which appears to come from the heater box joint. The approximate $1600 cost of tearing out the dash to get to the heater box to fix the joint is excessive given the value of the car.

My questions -- is this a common problem? It seems an unusual coincidence to have both these issues with the heater box/evap core. I don't see many postings on the topic. Did the prior replacement of the evaporator core require splitting open the heater box? Is it likely or possible that the heater box joint/seal could have been damaged during the replacement of the evaporator core? What is the nature of the material sealing the heater box joint in this particular model? The mechanic tried sticking some butyl tape on the small part of the joint he could reach (without removing the dash) but that didn't stop the leak.

What should I do? Are there any other possible causes or fixes for this problem? Has anyone else had this experience?


I had the same problem on my 2000 GS300. Took it several times to the Lexus dealership and at first they could not find the problem as the car would not leak while in their garage. once they finally found the problem they wanted over 2K for the repair. Apparently the seal will wear out and the only thing they can do is replace the entire unit.

I finally took it to another mechanic (who only works on Lexus and other imports) and had them pull it out and re-seal it. It cost me $600 for the entire job. It's now been 3 years since they fixed it and it is still working perfectly.

Good luck!



#4 FrankT

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Posted 11 June 2010 - 04:20 PM

This seams to be a common issue.

...a temporary solution to the wet carpet.

On my GS400, I made a tray by cutting up a gallon jug and slipping it under the leaking box, attached a tube so water can drain. At first I had it drain in to a small plastic soda bottle that was tucked under the carpet but last summer I redid the tray and now the tube is routed through a rubber grommet in the floor board.

I have so far forgotten about the issue until I read threads like this, but I promise one day I will pull the dash and reseal the evap box.


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