Know Leak Area
 

Recently I noticed the back passenger side floorboard is wet to point of having standing water after a good rain. Is there a know corrosion area that promote a leak in that are or is there a know source that will allow water in? Thank you and Merry Christmas and Happy New Year!

Windshield gasket often leaks. Can be stop gap caulked with black silicone or replaced with a new gasket.

Check the drains under the hood hinges, and also, their drains which are inside the front wheel wells.

My apologies, I was not clear which passenger side. It is the rear passenger floor board is getting soaked. The front was getting some water as well, that's when I found those drain holes mentioned along the hood hinge, plus I found a hole on the inner fender. I repaired that, the front passenger side is not dry. It is the rear, at times I do see a trace of water in the trunk on top of the wheel. I am thinking the leaky windshield might be the rear window seal that needs to sealed. Week end project for me to investigate.

Water migrates so that even leaks originating from the front can cause flooding of the rear floor. Besides those mentioned above also check for clogged sunroof drains and clogged drains in the trunk wells (there are a couple of rubber "nipples" on each side you can poke your finger up into, to open them up). Also cracked or loose tail light lenses can let water in.

An experiment for you
 

On my coupe, the major water incursion was from around the rear windshield gasket.

First, find someone you love and trust. Let them close you into the trunk with a flashlight and run water from a hose around the rear windshield first then the trunk gasket. I'll bet you find water coming in the upper left corner.

Now the bad news. In my case and probably yours, no amount of squirted-in sealer will result in a satisfactory repair. Upon removing the rear windshield and gasket you will probably find the gasket channel rusted through in a number of places. The better news is that this is not a complicated or especially expensive repair for a competent body shop. Body work cost me about $200. plus gasket (expensive for a coupe, much less for a sedan). I also invested $100 for a glass shop to R&R the rear windshield since I was afraid to crack it and coupe rear windshields are made of unobtainium.

Let us know how you do.

My 83 300D has similar problem.

Driver front floor - dry-ish
Pass front floor - dry
Driver rear floor - BIG puddle
Pass rear floor - dry

Could it be coming from the rear wind shield driver's lower corner? Any info on known leak entry points would be appreciated.

When I had this issue on my 85 300D, it was the evaporator drain hose. The original hose is made from a foam type material that degrades over time. Replacing the hose stopped the puddle.

I had rusted through Holes in the area that the Hood Springs are in and the Drains that would have drained that area were clogged. The Water that leaked in from the front ended up inside under the Rear Mats.

The first thing I did was to remove the Floor Plugs. That allows most of the Water to Drain out. Water does not seem to come back in through the Plugs unless I was fording some deep Water that was above the Plugs.

A common place for leaks in the rear is under the rear Window rubber where there is often holes rusted through.

Where does Water go when the Sunroof Drains are plugged?

I had this issue with my 83TD and a firewall grommet was poorly fed an alarm wire, and the bulkhead rubber was funneling water into the floor pan and settling in the rear footwell... THAT took a lot of investigation to find!

Check under the battery tray, you may find a rusted hole, a common leak area.

If this has been going on for any length of time, the water gets between the tar backing and the steel floor pan and eats it out from within. BTDT

So not only do you have to worry about the source of the leak being a rusted out spot, but also what is happening to the floor pan in that area.