I have clear, clean water leaking into the passenger side footwell. It's coming down both sides of the footwell:

Here you can see where the drops are falling from:

And here is an indication of just how much water is making its way inside the car:

I'm familiar with the AC vents getting blocked and causing water to drip down the side of the transmission tunnel but not so familiar with the leak on the other side of the footwell. I'm also doubtful that the AC drain would produce that much water over the course of a 20 minute minute journey.

The cleanliness of the water and the fact it's on both sides leads me to think it's not the heater matrix O-rings leaking.

I've only noticed it when it rains, which it has been doing a lot over the last few days,

Are there any common/known culprits for a leak like this or do I need to start stripping out bits of dashboard and pointing the hose at the car?

Plenum filter cover seals (use silicon) and sunroof drains blocked or hoses detached or more rarely, split, are common culprits.

Plenum cover seals - do you mean the covers for the pollen filters or something related to the plenum foam in the centre of the windscreen?

Those, and blocked sunroof drains, sound reasonably easy to check with minimal dismantling over a weekend.

Yep- the covers for the filters. Bead of silicon all round and a blob on the screw heads.

Covers for the pollen filters but there is also a self tapper that holds the plastic just above the pollen filter housing, water can get it through that. Or, mas OB says, sunroof drains. Water from the AC drips down on the passenger side next to the centre console and of course the O rings are on the other side.

It may still be the A/C drains...

This was after a 45 minute drive or so when it was particularly humid last summer - there was enough condensate to run over the carpet, out the door seal, down the side step and then off the back of the step. Certainly going to be more humid when its been raining too.

There are AC drains on BOTH sides of the transmission tunnel. O-rings are just RHS of (UK Drivers).

There'a a whole host of places you can seal up with silicone around the pollen filter inlet, plenum etc - but it is another place where random water can get in from, and then get blown by the blower fan into the heater box area.

I doubt it's sunroof drains, as they go down inside the A pillars at the front, and exit through the bodywork behind the front wing. If they overflow, or leak at the top where it joins the sunroof cartridge, then you usually get moisture in the headlining around the front corners of the sunroof area first, or drips from the centre dome light.

I'd check the AC drains first.. if the AC is running, as it's been pretty wet/humid lately, the AC will take the moisture out of the air, so entirely possible it's generated that much in 20 mins, or is could be a combination of plenum leaks and AC drains

Sounds like a trip over one of the pits at work and a prod around with some pokey wire is the first step then.

Looking at the pics and thinking about it some more, the water seems to be dripping off both ends of the black plastic below the glovebox - so the leak could just be in one location (near the transmission tunnel?) and running to both ends.

Cheers chaps.

On mine the foam stuff on the plenum grille was gone and the ends of the plenum were full of dead leaves and compost. The drain holes were thoroughly blocked where they run into the wheel wells.

If you're getting water from both ends of the duct in the passenger side from the blower motor to the heater box, then it will most likely be coming in from the plenum, through the blower motor.

I have heard of AC runoff making it that far back from the heater box before, but it's more common (especially with the we weather we seem to have at the moment, that it will be rainwater getting in

Undo the two bonnetstays and lift it all the way up.
Take off the pollencovers, remove filters and topdeck between wipers, you get a clear sight of all the places where the pollenfilter housings can leak.
And with the vac cleaner you can clean the housing from 20 years of leafs and other muck.

To take off the plastic bit covering the plenum foam do I have to take off the black trim on the A-pillars?

I have a new plenum foam to put in and it sounds like an ideal time to fit that, while taking it apart in search of a leak.

No, just the wiper arms and I think I ended up taking out the plate with the wiper mechanism on it.

The clip-on trim along the bottom needed to come off though.

RutlandRover wrote:

To take off the plastic bit covering the plenum foam do I have to take off the black trim on the A-pillars?>
I have a new plenum foam to put in and it sounds like an ideal time to fit that, while taking it apart in search of a leak

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You won't find a much better guide than this one. Has pictures too :)
If you're going to remove the plenum covers you'll probably find the self tappers that hold it in are at the point of disintegration so have some new ones handy and seal them where they go through the metalwork to stop micro leaks.

And now it's doing it on the driver's side too:

I'm starting think it probably is related to the plenum intake. There's a bunch of screws actually missing from the plenum trim:

That might have a lot to do with it. Standard self tappers but a dollop of RTV or a rubber washer under the heads is a good idea.

Probably time to get those carpets out too :(

I'm not sure I'll have much opportunity to take the whole carpet out - I might have to just try my luck airing it out with the windows open if we ever get a long enough period of sunlight.

With that much water in there it'll take an age parked in a desert to dry it out with just the windows open!
A good session with a wet vac, pop the door sill trims so you can get some blocks of wood between carpet foam and floor and warm air blowing under there will speed things up.
Most important thing will be to get and keep everything dry in the vicinity of the BECM (which is surrounded by carpet). If water gets in there you're in for a whole box of electrical gremlins to escape and wreak havoc.

Orangebean wrote:

A good session with a wet vac, pop the door sill trims so you can get some blocks of wood between carpet foam and floor and warm air blowing under there will speed things up.

Put some fluid absorbing sheets under the carpet, press the carpet down and repeat.
I've had it with a drowned BMW, amazing how much water the carpet can hold, press it as it's a sponge and buckets come out of it.