Just bought the 2009 RX 350
Looking under the hood, I saw a white plastic tank under the battery.
The tank didn't seem to have any liquid in it, but I could be wrong.
I looked to see if any hoses were connected to the tank but the view was
blocked by other things.

Does anyone know what I am talking about ?

Windshield washer fluid tank would be my best guess, without actually standing beside you having you point out the tank.

Windshield washer fluid tank is located on the passenger side.
The battery & the unknown tank are on the driver side.

Oh crap, you're right, I'm picturing my vehicle, which has the battery on the passenger side with the washer fluid tank right there.

Looking at page 455 of your owner's manual - it's hard for me to tell from the picture, but it's not the engine coolant reservoir is it?

Any chance you could pop off the engine cover and take a pic?

Here is the picture. The red arrow points to the tank in question. Air filter is top left. Radiator is on the right.


Click to view attachment

Here's the close-up picture. Red arrow points to tank.

Click to view attachment

Maybe the tea heater? Probably leads to a tap (faucet) in the cabin wherefrom hot water is delivered to tea drinkers? Ah! Lexus thinks fo everything! (Joke)

Coolant reservoir.

Like I said, take a look at page 455 of your owner's manual...or look at it in the link below if the link works. In the illustration in the manual, you can just make out that the coolant reservoir is under the air intake tube, which II can clearly make out in your pictures.

https://secure.drivers.lexus.com/lexusdrive...l&locale=en

The link above was for the 2010 RX 350 which showed the coolant reservoir is on the driver side, near to the battery.

Here the link for 2009 RX 350. Page 313 shows the coolant tank is on the passenger side, next to the engine oil dip stick.

https://secure.drivers.lexus.com/lexusdrive...l&locale=en

OK....I'm batting zero here...I thought you had a 2010...lemme check the 2009 manual....

Is that black plastic snorkel looking thing attached to the tank or is that some sort of black cap on the top of the tank?

Can you get a shot from below. I have OCD and this is really bugging me...hahaha

I verify the 2 black plastic snorkel things connect to the Air Filter housing, not to the mysterious tank.

There's no room the get my hand in and see if any hoses connect to the tank.
I knock on the tank to see if there's any liquid but it seems empty.
Who know if it should contain some liquid ?

This bugs me that I can't figure what the tank is for.

Does anyone have access to the repair manual for 2009 RX 350 (or 2008 RX 350) ?
Maybe you can figure out the purpose of the white tank below the battery.

PS: My 2009 RX 350 has Tow Prep package which has Heavy Duty Transmission Cooler, Heavy Duty Alternator and Radiator.
I don't know if the mysterious tank is related to the Tow Prep pkg.

I just looked at my 08 RX and it looks to me like this tank is connected to the bottom of the cold air intake coming from the front of the car, my guess is since it is at the lowest point of the air intake it has something to do with keeping any water or moisture from entering the electronics of the air intake system. This is just a guess but it is one thing I came up with. I thought it might also be meant to gather gas fumes that escape the intake when you turn off the engine.
Jeff

Looks like this forum is not used by any Lexus mechanics.....

You could always drop by the dealership on Monday...I won't be able to sleep at night till I know what this is...

According to another poster on the Club Lexus forum, the tank could be the intake resonator.
It's intended to reduce intake air noise.