My 03 Montero XLS is been leaking coolant for couple of weeks. Yesterday I finally saw clearly where it's coming from, but can't figure out what causes it (see picture). Could it be from the water pump?

 

a leak at the water pump or at the thermostat is most likely in this case. Since thermostat housing is pretty visible you should be able to tell if that is responsible. If not, its probably a water pump.

 

It looks like it's the water pipe assembly (12 on the attached drawing). Is Mitsubishi dealer the only place where I can get one?

 

Is your pipe cracked or just a bad o-ring(s)? You can get either one at the dealer or order it online at various Mitsubishi parts dealers. You can try Mitsubishi Parts » Online Catalog or Mitsubishi Auto Parts, Up to 70 % off | Discount OEM Factory Direct Wholesale Mitsubishi Parts Warehouse
It is not likely that you'll find these parts in your local parts store.

 

If the leak is where you say it is, that is a pain in the butt. Assuming your 2003 XLS is the same as my 2002 3.5 XLS Montero Sport, you will have to remove the plenum, the intake manifold (leave fuel injection rail with injectors bolted to the manifold,) to get access to the water pipe.

The water pipe has an o-ring that slips into the back of the water pump. The pipe at the rear has tabs that are bolted down.

Post 80 of this thread, I show the water pump and o-ring, I had a new water pump and it came with the o-ring. https://mitsubishiforum.com/forum/mi...l-48920/page8/

Post 82 of the same thread you can see the water pipe.

Post 83 of the same thread you can see where the water pipe tabs bolt to the drivers head and passengers cam thrust plate.

 

Well, I took the car to one of my friends who's a mechanic and he figured out that somewhere around that water pipe assy is leaking, so he told me to get the pipe and the O-ring.

 

Tough call.

If it's just the o-ring, then you could remove the water pump from the front, but that requires removal of the accessory drive and you are well on your way to doing a timing belt service. But, the plenum and intake is not removed. Then if you find the pipe is damaged or corroded, well then off comes the plenum and intake to gain access to the pipe.