That pipe runs from the air pump (mounted on the left side along with the alternator), across to the diverter valve on the right side (mounted just left of the AC compressor). I replaced the water pump a few years back. Its a chore, but its just nuts and bolts, I think I was about 3 hours teardown and 3 hours reassemble. I recall having to grind down a socket to torque the right side bolts as space is limited. Also be careful on removal of the serpentine belt. A 1/2 drive ratchet will fit into the idler to take off tension, but when you release the idler after belt removal, it springs down and may jam your ratchet at the frame crossmember. Good luck.