Find pumping station valve was a mission in Jak II. After having joined the Underground Torn send them on a mission to restore water to the Slums, by turning a valve outside Haven City at the pumping station.
The first mission to introduce the Metal Heads, specifically the manic head enemy. Beat the couple at the beach and then head up the pumps and to the right, going around the cliff. You'll encounter some more enemies, including glubs and znorkle tooths. It's a fairly straightforward mission where you fight some enemies, cross a gap, cross a pipe (the ones you encounter can either make a 90 degree turn periodically or have a moving electrified device) while climbing slowly higher.
You'll generally avoid the low-ground area in this mission and you don't really encounter more than four enemies at once either. Eventually you'll reach a metal bridge, cross it and fight a few more Metal Heads to reach the valve (and end up at the beginning after the cutscene).