From what I've read on this forum, a CP3 failing is extremely rare. More often the reason for low rail pressure is one of these:
- air in fuel (almost certainly not the issue if you have 15psi from your lift pump)
- bad fuel quantity solenoid (also called MPROP, located on the back of the CP3)
- bad injector
- bad fuel pressure solenoid (located on the back of the fuel rail, up against the firewall)
If the fuel quantity solenoid has failed, it will be sending the majority of the fuel though the return line before it even reaches the high pressure part of the CP3. To test, disconnect the fuel return line from the back of the CP3 and see if an excessive amount of fuel comes out.
If an injector has failed, it can be sending an excessive amount of fuel back through its return line, preventing the rail from building up pressure. To test, remove the return line from the top of all four injectors (don't lose the metal clips!) and connect a vertical piece of clear tubing to each. Crank the engine for 15-30 seconds and compare the fuel levels in each tube. If one has substantially more fuel than the others, that injector likely has a problem.
If the fuel pressure solenoid has failed, it can be sending an excessive amount of fuel back through its return line, preventing the rail from building up pressure. To test, disconnect the return line and crank the engine. If an excessive amount of fuel comes out during cranking, it probably isn't functioning properly.
CP3s aren't cheap and I definitely suggest doing some diagnostic work before rebuilding or replacing it.
Some other threads that may be helpful:
Jeep won't start, being told it needs a new fuel pumpcrank, but no start....HELP