My OBD shows that I have low fuel pressure in a 1997 Ford Mercury (tracer). Do I have to replace the fuel pump? If so, do I have to replace the whole thing or is it enough to replace just the motor unit? What else might cause low fuel pressure reading?
Generally speaking (not familiar with that model), it could be due to:
- Bad fuel pump (fairly rare, fuel pumps are one thing that usually last forever)
- Clogged fuel filter (has it been changed recently/ever?)
- Bad fuel pressure regulator (although, they usually fail the opposite way)
- Leaking fuel injectors (unlikely, would require a pump that's marginal to begin with)
The 97 Mercury Tracer doesn't use a Fuel Rail Pressure sensor and any reading you're getting from your OBD2 application is false.
If you believe that the fuel pressure is low, hook up a fuel pressure gauge and verify before replacing anything.
Refer to Brian Knoblauch's answer for potential causes.