2003 Ford Focus

Could be all these things, but it could be many other things as well.
Is your 'check engine' light on?
I would start by checking for DTCs.
You could have a misfire going on (in which case further diagnostic will be necessary).

Possible causes include:

- Dirty or leaky fuel injectors
- Bad MAP (manifold absolute pressure) sensor (I'm not sure if Focus have this sensor though)
- Bad TPS (throttle position) sensor
- Bad or dirty MAF (mass airflow) sensor
- Low fuel pressure
- Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)

and in some instances. Bad fuel!

There is a few easy things that you can do to help diagnose and maybe fix your problem:

- Do a fuel pressure test. This will tell you if your problem comes from a lack of pressure (due to a bad pump, filter, damage fuel line, regulator etc).

If pressure is too low:
- your pump output could be too low (weak pump, low voltage to the pump)
- or your filter is clogged (when was the last time you replaced it? I think Ford recommends replacing the fuel filter on focus every 30,000- 35,000 miles). Although it's not a very accurate test, you can try to remove it and blow thru it to check.
- or your fuel pressure regulator is messed up (disconnect the vacuum line from the regulator and check for gas in it; if there's gas in it, the diaphragm in the regulator is messed up and you need to replace the regulator).
- you could also have a leak somewhere including leaky injectors.
- it could be your inlet filter sock that's clogged. (If you replace your fuel pump, make sure you replace the sock as well)

Other things you can do:

-Buy some electronic cleaner, remove the air duct from the throttle body and clean your MAF.
- Put some fuel system cleaner in your tank to clean fuel injector and fuel system (put it in a full tank).

- check for a vacuum leak (intake manifold, and vacuum hoses, throttle body, EGR valve etc)

Good luck!