I have a 2003 Ford Focus and it started sputtering a month ago. I've put gas treatment in and if I keep a full tank it's fine, but once it gets around half a tank it sputters. Now it sputters the whole time you're driving at low speeds and has started jerking along with the constant sputter. Any idea why?
It sounds like the fuel supply is being interrupted. This usually means that either the filter is blocked, one of the lines is blocked, or the breather is blocked, or as zhang says, air is getting into the system
Try taking the fuel cap off while it is spluttering - if it suddenly clears up, it's the breather (put simply, if the breather is blocked, air can't get in to the tank to replace the fuel, so the fuel can't flow out).
If that doesn't work, try replacing the fuel filter(s). I'm not an expert of Fords, but it should be a relatively simple procedure - the Haynes manual should detail it. Blocked lines is unusual on modern cars, but you do still have rubber lines in the system that can get blocked, perish or develop pinholes - if the first two tricks don't work, you may find they need replacing...
A sputtering (misfiring) engine indicates insufficient fuel/too much air
In the situation described, this sounds like an issue pertaining to low fuel pressure since the car doesn't suffer as much when the fuel tank is full (higher in-tank fuel pressure).
The following culprits spring to mind:
a clogged fuel filter
If enough gunk accumulates in the fuel filter the pressure at the fuel rail can get low enough to cause engine misfires. On most fuel delivery systems this should be replaceable standalone though filters that are integrated into the fuel pump are also commonplace.
a failing fuel pump
As fuel pumps age their ability to pressurize fuel diminishes because the brushes in the electric motor wear out. Replacement/reconditioning is the only real option in such circumstances.
a bad fuel pressure regulator
Not likely in this situation. I'd expect issues across the board if this went bad and not just with a part-filled fuel tank.
My recommended course of action
- confirm that the fuel rail pressure is lower than expected
- if low, go for the low cost fix first by replacing the fuel filter if it hasn't been done already
- if that doesn't solve it, it is likely time to invest in a new fuel pump
Intermittent misfires could also be down to bad fuel (specifically water in the fuel). You may want to confirm that fuel quality is not to blame here by emptying the fuel tank and visually inspecting it.