First, is your "check engine" light coming on? If so, then that's the first place to check. AutoZone will scan your computer for free, as well as give you the definitions to the codes.
Also, I doubt that you have water in your gas. If this was the case, your car wouldn't smooth out as you increase speed. It would get worse.
If your check engine light IS NOT illuminated, then I'd suspect you have a leaking fuel pressure regulator. These hardly ever set a computer code, but always give drivability problems. (For example, if it were the E.G.R. Valve, the computer would see this, as well as an ignition misfire/lean-fuel misfire)
What I believe is happening:
The diaphram inside your FPR is leaking fuel into your intake manifold. Since your engine can't account for this extra fuel, it'll run rich. (Too much fuel)
While driving, the computer can usually compensate for this, since there's a lot of air pumping through your cylinders at high r.P.M.S. But when your engine is idling, the cylinders can't burn this extra fuel, and sometimes that fuel will foul a spark plug. (Prevent it from firing) But as soon as you hit the gas pedal, the computer can now compensate for this again, and the engine smooths out.
Now keep in mind, there are other possible causes to your drivability issue. Your computer will be the key to solving it. If the check engine light is on, then it's a system the computer can "see", and it needs to be scanned. If the check engine light is not coming on, then it's something in the system the computer can't see.
If your FPR turns out not to be the cause, heres a list of other things to check.
Spark plugs/wires, ignition coils, fuel pump/filter, injectors, vacuum lines (for a leak).
Sunday, February 14th, 2010 AT 7:23 AM