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I would suspect a vacuum leak to be causing the rough idle. When you turn the A/C on the idle control solenoid raises the RPM's in order to compensate for the added load to the engine. Take a can of WD-40 or Carb cleaner and spray it around the intake manifold and any accessible vacuum lines, when the spray comes across a vacuum leak the engine idle will smooth out and usually increase in speed. Once this is done replace or reconnect whatever the fault may have been.
There is a TSB for the PCV hose. The elbows at both ends get holes in the underside that you can't see unless you remove them and look. This is a very, very common issue on the ZX2. You can get a new PCV hose with both elbows for about $30 from the Ford dealership. You only need to remove the air intake to gain access to the PCV hose to replace it.
This is only a guess, because rough idle can be due to an infinity of reasons. On the Focus, assuming that the spark plugs and wires are all good (check that the wires are seated securely), then very often this is due to a problem with the vacuum lines or the Idle Air Control (IAC) valve. With the engine running, listen closely for any tell-tale hissing sound and try to locate it. Check that the vacuum hoses are all in good condition and that they are securely in place. The IAC is a little more complicated to check. Its job is to let air in when the throttle is closed. If it is not opening as it should then the engine will idle rough. One thing you can try is to remove the IAC entirely, leaving an opening for air to get into the intake manifold. If the engine idles better with the IAC removed, you will know that the IAC has a problem. It could be a problem with the IAC itself or with its circuit. Good luck, if you have any more questions let me know.
If it ran out of gas you may have sediment in the fuel lines causing rough running which may trigger a transmission code due to improper vacuum due to the rough running.
Check the exhaust manifold gasket. I have a 2003 SVT Focus I had a problem with it loping at low RPMs. I had a free flow exhaust system. When I bought a new exhaust manifold I replaced the gasket and the problem went away.
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