My focus has a light vibration and the rpm goes slightly up and down but it only does it when its on idle or when iam in a stop light. When it does that, my panel lights kind of dim out as the rpm goes up and down. I already put new motorcraft sparkplugs and new wires but still the same. I just went to check the alternator, they test it with the machine and they told me it was overcharging, they told me i need to fix the regulator. do you think that will fixed the problem? thanks

1st your car should not idle below 110 rpm even in gear

idle problems on these are caused by both broken or open vac ports or a dirty throttle body the A/F ratio cant get a correct reading

run the fault codes as the engine idle is being affected by a faulty sensor . The vibrations are probably because the idle rpm's are too low.

Problem on focus too . Without A/C on it idle's smooth . With A/C on the PCM - engine computer should raise the idle rpm. 850 . The only way to raise the idle would be to have PCM flashed programmed with up dated software . Adjusting the idle anyother way would cause check engine light problem , Wouldn't be able to get it to go out. Your best bet ,take it to the ford dealer and have the PCM reflashed .

if the car is running does it have the check engine light on? if yes what is the code if no then check to see if temp sensor is reading the right temp by plugging in a scanner that can read all data to give you an accurate reading... if it reads wrong temp it can cause running issues

Possible Idle air control, simple to change on the throttle body

check the breather hose behind the manifold front right side of engine

Check and lube the Idle Air Control (IAC) valve,

This tiny electric motor is mount on top valve cover and close to the accelerator cable.

The function of IAC motor is to idle up the RPM when more load is added.

Example: A/C on,light on,stop light on and radio on.

A stuck IAC can be free by few soft tape on the body of the motor.

To lube the worm gear,you will need WD-40 to free up the old grease and lube white lithium grease.

Clean the electrical contacts with CRC electric part cleaner.

This electric motor is very expensive.

Please handle it with care.

are you sure it is a miss or a vibration? Focuses are famous for a vibration at idle because of the upper motor mounts. some get so bad that the whole dash vibrates at stop lights.

since this is a standard and you need to engage the clutch which basically shifts you into neutral to shift gears, this ALMOST sounds like the idle air control valve.

Sounds like the ISC (Idle Speed Control) which is adjustable.  The secondary idle needs adjustment to prevent the car from wanting to stall at low speeds.
Greg