i know this is a ranger forum but im just asking everywehre.. my friend has a 2001 ford focus it idles fine at the begining but after awhile the idle starts going to 300 rpm if he shuts of the car n restarts it it works fine for awhile then does the same thing ??

 

intake temperature sensor, idle air control valve.

 

Don't just throw parts at it.

Clean the IAC, see if that helps. One way to check it is to remove its electrical connector while it is idling. Listen for an idle change. No change=bad IAC.

IAT is also testable, but I don't know its ohm range. Simply remove it, attach an ohmmeter and heat it with a hair dryer. If its ohms vary with temp change, it's probably good. That year Focus, IAT MAY be integrated into the MAF, should you be unable to locate it on the intake tube.

You might also clean the MAF, check/clean/gap or replace the spark plugs.

 

is the check engine light on?

 

The MAFS get dirty easily on the focus if you are using an oiled filter like K&N. The oil collects on the MAFS wire.

Might even be as simple as a fuel filter. (pass side just in front of the fuel tank)

 

focusfanatics and focaljet have tons of people/mechanics who have had the same problem