'91 Ranger 4.0L Cold start problem

My Ranger will only come off base idle for about 5-10 seconds and then idle at 900- 1000rpm no matter how cold or warm it is. Any ideas what is at fault? My mileage has also become noticably worse the last few weeks .

My Ranger will only come off base idle for about 5-10 seconds and then idle at 900- 1000rpm no matter how cold or warm it is. Any ideas what is at fault? My mileage has also become noticably worse the last few weeks .

Not sure about the idle problem.

As for the cold starting:
-Change fuel filter
-Change those sparking plugs (do not use platinum) set the plug gap shown on the engine compartment, it may differ from tech manuals.
-Check air filter
-Use premium gasoline and fuel enjector cleaner every other tank