The erratic rpm is commonly termed as "surging" and mostly due to a fault in the idling system, adjustment or vacumn leakages.
1. Check for disconnected vacumn hoses. Faulty PCV would allow uncontrolled amount of air into cylinders.
2. Check radiator coolant level.
3. Check for clogged cooling hoses to fast idle valve.
4. Check idle adjust screw.
Apr 1, 2009 at 1:28 AM