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The main reason for surging is excessive air getting past the throttle body and causes are :
1. Vacumn leakages. Check for disconnected vacumn hoses. Faulty PCV, leaking brake booster or hoses.
2. Idling speed adjusted too high. Check the idle adjust screw.
3. Throttle cable adjustment. Must have sufficient free travel.
4. Leaking manifold gaskets.
5. Idle Air Control valve. Stuck and not closing correctly. Cleaning the throttle body and IAC helps.
Is the Check Engine Light showing while driving?
Check the coolant hoses to IAC and level in radiator as well. Clogging of coolant hoses would cause the problem.
At operating temperature, the specs are as follows.
The idling rpm sgould be 750. At 1000 it would surge when it is just reaching warmed up temperature. Recheck the coolant level again.