Hi:
Usually when the something electrical is wrong, the check engine light will come on. From that point, mechanics use a scanner to retrieve a code from the car's computer that identifies where the problem is. If you had no light, then there wouldn't have been a code.
To answer you question, yes a coil pack can cause a bad idle. However, confirm with him that he actually checked the coil and knows for sure it is bad. He may be guessing that is the problem which leads to not having it fixed, and you still get stuck with the bill. Maybe ask him to guarantee it will fix the problem.
Let me know what you find or if you have other questions.
Joe
Saturday, August 1st, 2009 AT 2:15 PM