Q: I have a 1998 Honda Accord ex. What will cause it to idle up and down

This problem of erratic idle speed is often referred to as "surging" or "hunting". The causes include vacuum leaks and EGR system faults, a faulty idle air control (IAC) valve or circuit, faults in the fuel pump control circuit, defective spark plug(s) or ignition system components, faulty fuel injector(s) or injector circuit faults, PCV system faults or a faulty ECM (not common). Often, the application of a real time automotive scanner can find the fault as actual engine operating parameters can be viewed during the "surging". I would recommend that you request a rough operation/rough idle diagnostic and the responding certified mechanic will perform the required diagnostic steps to get this resolved. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.