Q: Car engine revs way up immediately upon starting engine.

Common causes of high RPM excursion at start-up are typically: large vacuum leak, throttle plate stuck open, cruise control stepper motor stuck in one position; faulty throttle position sensor, and/or faulty idle air control motor or solenoid. If you request a diagnostic by a certified YourMechanic technician, he or she would first check to see if any Diagnostic Trouble Codes have been set. If no codes were set, the technician would go through the referenced possibilities until the root cause of the high idle is found. Once the idle excursion is resolved, the rough running condition would be evaluated. The causes of rough idle "slightly" overlap the problem areas cited above but there are lots of additional causes such as faulty fuel and ignition system components and, of course, mechanical problems such as uneven cylinder compression, burnt valves and so forth. If you desire to set up a diagnostic to resolve both the high idle and rough running condition, please schedule with YourMechanic.