Mitsubishi Repair: P0505 - idle control system malfunction - Check engine, vacuum hoses, mitsubishi galant

Question
Hi,

Can you please help me find what could be a possible fix to my problem?

Error code was: P0505 for my Mitsubishi Galant-S 1996.

Symptom: Fluctuating RPM during the start up.

Also, I haven't done any tune up on this car since 2 yrs.


Please help!

Thanks.

Answer
Hardik,
There is a special component connected to your throttle body which regulates the idle speed. It's called the Idle Speed Controller (ISC).  It's responsible for maintaining a constant idle speed when your foot is off the gas pedal.  If the ISC is bad, it will bounce your idle around; a condition commonly referred to as "idle surge".  It's usually remedied by replacing the ISC.  A new ISC for your car costs between $230 - $290 plus labor cost to install.  So have it fully inspected first before replacing it, because it can be expensive.

The ISC gets it's commands from your car's computer; the ECU.  If the ECU is bad, or it's getting a bad signal from another component, it may be telling your ISC to idle improperly.  So it's a good idea to have your ECU inspected as well if no problems are found with your ISC.  Also have your mechanic check for loose or broken intake and vacuum hoses.  An air leak can most definitely cause a racing idle upon start up.
Good luck!