1999 Mitsubishi Eclipse Rough Idle

BILL57

MEMBER
1999 MITSUBISHI ECLIPSE
2.0L
4 CYL
FWD
AUTOMATIC
245,000 MILES

Engine idles rough and a lot of times will kill if I don't keep RPM's up by holding the brakes and pressing on the accelerator to keep RPM's around 800. RPM's fluctuate from1200 down as low as 400, back and fourth until it kills. I have used a code analyzer and it shows P0505 which is Idle air control system malfunction. I have replaced the idle air control module and the absolute air pressure sensor but it still does the same thing. I was told there may be something else that needs to be replaced that makes up the whole idle air control system. Is there anything else I should be looking at?

1 Reply

JACOBANDNICKOLAS

MECHANIC
108,164 POSTS

There really isn't much else. Have you checked for engine vacuum leaks? Also, is the code still there? If it is, I believe you either have an open circuit or short in the wiring to the IAC. Check the wiring for continuity. Also, make sure there is power to the IAC. Disconnect the connector to the IAC and check for power to terminal pin #2. Is there power? Also, check for power between ground and terminal pin #5. If no battery voltage exists, check wiring between MFI relay connector and IAC.

If there is no power to the items listed above, you need to remove the 26 pin connector at the ECM. Once disconnected, turn the key on. Check for power at ECM terminals 4, 5, 17, 18. If there is battery voltage there, check for ECM connector issues. If no power at these pins, the ECM is bad.

Let me know if this helps or causes more confusion. Also, with that many miles on the vehicle, you may want to check compression. If you have a couple weak cylinders, it can cause this issue.