1999 Jeep Grand Cherokee P0505 Code: High Idle After Freeway Stops (No Vacuum Leak Found)
Hi everyone, I'm running into a persistent issue with my 1999 Jeep Grand Cherokee 4.7L V8, stock setup, with approximately 173,000 miles. I've recently picked up the P0505 diagnostic code (Idle Air Control System Malfunction) using a basic OBD-II code reader — no scanner available. The symptom is not a complete engine stall or misfire. Instead, when coming off the freeway onto an off-ramp, especially after driving for about 10 miles, the engine idles at an abnormally high rate — reaching up to 2,000 RPM — and remains stuck in this 'fast idle' state. This behavior is intermittent: it doesn't occur when cold or during normal city driving. It only happens consistently after highway travel. I've already replaced the AIC (Idle Air Control) module, TPS (Throttle Position Sensor), and cleaned the throttle body. I also recently replaced both camshaft and crankshaft position sensors due to a different code — which resolved that issue temporarily. Each time I drive the vehicle to operating temperature, clear the P0505 code, but it returns within minutes or after just a few miles. I've tested for vacuum leaks using Brakleen and butane torch — probing every accessible area on top of the engine. No leaks were detected. The engine head appears remarkably clean at this mileage. I have not yet performed a full wiring harness inspection, nor have I tried unplugging the IAC module to see if the code persists. The vehicle is only available in the evenings for about an hour, so I plan to conduct further testing tonight. Given that all known symptoms point to a throttle or idle control issue, but no vacuum leaks are found and the AIC has been replaced, what other components should I inspect? I've reviewed multiple threads in the Jeep/Dodge section (13 pages), but none match my specific scenario. Thanks for any insights — Scott aka Unclepsycho
First thought: check the throttle linkage thoroughly — could it be binding or sticking? Second possibility: vacuum leak, especially in the EVAP system. Since this is active during hot engine operation, a partial vacuum loss might disable certain systems (EVAP, PCV), leading to erratic idle behavior. From the service manual: P0505 occurs when the PCM detects a short to ground or battery voltage on any of the four IAC driver circuits for 100 milliseconds while the IAC motor is active. So perform a wiggle test on all wiring harnesses connected to the IAC — gently tug and observe if the code returns. Any other symptoms? Horn, cruise control, speedometer, tachometer? Do you have access to a scanner with freeze frame data? That could provide more context.