2004 Jeep Grand Cherokee Laredo 4.7L P0121 Code: RPM Surging at 60+ MPH and Unstable Idle
My 2004 Jeep Grand Cherokee Laredo with a 4.7L engine is experiencing consistent RPM surges when cruising at speeds above 60 mph. The issue began while using cruise control, so I've since disabled it entirely. However, the problem persists even on flat roads with steady pedal input at constant speeds over 60 mph. The vehicle now displays a P0121 diagnostic code — indicating that the throttle position sensor (TPS) voltage does not match the manifold absolute pressure (MAP) sensor readings. I recently performed several maintenance steps to address potential airflow and sensor issues: I removed and cleaned the Air Intake Control (AIC) unit, including its mounting hole using CRC Mass Air Flow Sensor Cleaner. I also cleaned the throttle body in place while the engine was idling, following instructions from http://www.wjjeeps.com/service/tbody.htm. Additionally, I inspected and reinstalled the Throttle Position Sensor (TPS), which appeared clean and free of buildup. After these actions, the RPM surging temporarily improved. However, during a 120-mile drive, the issue returned — with engine RPMs fluctuating between 1900 and 2600 every 15 to 20 seconds while cruising. The idle in Park is now unstable, swinging from 1000 to 1600 RPM instead of its normal range of 700–800 RPM. On one occasion, I floor-pressed the accelerator and revved the engine to around 5000–5500 RPM; after that, surging stopped for approximately ten minutes. I am seeking advice on how to resolve this persistent RPM surge and idle instability. Should I replace the TPS or AIC? Is there a known issue with the MAP sensor in this model year? Please note: I have ruled out transmission-related causes. Any technical insights, especially regarding diagnostic procedures using an OBD2 scanner or digital multimeter (DVOM), would be greatly appreciated. CAHOU812
P0121 - Throttle Position Sensor (TPS) voltage does not match MAP sensor readings. This is a known issue in the 2004 Jeep Grand Cherokee Laredo 4.7L. The root cause likely lies in the correlation between TPS and MAP signals. Since there are no other diagnostic codes, and the vehicle is stock, it's important to verify both sensors using an OBD2 scanner or a digital multimeter (DVOM). Key checks include: - Confirming proper connection of the MAP sensor (located on top of the intake manifold) — ensure connector is tight, clean, free from corrosion, and without leaks. - Testing for high resistance in the MAP 5V power circuit. - Checking for short circuits between the MAP 5V supply and ground. - Evaluating the MAP sensor itself for internal faults (e.g., high resistance in signal or ground circuits). - Assessing TPS voltage supply and signal integrity — including possible high resistance in the TPS signal circuit, grounding issues, or high resistance to ground. A common diagnostic SWAG (suspicion) is a failing MAP sensor. However, both the TPS and PCM could also be contributing factors. A thorough test of both sensors under load conditions is recommended before replacement.