Owner's Problem Description
The owner reports that the 1995 Saturn SL1 revs to 2500 RPM upon starting, then gradually slows down. The vehicle can only be started with the gas pedal pressed to the floor, and it idles high while driving, causing the car to push forward at stoplights. The owner doubts the transmission is the issue as the car accelerates smoothly. They suspect it could be an idle air control valve (IACV) or a throttle issue, alongside mentioning a rougher idle after replacing the PCV valve.
Professional Technical Analysis
The issue with this 1995 Saturn SL1, equipped with a 1.9L SOHC engine (approximately 150,000 miles), is characterized by high idle RPM upon starting and a tendency to push forward at stoplights due to elevated idle. The owner's description indicates potential concerns with the idle air control valve (IACV) and throttle body. Given that the vehicle requires the gas pedal to be pressed to the floor for starting, this suggests a possible malfunction in the IACV, which regulates air intake during idle conditions. A faulty IACV could lead to excessive air entering the engine, resulting in a high idle situation. The engagement of the gas pedal during starting could be compensating for inadequate air control or fuel delivery issues, including a potential failure of the coolant temperature sensor affecting fuel mixture. Additionally, the recent replacement of the PCV valve might have introduced a vacuum leak or improper installation, contributing to the rough idle. The IACV is often located on the throttle body and is critical for maintaining the correct idle speed. Diagnostic steps should include checking the IACV operation by measuring the resistance (typically 10-14 ohms) and ensuring proper voltage supply during key-on engine-off conditions. The coolant temperature sensor should also be tested for proper operation, as it influences fuel calculations when the engine is cold. Given the vehicle's age and typical wear patterns, the throttle body may also require cleaning to ensure smooth operation. The current condition raises safety concerns due to the vehicle's tendency to accelerate unexpectedly, necessitating immediate diagnostic attention.
Possible Causes
Most common causes (ordered by frequency):
- Idle Air Control Valve (IACV) Malfunction: The IACV may be stuck open or faulty, allowing excessive air into the intake manifold, leading to high idle RPM. Typical resistance for IACV is 10-14 ohms. If the IACV is unable to effectively control airflow, it could cause erratic idle patterns and starting difficulties, necessitating further testing and potential replacement. - High idle speed upon start, difficulty starting without pressing accelerator, potential stored codes related to IACV malfunction.
- Throttle Body Issues: The throttle body may be dirty or malfunctioning, leading to improper air-fuel mixture and high idle conditions. Cleaning the throttle body can help restore proper operation. If the throttle position sensor (TPS) is faulty, it can also contribute to erratic idling and starting problems. Typical TPS voltage should be around 0.5-1.0V at idle and 4.5V at wide open throttle. - Rough idle after PCV replacement, potential throttle body contamination affecting air flow.
- Coolant Temperature Sensor (CTS) Failure: A faulty CTS can send incorrect temperature readings to the engine control module (ECM), causing improper fuel adjustments during cold starts, which can lead to high idle. Testing the CTS for resistance (should be around 5,000 ohms when cold) can help identify if it is functioning correctly. - Starting difficulties and high idle when cold, potential stored codes related to CTS malfunction.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Scan: Utilize the GeekOBD APP or a professional OBD-II scanner to retrieve any stored diagnostic trouble codes (DTCs) related to the IACV, throttle position sensor, or coolant temperature sensor. Pay attention to freeze frame data that may indicate conditions leading to the high RPM issue.
- STEP 2 - IACV Testing: Measure the resistance of the IACV using a multimeter. Confirm it falls within the normal range (10-14 ohms). Check the voltage supply to the IACV during key-on engine-off condition, which should be around 12V. If values are out of range, replacement may be necessary.
- STEP 3 - Throttle Body Inspection: Inspect the throttle body for carbon buildup and clean if necessary. Check the throttle position sensor voltage at idle and wide open throttle to ensure proper operation. This step is crucial as a dirty throttle body can significantly affect idle performance.
- STEP 4 - CTS Testing: Test the coolant temperature sensor for resistance and ensure it provides accurate readings when the engine is cold. If the readings are incorrect, replace the sensor. Monitoring the engine parameters with the GeekOBD APP can also provide insights into the engine's performance during cold starts.
