Owner's Problem Description
The owner has a 1995 Subaru Legacy LS sedan with a 2.2L engine and automatic transmission. After sitting idle for about a year, the vehicle was driven for approximately 270 miles without issue. However, the next day, when the owner shifts the transmission from neutral or park into any gear, the engine kicks in gear and then shuts off. The owner is seeking assistance to diagnose and resolve the issue.
Professional Technical Analysis
The 1995 Subaru Legacy LS sedan is equipped with a 2.2L engine and an automatic transmission. Given the vehicle's history of sitting idle for approximately a year, it is crucial to consider the effects of this prolonged inactivity on various systems, particularly the transmission and fuel system. The described symptom of the engine shutting off when shifted from neutral or park into gear could indicate several potential issues. It is important to assess the vehicle's idle speed, which was reported to be at 750 RPM, within the normal range. However, if the idle speed drops significantly under load, this could lead to stalling. Environmental conditions, such as humidity, could affect the fuel system and ignition components after extended sitting. Additionally, checking the transmission fluid level is essential as low fluid can lead to poor engagement and stalling. The transmission may also be experiencing issues related to the torque converter, which can become stuck after prolonged inactivity. This could cause the engine to stall when put into gear. A thorough diagnostic approach using the GeekOBD APP for OBD-II scanning can help retrieve any fault codes and monitor live data to identify if the engine management system is functioning correctly. Furthermore, it is vital to conduct a visual inspection of all connections and components related to the transmission and engine management systems to ensure they are in good working condition.
Possible Causes
Most common causes (ordered by frequency):
- Torque Converter Lock-Up Failure: The torque converter may become stuck due to prolonged inactivity, causing the engine to stall when shifted into gear. This situation is often exacerbated when the vehicle has been sitting unused for an extended period, as internal components may stick or seize. Testing the torque converter operation during gear changes can reveal if this is the cause. The torque converter operates by allowing the engine to remain at idle while providing enough power to engage the vehicle without stalling. A stuck torque converter will not disengage properly, leading to engine shutdown. If this is suspected, a test can be performed by applying a breaker bar to the crankshaft pulley to attempt to free the converter. - Engine shuts off upon shifting into gear, possible unusual noises from transmission area, inability to engage properly in drive or reverse.
- Transmission Fluid Issues: Prolonged inactivity can lead to sediment buildup in the transmission fluid, which can affect fluid flow and pressure. If the transmission fluid is low or contaminated, it can lead to improper shifting and stalling. The recommended fluid level should be checked with the engine running and the vehicle on a level surface. If the fluid appears dark or has a burnt smell, it should be replaced. The correct type of fluid for this model is typically Subaru ATF, ensuring proper lubrication and operation. - Transmission fluid level checks showing low or contaminated fluid, potential slipping during gear engagement.
- Idle Air Control Valve (IAC) Malfunction: The IAC valve regulates the engine's idle speed by controlling the amount of air entering the intake manifold. A malfunctioning IAC can cause irregular idle speeds, especially when the engine is under load. If the IAC is stuck or failing, it may not allow sufficient air into the engine when shifted into gear, causing a stall. Testing the IAC operation by monitoring the engine RPM during gear shifts can help diagnose this issue. The IAC should operate smoothly with no sticking, maintaining idle around 750 RPM when in park or neutral. - Erratic idle when shifted into gear, possible IAC codes stored in the ECU, engine stalling under load.
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 Diagnosis: Use a professional OBD-II scanner or GeekOBD APP to conduct a thorough scan of the vehicle's computer systems. Retrieve any stored and pending fault codes which may indicate issues related to the engine management or transmission systems. Pay attention to any fault codes related to the torque converter, transmission fluid pressure, or idle air control valve. This step is crucial for pinpointing potential issues before proceeding with further diagnostics.
- STEP 2 - Transmission Fluid Check: With the engine running, check the transmission fluid level using the dipstick to ensure it is within the proper range. If the fluid is low, top it up with the recommended Subaru ATF. Inspect the fluid for color and odor; if it appears dark or smells burnt, it should be replaced. This step is vital as low or contaminated fluid can lead to poor shifting and stalling under load.
- STEP 3 - Idle Air Control Valve Testing: Monitor the IAC valve operation by observing the engine RPM when shifting into gear. If the idle drops below 500 RPM, the IAC may be malfunctioning. Use the GeekOBD APP to check for any stored trouble codes related to the IAC. If necessary, test the IAC for proper operation by applying a 12V power supply to see if it moves freely.
- STEP 4 - Torque Converter Functionality Test: Attempt to manually rotate the crankshaft pulley with a breaker bar to see if the torque converter is stuck. If it is difficult to turn, this may indicate a problem with the torque converter. If the converter does not disengage properly, it will cause stalling when shifting into drive or reverse. Testing the torque converter's function is critical for confirming if this is the root cause of the stalling issue.
