Owner's Problem Description
The owner reports that their 1987 Chrysler New Yorker stalls frequently when coming to a stop in drive, occurring 9 out of 10 times. However, if the owner shifts to neutral while slowing down, the engine does not stall. They are unsure where to begin diagnostics and seek assistance with this issue.
Professional Technical Analysis
The 1987 Chrysler New Yorker is equipped with a 3.0L V6 engine and an automatic transmission, with an estimated mileage of around 150,000. The reported issue of stalling during braking while in drive suggests potential problems related to the engine management system or transmission performance. The owner notes that the engine stalls most often when stopping at red lights, but shifting to neutral allows the vehicle to maintain operation without stalling. This behavior could indicate a failing Idle Air Control (IAC) valve or Throttle Position Sensor (TPS) issue. Both components are critical for maintaining proper idle speed and engine response during deceleration. Additionally, the possibility of a transmission issue should not be overlooked; if the transmission fails to downshift correctly, it could lead to stalling as the engine load increases during braking. The vehicle's idle should be monitored closely, especially during the transition from acceleration to deceleration. A thorough inspection of fuel pressure levels is essential, as low fuel pressure can contribute to stalling conditions. As the owner has mentioned recent maintenance, including a tune-up and fuel system cleaning, it's important to consider the potential for residual effects from those procedures. The current vehicle condition appears stable with no warning lights illuminated, but the intermittent stalling raises concerns regarding safety and reliability, especially in traffic. A comprehensive diagnostic approach is warranted to identify the root cause of this issue.
Possible Causes
Most common causes (ordered by frequency):
- Throttle Position Sensor (TPS) malfunction: A faulty TPS can provide incorrect input to the Engine Control Module (ECM), causing improper fuel trim adjustments and leading to stalling during deceleration. The TPS should show a voltage change from 0.5V at closed throttle to approximately 4.5V at wide open throttle. If the TPS is not functioning properly, it may not correctly compensate for engine load during braking. - Intermittent stalling symptoms, voltage readings outside specification when tested, possible issues with throttle response during deceleration.
- Idle Air Control (IAC) valve failure: The IAC valve regulates the engine's idle speed. If it is stuck or malfunctioning, it may not allow adequate air flow during braking, causing the engine to stall. IAC should maintain a steady idle speed around 600-800 RPM. If idle speed drops below this range when coming to a stop, it indicates potential IAC failure. - Erratic idle during stopping, IAC operation should be verified by monitoring idle speed with a scan tool.
- Transmission downshift delay: If the transmission does not downshift quickly enough during braking, it can lead to stalling as the engine is unable to maintain the necessary RPM. Ensuring the transmission fluid is at the correct level and that the transmission has been recently serviced is critical. Transmission fluid should be checked for proper level and condition, and the transmission control module may need to be scanned for any fault codes. - Symptoms of stalling when stopping, possible delay in downshifting noted during test drives.
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: Use a professional OBD-II scanner or the GeekOBD APP to perform a thorough scan of the vehicle's engine control system. This will help retrieve any stored trouble codes and freeze frame data that can provide insight into the conditions leading up to the stalling issue. Even without a check engine light, this step is crucial in identifying any hidden issues.
- STEP 2 - Live Data Monitoring: Utilize the GeekOBD APP to monitor live data, focusing on TPS voltage readings and IAC valve operation. The TPS voltage should vary smoothly between 0.5V and 4.5V with throttle position changes. Idle speeds should be monitored to ensure that they remain stable around 600-800 RPM during deceleration and stopping.
- STEP 3 - Component Testing: Test the TPS and IAC valve for proper operation. Check TPS resistance values (should be within 1k-5k ohms) and IAC valve response to changes in the idle speed. Additionally, check transmission fluid level and condition, as well as any diagnostic codes from the transmission control module.
- STEP 4 - Road Test with Observation: Conduct a controlled road test while monitoring live data through the GeekOBD APP. Document any stalling occurrences and correlate with throttle and braking patterns. Pay attention to how the vehicle behaves when shifting between drive and neutral during stops.
