Owner's Problem Description
The owner has a 1993 Plymouth Laser with a 1.8L engine and a 5-speed manual transmission. After replacing the transmission due to a hole causing loss of 5th gear, the vehicle now lurches in 1st and 2nd gears when cold. The issue resolves after driving for 5-10 miles, with no other problems noted. The owner used the correct fluid type and viscosity as per the manual, leading to confusion about the cause of the lurching.
Professional Technical Analysis
This 1993 Plymouth Laser is equipped with a 1.8L 4-cylinder engine and a 5-speed manual transmission, likely having 120,000+ miles on it. The intermittent lurching in 1st and 2nd gears when cold suggests possible issues related to the transmission fluid or mechanical components. Since the transmission was replaced, this can lead to several interconnected systems at play. The new transmission may have a different internal condition, and the fluid type/viscosity could impact the operation. The use of 80W90 gear oil instead of the recommended ATF (Automatic Transmission Fluid) can lead to poor lubrication at lower temperatures, contributing to lurching due to inadequate fluid flow characteristics. Additionally, the engine's performance may be affected by other components such as the distributor cap or ignition system. If the engine timing is off or there's an issue with the ignition spark delivery, particularly when cold, it can cause power loss and lurching. It is critical to assess the fluid's temperature behavior and ensure proper ignition timing and fuel delivery under cold start conditions.
Possible Causes
Most common causes (ordered by frequency):
- Incorrect Transmission Fluid Type: Using 80W90 gear oil instead of the recommended ATF could lead to inadequate lubrication and poor performance, especially when cold. The viscosity may not provide the necessary flow characteristics that a manual transmission requires during low-temperature operation, causing lurching in gears until the fluid warms up. - Lurching in 1st and 2nd gears only when cold, correct fluid type not being used.
- Mechanical Issues in Replacement Transmission: The replacement transmission may have internal wear or issues that only manifest under certain conditions, such as cold temperatures. This could include sticking synchronizers, worn clutches, or other mechanical deficiencies that affect power delivery. - Lurching occurs only when cold, suggesting potential mechanical factors in the new transmission.
- Ignition Timing or Fuel Delivery Issues: If the ignition system, such as the distributor cap, is malfunctioning, it could lead to inconsistent ignition timing, particularly when cold. This would cause poor acceleration and lurching until the engine warms up sufficiently. - Power loss in 1st gear, intermittent engine performance issues when cold.
- Fluid Temperature Characteristics: The transmission fluid may not be performing optimally at low temperatures. A viscosity that is too high can lead to poor fluid flow through the transmission, resulting in lurching until the fluid reaches its optimal operating temperature. - Lurching resolves after 5-10 miles of driving, indicating temperature-related fluid behavior.
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 - Fluid Type Verification: Begin by confirming the type of transmission fluid used. Check the service manual for the manufacturer’s specifications. If 80W90 gear oil is confirmed, consider replacing it with the correct ATF recommended for this vehicle, as improper fluid can significantly affect transmission performance.
- STEP 2 - Transmission Inspection: Inspect the new transmission for mechanical issues. Check for proper installation, any signs of leaks, and unusual noises when the vehicle is cold. Look for signs of sticking synchronizers or worn clutches that could explain the lurching.
- STEP 3 - Ignition System Check: Inspect the ignition system components, including the distributor cap, to ensure they are in good condition. Test for proper spark delivery and verify ignition timing with a timing light if necessary. Address any issues found during this inspection.
- STEP 4 - Road Test and Data Logging: Conduct a controlled road test after making any necessary adjustments. Monitor the vehicle's performance during the drive, paying close attention to any changes in lurching behavior as the transmission warms up. Use live data monitoring if available to assess transmission fluid temperature.
