Owner's Problem Description
The owner reports that the 1990 Jeep Laredo drives smoothly when accelerating gently. However, when they apply more throttle, the rear end jerks violently back and forth, which stops when they ease off the gas pedal. There are no warning lights present, and the vehicle is generally in fair condition. The owner is seeking advice on potential causes and solutions for this jerking issue.
Professional Technical Analysis
The 1990 Jeep Laredo is equipped with a 4.0L inline-six engine and an automatic transmission, with an estimated mileage of around 150,000 miles. The reported jerking during acceleration suggests possible issues with the throttle position sensor (TPS), ignition system, or drivetrain components. When the throttle is opened abruptly, the PCM (Powertrain Control Module) relies on the TPS to regulate fuel delivery and engine response. A malfunctioning TPS could send incorrect signals, leading to fuel delivery inconsistencies and resulting in the rear end jerking. Additionally, worn ignition components such as spark plugs, wires, and the cap and rotor may cause misfires under load, contributing to this symptom. The vehicle's current condition is fair, but safety concerns arise from potential driveline instability during acceleration. It is crucial to assess the TPS functionality, as well as the overall ignition system health, to ensure smooth operation. Specific voltage checks of the TPS should read between 0.5V at closed throttle to 4.5V at wide open throttle (WOT). The ignition system components should be checked for resistance values: spark plug wires should measure around 1,000-3,000 ohms per foot, and the cap and rotor should be inspected for wear and carbon tracking.
Possible Causes
Most common causes (ordered by frequency):
- Throttle Position Sensor (TPS) malfunction: The TPS may be failing to provide accurate voltage readings to the PCM. A properly functioning TPS should output 0.5V at closed throttle and ramp up to 4.5V at WOT. A faulty TPS can lead to erratic fuel mixture adjustments, causing hesitation or jerking during acceleration. Testing with a multimeter should confirm the voltage outputs, and any significant deviation may indicate a need for replacement. The OEM part number for the TPS is typically 56040905. - Symptoms include jerking during acceleration, potential TPS voltage readings outside normal range, and no check engine light indicating immediate faults.
- Ignition System Issues: Worn spark plugs, faulty ignition wires, or a degraded cap and rotor can lead to misfires under load. A weak ignition system struggles to provide consistent spark, especially during acceleration, which can cause the engine to stumble, contributing to the jerking sensation. Spark plug resistance should be around 5,000-15,000 ohms, and wires should be inspected for cracks or carbon tracking. Replacement of these components can resolve the jerking issue. - Symptoms may include engine stumbling, rough idle, and visible wear on ignition components.
- Drivetrain Components: If the U-joints or the driveshaft are worn, they can cause jerking during acceleration. Inspecting the driveshaft for play and ensuring U-joints are properly lubricated can prevent jerking. Any unusual noises during acceleration may indicate a failing component in the drivetrain. - Symptoms may include unusual noises from the drivetrain or noticeable play in the driveshaft.
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 - TPS Testing: Use a digital multimeter to back probe the TPS connector. With the ignition on but the engine off, check the voltage at the signal wire. At closed throttle, the voltage should be around 0.5V, increasing smoothly to 4.5V at wide open throttle. If the readings are inconsistent or out of specification, the TPS may need replacement. Ensure connections are secure and clean to prevent false readings.
- STEP 2 - Ignition System Inspection: Visually inspect spark plugs, wires, cap, and rotor for wear or damage. Use an ohmmeter to measure the resistance of the spark plug wires; they should ideally be below 3,000 ohms per foot. Replace any components showing signs of wear or contamination. After replacement, perform a test drive to check for improvements in performance.
- STEP 3 - Drivetrain Inspection: Check the condition of the driveshaft and U-joints for wear. Look for any play in the driveshaft by manually rotating it and checking for any unusual movement. Any unusual noises during acceleration should be noted and further investigated. Lubricate U-joints if necessary.
