Owner's Problem Description
The owner has a 1973 Chevy truck that was stored for about 1.5 years. Upon attempting to start the engine, it started knocking. After removing the valve cover, it was discovered that some rocker arms were completely off the push rods. The owner is seeking guidance on how to adjust the rocker arms properly.
Professional Technical Analysis
The 1973 Chevrolet C/K Pickup is equipped with a small block engine that is known for its robust performance but can suffer from wear issues, particularly on the camshaft and lifters due to age and lack of use. The reported symptoms of knocking during startup suggest possible lifter failure, rocker arm misalignment, or camshaft wear. Given that the vehicle has been inactive for over a year, oil degradation and potential rust on internal components can exacerbate these issues. The primary concern here involves ensuring the rocker arms are properly seated on the push rods and verifying the integrity of the associated components. In this scenario, the lifters need to be checked for proper operation as they play a critical role in maintaining the correct push rod height and rocker arm function. If a rocker arm remains loose despite tightening, it may indicate underlying issues such as a worn lifter or camshaft. Proper torque specifications for the rocker arm bolts should be adhered to; typically, they are tightened to about 22 lb-ft, followed by a quarter turn additional. It's crucial to adjust them in the correct order and ensure that the push rods rotate freely before final tightening. Additionally, checking the ignition timing post-adjustment is advisable to ensure optimal engine performance.
Possible Causes
Most common causes (ordered by frequency):
- Worn Camshaft: Common in older small block engines, camshaft wear can lead to improper lifter operation, causing rocker arms to be misaligned or loose. If the cam lobes are worn down, they may not provide sufficient lift for the lifters, affecting the push rod alignment. - Knocking sound upon startup, rocker arms not retaining proper position, potential oil starvation effects.
- Faulty Lifters: Hydraulic lifters can fail, especially in engines that have sat idle for a long time. If a lifter fails to pump up, it may not maintain the necessary pressure to keep the rocker arm engaged. - Sound of excessive clearance in the valve train, visible irregularities in rocker arm positioning.
- Improper Rocker Arm Adjustment: If rocker arms are not adjusted correctly, they can lead to knocking sounds and poor engine performance. Adjustments should be done while ensuring the push rods are properly seated. - Rockers not seated properly on push rods, potential for increased wear on components.
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 - Visual Inspection: Begin with a thorough visual inspection of the engine's top end, focusing on the rocker arms, push rods, and lifters. Look for any signs of dislodged components, excessive play, or wear patterns that might indicate failure.
- STEP 2 - Adjust Rocker Arms: To adjust the rocker arms, ensure the engine is at TDC on the compression stroke for the cylinder being adjusted. Loosen the rocker arm nuts and turn the push rod until it has a slight resistance, then tighten the rocker arm nut to the specified torque (22 lb-ft) followed by an additional quarter turn.
- STEP 3 - Check Lifters: If a rocker arm remains loose, check the corresponding lifter for proper operation. This may require removing the intake manifold to access the lifter. Use a lifter puller tool to remove the lifter for inspection.
- STEP 4 - Test Engine: After adjustments are made, test the engine by starting it and listening for any abnormal sounds. Ensure that the knocking noise has been resolved and monitor for any signs of continued issues during operation.
