Owner's Problem Description
The owner is experiencing engine performance issues with their 1985 Chevrolet truck equipped with a 5.7L 350 V8 engine. The vehicle starts but runs poorly, and the owner is confused about the vacuum system setup, as the truck was not originally configured as intended. They have a vacuum diagram but are struggling to follow it due to modifications made since purchase. They've confirmed that the hose from the canister to the carburetor is correctly connected, but they're uncertain about the connections related to the EGR, purge, TCC, and TVS systems.
Professional Technical Analysis
The 1985 Chevrolet truck equipped with a 5.7L 350 V8 engine has been reported to have poor engine performance, particularly related to the vacuum system. Given the modifications made to the vehicle since its original setup, understanding the vacuum routing is crucial for diagnosis. The truck starts but produces rough running conditions, indicating potential vacuum leaks or incorrect hose routing. The EGR (Exhaust Gas Recirculation) system and its associated hoses are critical in maintaining proper engine performance and emissions. It is common for vacuum lines to deteriorate or get disconnected, especially in older vehicles, leading to performance issues. The vehicle’s age and the potential for wear and tear on rubber hoses should be taken into account. Systematic testing of vacuum lines using a vacuum gauge will help identify leaks, with normal readings expected to be steady under idle conditions. A typical vacuum level for a healthy 5.7L engine at idle should be approximately 18-22 inHg. Any significant drop in this reading can indicate leaks or obstructions. Additionally, a thorough check of the EGR system operation is essential, as a malfunctioning EGR valve can lead to poor engine performance and increased emissions. It is important to ensure all connections align with the provided vacuum diagram, especially for components like the TCC (Torque Converter Clutch) and TVS (Throttle Valve Switch), which influence engine performance.
Possible Causes
Most common causes (ordered by frequency):
- Vacuum Leak: A vacuum leak in any of the hoses connected to the intake manifold can cause a lean condition, leading to poor engine performance and rough idle. The most common areas to check are the hoses connected to the EGR valve and any PCV (Positive Crankcase Ventilation) connections. Inspect for cracks or disconnections, as these can significantly affect the air-fuel mixture. - Symptoms include rough idle, hissing noise from vacuum lines, and fluctuating RPMs during idle.
- Incorrect Hose Connections: Given the vehicle's modifications, it is possible that the hoses are not connected according to the vacuum diagram. This could lead to improper routing of air and exhaust gases, affecting engine performance. It is essential to verify that all hoses are connected correctly as per the diagram. - Visual inspection of hoses shows mismatched connections and potential blockages.
- EGR Valve Malfunction: A malfunctioning EGR valve may be stuck open or closed, leading to poor engine performance. If the EGR valve does not open when required, this can lead to increased NOx emissions and poor fuel economy. - Check engine light may be illuminated, and codes related to EGR functionality may be present.
- PCV Valve Failure: A failed PCV valve can disrupt the proper flow of crankcase gases, leading to increased pressure and potential vacuum leaks. Ensure the PCV valve is functioning correctly for optimal engine performance. - Rough idle and oil leaks may indicate PCV valve issues.
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: Start by visually inspecting all vacuum hoses connected to the intake manifold. Look for any signs of wear, cracking, or disconnection. Pay close attention to connections at the EGR valve, PCV valve, and throttle body. A vacuum leak can often be visually identified by listening for hissing sounds or seeing damaged hoses.
- STEP 2 - Vacuum Testing: Use a vacuum gauge to test the vacuum levels at the intake manifold and other relevant points. Normal readings should be between 18-22 inHg at idle. Any significant deviation indicates potential leaks or blockages. Additionally, check the EGR valve operation by applying vacuum to the diaphragm and ensuring it opens and closes properly.
- STEP 3 - EGR and PCV Valve Testing: Remove the EGR valve and inspect it for carbon buildup or mechanical failure. Clean or replace as necessary. Similarly, inspect the PCV valve for proper operation. It should rattle when shaken, indicating it is not stuck. Replace the valve if it fails this test.
- STEP 4 - Reconnect and Test: After completing the inspection and any required repairs, reconnect all hoses and perform a road test. Monitor engine performance during the test, looking for any signs of improvement or persistent issues. Use the GeekOBD APP to scan for any stored trouble codes post-repair to ensure all systems are functioning correctly.
