Owner's Problem Description
The owner replaced the timing belt on a 1991 Dodge Dynasty 2.5L after it broke while driving. After installation, the car starts but runs poorly with low power and timing set at 12 BTDC. The owner is concerned about potential bent valves due to the timing belt failure, despite it being a non-interference engine.
Professional Technical Analysis
This 1991 Dodge Dynasty with a 2.5L engine has undergone a timing belt replacement after a breakage incident. The timing belt is correctly aligned, and the engine starts, but exhibits poor performance and low power output. Given that this engine is a non-interference design, the probability of bent valves is significantly reduced. However, the symptoms suggest potential misalignment of the timing belt. A common issue in such scenarios is that the timing may be off by one tooth, especially when the tensioning tool is not used during installation. Without this tool, it is easy to accidentally overtighten the belt, leading to misalignment. The owner also reported broken vacuum lines, which could contribute to low power by affecting the engine's ability to maintain proper pressure and air-fuel mixture. It's crucial to verify the timing marks after turning the crankshaft through two complete revolutions to ensure proper alignment. Additionally, the absence of the tensioning tool may have resulted in improper tension, further complicating the performance issues. Therefore, a thorough examination of the timing alignment, tension, and vacuum systems is warranted for accurate diagnosis and resolution.
Possible Causes
Most common causes (ordered by frequency):
- Timing Belt Misalignment: The absence of a tensioning tool during installation may have led to the timing belt being off by one tooth. This common issue can cause poor engine performance despite proper alignment of timing marks. A careful re-checking of the timing marks after two full rotations of the crankshaft is recommended to confirm correct alignment. - Poor engine performance, low power output, timing set at 12 BTDC, absence of tensioning tool during installation.
- Vacuum Leak: The presence of broken vacuum lines can lead to improper air-fuel mixture, causing low power and rough engine operation. This should be addressed to restore engine performance. - Reported broken vacuum lines, symptoms of low power and rough idle.
- Improper Tensioning: Without using the proper tensioning tool, the timing belt could be over-tightened, leading to premature wear or failure of the belt, which may affect timing and engine performance. - Engine running poorly, potential premature wear on timing belt due to improper installation practices.
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 timing belt installation, ensuring that the timing marks align correctly with the manufacturer's specifications. Check for any signs of wear on the belt and verify the condition of the tensioner and idler pulleys.
- STEP 2 - Verify Timing: Rotate the crankshaft two complete revolutions in the normal direction and recheck the timing marks. This is crucial to confirm whether the timing is indeed correct and to identify any potential misalignment.
- STEP 3 - Pressure Testing: Conduct a pressure test on the vacuum system to identify any leaks. A smoke test can be particularly effective in locating broken or disconnected vacuum lines.
- STEP 4 - Performance Monitoring: After adjustments, monitor engine performance and check for any persistent issues. Use a diagnostic scanner to check for stored codes that may indicate underlying problems.
