* Oil Pump Failure: This is a prime suspect. A new pump could be defective (rare but possible), or more likely, something is hindering its operation. This could be:
* Incorrect pump installation: Was the pump installed correctly? Were the gears properly meshed and seated? A slight misalignment can drastically reduce output.
* Pump Pickup Issue: The pump might not be drawing oil properly from the pan. This could be due to a clogged screen on the pickup tube, a poorly seated pickup tube, insufficient oil in the pan, or even a collapsed pickup tube.
* Worn or Damaged Pump Gears: Although less likely with a new pump, manufacturing defects can still occur.
* Bearing Failure: High initial pressure dropping dramatically suggests bearings are failing. This is a critical issue. As the engine warms, the oil thins and the failing bearings allow increased oil leakage, leading to low pressure. The rough idle is a clear symptom of this. Possible causes:
* Insufficient bearing clearance: Incorrect bearing clearances during the rebuild will cause this. This is a major rebuild error.
* Damaged bearings: Perhaps damaged bearings were installed, or the bearings were damaged during installation.
* Oil Viscosity: While less likely the culprit with a fresh rebuild, using the incorrect oil weight can exaggerate the problem.
* Oil Leak: A significant oil leak, especially internal (e.g., a main seal failure, or a significant rear main seal leak) will quickly reduce oil pressure as the oil level drops. Check for leaks carefully – this is easier to spot than some of the other problems.
* Relief Valve Issues: Though less likely, a stuck-open relief valve in the oil pump will prevent pressure from building.
* Incorrect Oil Pressure Gauge: It's possible (though less likely) the gauge itself is faulty. Try another gauge to rule this out.
Troubleshooting Steps:
1. Check the oil level: Is there enough oil in the pan? Low oil level will dramatically affect pressure.
2. Check for leaks: Look carefully for external leaks at all seals and gaskets.
3. Remove the oil pan: This is probably the most important step. Inspect the oil pump pickup, screen, and the pump itself for damage or debris. Check the condition of the bearings.
4. Inspect the oil filter: Make sure it's installed correctly and hasn't collapsed. A collapsed filter can restrict oil flow.
5. Use a mechanical oil pressure gauge: Don't rely solely on the factory gauge. A mechanical gauge provides a more reliable reading.
Important Note: Running this engine with low oil pressure will cause catastrophic engine failure very quickly. Do not run the engine further until the problem is identified and corrected. The consequences of ignoring this are extremely expensive. A professional mechanic specializing in engine rebuilds should be consulted immediately.
