* Oil Pump Installation: The most likely culprit is an incorrectly installed oil pump. It needs to be seated correctly and properly aligned with the drive shaft. A slightly off-alignment or improperly seated pump won't prime. This is a critical check. Did you use a new oil pump? Even a slightly damaged one could fail.
* Oil Pump Drive: The oil pump is driven by the crankshaft. Ensure the drive shaft is properly installed and undamaged. A bent or broken shaft would prevent the pump from working. This is often overlooked but important.
* Oil Pickup Tube: Make sure the oil pickup tube is correctly installed and unobstructed. If it's bent, clogged, or improperly seated in the oil pan, it won't draw oil. Debris in the pan is also possible.
* Blocked Oil Passages: It's possible that during the rebuild, some debris or sealant blocked oil passages within the engine. This is less likely if you meticulously cleaned everything but still possible.
* Oil Pump Relief Valve: While less common, a problem with the oil pump's relief valve could prevent priming. This is usually a mechanical issue within the pump itself.
* Low Oil Level: Though you've mentioned a rebuild, double-check you have the correct amount of oil added. Insufficient oil can prevent priming.
Troubleshooting Steps:
1. Re-Inspect Oil Pump Installation: This is the most critical step. Carefully remove the oil pan again and verify the pump's installation. Check the alignment and seating. A new oil pump is always best after a long block rebuild.
2. Check Oil Pickup Tube: Inspect the pickup tube for damage, blockages, or incorrect placement.
3. Check Crankshaft and Oil Pump Drive: Visually inspect the crankshaft and the oil pump drive for any damage.
4. Check for Debris: Carefully inspect the oil pan and oil passages for any debris that might be blocking the oil flow.
5. Prime Manually (if possible): Some oil pumps have priming mechanisms; check your service manual. You might be able to manually prime the pump with a drill (carefully and with the proper adaptor, if possible on your design.) This helps rule out the pump itself.
6. Consult a Mechanic: If you've checked all of the above and still have problems, consulting a mechanic specializing in engine rebuilds is highly recommended.
Important Note: Continuing to crank the engine without oil pressure is extremely damaging. If the oil pump isn't working, stop cranking immediately to avoid catastrophic engine failure. Dry cranking is risky.
