* Clogged or Restricted Oil Passage: The most likely culprit. There's a passageway in the engine block that feeds oil to the rocker arms. Sediment, sludge, or a piece of debris could be blocking the passage on the left side. This is especially likely if the engine has high mileage or hasn't had regular maintenance.
* Faulty or Partially Blocked Oil Gallery Plug: These plugs seal the oil galleries. If one is damaged or partially blocked on the left side, oil flow will be restricted.
* Problem with the Oil Pump: While less likely if the right side is getting oil, a weak or failing oil pump might not have enough pressure to push oil through restricted passages on the left. It could be delivering sufficient oil to the right, but not enough to overcome the blockage on the left.
* Broken or Damaged Oil Line: Though less common, a damaged oil line feeding that specific side could be the issue. This is less probable if you're only seeing the issue on one bank of rockers.
* Incorrect Lifter Installation: While you replaced the lifters, it's possible one or more are improperly installed on the left side, somehow obstructing oil flow *locally* around those lifters. This is less probable given your statement that the rods and lifters themselves are clean, suggesting oil is reaching the area, but not distributing properly.
Troubleshooting Steps:
1. Oil Pressure Check: First, verify you have adequate oil pressure. Low pressure points to a pump or other significant issue.
2. Visual Inspection: Carefully inspect the rocker arm oil passages from the top and if possible, visually inspect the oil galleries (may require removing components). Look for any signs of blockage or debris.
3. Remove the Valve Cover (Left Side): A close examination of the oil feed holes to the rocker shaft might reveal a blockage. Pay close attention to any restrictions.
4. Professional Help: If you're not comfortable disassembling engine components, take it to a qualified mechanic. They have the tools and expertise to diagnose and fix the issue, potentially using a pressure test to pinpoint blocked galleries.
It's crucial to address this immediately. Running an engine with insufficient lubrication can lead to catastrophic engine damage.
