* Relief Valve: The oil pressure relief valve might be sticking or improperly calibrated. When under load (accelerating in 5th gear), the demand for oil is higher, and a sticking relief valve might prematurely open, releasing oil and lowering pressure. When you let off the gas, the demand decreases, and the valve might close properly, allowing pressure to build up.
* Oil Pump Installation: While you mention a new pump, there's a chance it wasn't installed correctly. A misaligned pump or incorrect priming can affect its performance.
* Worn Engine Bearings: Worn crankshaft or connecting rod bearings can increase oil clearances, requiring more oil volume to maintain pressure, especially under load. This would cause low pressure under load but higher pressure when the load is reduced.
* Clogged or Restricted Oil Passages: Internal oil passages in the engine block or oil filter housing could be partially blocked, restricting oil flow, especially when the engine is under load.
* Oil Viscosity: While less likely, using oil that's too thin for the operating temperature could contribute to the problem. Check your owner's manual for the recommended oil viscosity.
* Oil Pressure Gauge Accuracy: Though less likely, there's a small chance your oil pressure gauge is faulty, and the readings are inaccurate.
What to do next:
1. Check the Oil: Ensure you're using the correct weight and type of oil specified for your 318 engine.
2. Inspect the Oil Filter: Make sure it's not excessively clogged. Consider replacing it as a precautionary measure.
3. Mechanical Inspection: This is crucial. You'll likely need a mechanic to inspect the engine for worn bearings, clogged oil passages, and the proper functioning of the oil pressure relief valve. They can also verify the oil pump installation.
4. Consider a Mechanical Oil Pressure Gauge: If possible, temporarily connect a mechanical oil pressure gauge to ensure the electrical gauge is functioning correctly. Mechanical gauges are generally more reliable.
Don't continue driving the truck with low oil pressure under load. This can lead to serious engine damage, potentially requiring a rebuild. Get it checked out by a qualified mechanic as soon as possible.
