* Oil Pump: A failing oil pump might be able to provide enough pressure at higher RPMs (3500+), but struggles at lower RPMs and when the engine is hot (thinner oil). The noise is likely from increased engine wear due to insufficient lubrication. A worn oil pump will eventually fail completely.
* Oil Pressure Relief Valve: This valve regulates oil pressure. If it's sticking or malfunctioning, it might not open properly when the engine is hot, leading to lower pressure. Once the RPMs are high enough, the increased oil pressure forces the valve open, resolving the issue temporarily.
* Oil Pressure Sensor: Less likely, but a faulty oil pressure sensor could give a false reading. However, the accompanying noise makes this less probable. It's worth considering only *after* checking the oil pump and relief valve.
What to do:
1. Check the oil level: Make absolutely sure your oil level is correct. Low oil will cause low oil pressure. If low, *do not* run the engine; add the correct type and amount of oil immediately.
2. Check the oil pressure: This requires a mechanical oil pressure gauge. Don't rely solely on the dash light. A gauge will give a much more accurate reading across the RPM range, both when cold and hot. Borrow or rent one if needed. This is crucial for diagnosis.
3. Listen carefully to the noise: Try to pinpoint where the noise is coming from. This could help diagnose further. A bearing noise often changes in character as the engine warms up.
4. Inspect the oil: Check the oil's color and consistency. Is it milky (indicating coolant leak)? Is it excessively dark or dirty? This can provide clues about potential internal engine issues.
5. Consider the age and condition: A 1989 Jetta has likely accumulated significant mileage. Wear and tear on the oil pump or relief valve is a very real possibility.
If the oil pressure is low even with a proper oil level:
* Oil Pump Replacement: This is likely the most significant repair needed. It's a relatively involved job, requiring engine disassembly.
* Oil Pressure Relief Valve Replacement: This is a less extensive repair but might require some engine work to access the valve.
Important Note: Continuing to drive the car with low oil pressure will cause catastrophic engine damage. Get this addressed promptly, especially before pushing it to high RPMs. It's best to have a qualified mechanic inspect the vehicle as soon as possible. They can diagnose the exact problem and recommend the most appropriate solution.
