For 1990-2009 cars

Complaint/Condition | Possible Cause(s) | Correction(s)
--- | --- | ---
Low Oil Pressure or Fluctuating (Oil Light may be On or Flashing) | Improper viscosity engine oil. | Replace engine oil with correct viscosity.
Faulty gauge. | Verify oil pressure and check sending unit and gauge. Refer to Instrument Cluster.
High ambient temperature. | Check cooling system and verify correct engine oil viscosity.
Contaminated engine oil. | Verify engine oil is diluted with gasoline, coolant, etc.
Frozen water in oil pan. | Thaw water and change oil.
Cracked oil pickup tube, damaged or missing oil seal or gallery plug. | Verify oil pickup tube is not cracked and all seals/plugs are not damaged and installed.
Faulty regulator valve. | Replace valve.
Faulty oil pump. | Replace oil pump.
Engine running clearances excessive. | Refer to the Basic Engine section and check bearing clearances.
Crankcase over/under filled. | Verify oil level and quantity in crankcase.
Orifice in cylinder block missing. | Inspect cylinder block for orifice.
High Oil Pressure | Improper viscosity. | Replace engine oil with correct viscosity.
Faulty oil pressure gauge. | Verify oil pressure and check sending unit and gauge.
Plugged oil filter. | Replace oil filter.
Ambient temperatures to low. | Verify correct viscosity engine oil.
Faulty regulator valve. | Replace valve.
Intermittent Squeal or Whistle Noise on Start-up or between 1500 and 2500 rpm. | Air purging from pump cavity. Faulty oil pump regulator valve. | Normal. Replace oil pump regulator valve.
Oil Light On or Flashing | Low oil. | Add oil.
Coolant in Oil | Cylinder head plug or gasket. Cylinder head/block coating porosity. | Inspect front cylinder head plug (DOHC [LL0] engine). Replace appropriate casting.

Important: Always replace the PCV valve after an incident of coolant in the oil has been corrected and the system has been flushed. Refer to engine diagnostic section for excessive lube oil loss diagnostics.