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Oil Pressure - Eerratic Reading

  • 1971
  • 911T
120 000

1971 Porsche 911 T, with original 2.2 liter engine. The car has approximately 120,000 miles with a full engine overhaul about 4,000 miles ago. 

Approximately 200 miles ago, the oil pressure gage began showing erratic readings.  From a cold start the oil pressure shows 60 – 100 lbs of pressure, which has been normal depending on engine speed.  During engine warm up, the pressure will jump from 65 to 30 and then back to 65 while the engine speed remains constant at approximately 1500 to 2200 RPMs. When the engine warms up the pressure drops to between 40 to 70 lbs of pressure depending on engine speed.  The pressure continues to jump from the 40-70 lbs of pressure to between 25-30 and then back.

I have changed the oil, filter and checked the oil sump screen and bottom of the engine for debris. The oil is clean and free of any debris.  I have replaced the oil pressure sending unit, checked the electrical ground, I have temporarily installed a mechanical oil pressure gage and have also replaced the two oil pressure relief valves and springs with no change in the erratic oil pressure reading. – after each step, the same erratic pressure readings are observed.

During the engine rebuild, the machine shop performed the oil pump bypass modification, installed the larger Turbo piston squitter’s and checked the oil pump.  During the reassembly, the original chain tensioners were replaced with the Carrera pressure fed chain tensioners, the external oil cooler was cleaned and checked, the oil tank was cleaned and checked and new oil hoses were installed.

Any suggestions on where I might look next?  It seems like a pressure relief valve is sticking or something temporally blocking oil flow. 

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