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I have a 2006 Subaru STI. I started getting a low oil pressure warning (factory pressure switch) on cold startup. It stays on after cranking for 2-10 seconds and then turns off. It doesn't happen after the engine gets to operating temperature. I installed a pressure sensor and logged the oil pressure at various temperatures. When warm everything seems fine. ~14 PSI at idle and ramps to 85 PSI (the recirculation valve opening pressure) by ~2500 RPM. On a cold start though the pressure starts at ~2 PSI and ramps to ~35 PSI over a few seconds. What could be causing this? It seems to be the opposite of what should be happening given the viscosity of engine oil vs temperature and how positive displacement lobe pumps work. Any suggestions about what to start looking at would be greatly appreciated. Oh, and the short block and oil pump are both factory and fairly new (
Could be sucking air through a crack or badly seated gasket, could be a faulty pressure relief valve that is sticking slightly open, could be several things. Regardless, cold should ALWAYS be higher than hot, and it needs investigation!
I'd be looking at the bottom end from the pickup mesh - not unknown to be partially blocked by silicone sealer and to show up most when the more viscous cold oil has more difficulty getting past than the hot, thinner oil - to the pressure relief valve that might have a little bit of debris causing it to stick open when cold.
Thanks. I came to the conclusion that it must be something on the suction side of the pump. I will start investigating there.