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I have a '08 Grand Prix GXP with LS4. Car has 165,000 miles Oil consumption about a quart per 1,200 miles. Oil is 5w-30 synthetic with AC PF63 filter. The Service Engine light comes on intermittently. Code is always "oil pressure sensor malfunction."
I installed a pressure gauge today in place of the sensor. At startup, it shows 40 psi. After a few minutes of warmup, pressure drops to 14 psi with a fairly loud lifter tick. Above 2000 rpm, it's at 40 psi.
Took the car for a 10 mile drive and looked at the gauge again at idle--it's 35-40 psi, no lifter tick. At 2000 rpm, still 40 psi. I thought the gage was stuck, so I shut the engine off and saw zero on the gage. Startup shows the same 35-40 psi at idle.
Can anyone guess why a cold to lukewarm engine would show less oil pressure at idle than a hot engine at idle?
I have two ideas:
First is excessive clearance on crank bearings. Bleeding off pressure when the engine is cool but tightening up when the engine goes to work and the heat tightens the clearances up thus holding more pressure.
Second, and I think the most likely reason, because it seems to be the same problem my car and a lot of others have is the oil pick-up o-ring. The o-ring gets hard and doesn't seal very well allowing the oil pump to pull air. Think of trying to drink through a straw with a hole in it. When the car is cold the oil is thicker (think milk shake) which makes the oil even harder for the pump to drink plus at idle the pump is not pulling as hard and fast so less oil to the top of the engine. Oil warms up and starts flowing back to the pan faster than the pump can push it back up. Result, lifter tick. Drive it a little and you get the lifters full and all of the passages full so it takes a little while to bleed back down but idle long enough it will get back to lifter tick. I have no idea about the sensor code unless it is related.
Oh, third theory. Your car is haunted. Sell it or find the nearest Priest to perform an exorcism.
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LS4 Oil Pressure Dilemma 
