Up to this point very tight car ,suddenly without notice water in oil

head gasket failure

Simple check first. Check your oil and make sure it is reading correctly on the dipstick. If it is low then add oil and watch everyday to see if the oil drops. If it is dropping quick enough to notice in a day or two then you have a leak somewhere an need to find that. A pressure test may need to be done as well to make sure oil pressure is high enough.

bad connection on oil pressure sensor, they just go bad

it is never a good idea to guess take it to a mechanic or 4 get several different quotes if 3 are same the it is the likely problem and you know where you stand

Your head gasket is leaking, or you have a cracked cylinder wall.

chances are its the head gasket moreso than a cracked head.

grand prix are notorious for leaking head gaskets. have the block mic'd when you get it apart to make sure its not warped - if you put a new gasket in the same thing will happen.

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It could be that your oil or coolant level is still low. When your oil or cooant level is low, your computer senses that and either shuts the fuel pump off or sets limits to it's operation.

When an engine is overheating and then the temp drops on the guage, it's usually a sign that the coolant is not touching the temp sensor. There's no coolant in the engine. Putting water in the overflow tank won't solve this. Take the radiator cap off after the engine cools and put coolant in the radiator. you might have to crank the engine and do this several times, as the engine can develop air pockets that won't allow the coolant to travel inside the engine. If you don't see any leaks, and it still uses coolant, you've probably got a blown head gasket. Check for water on the plugs, check cylinder compression. If it slowly uses coolant, try putting a large box of pepper in the radiator or some of that radiator seal copper flakes.

Sounds like the bearings in your turbo, or your return pipe for the turbo is blocked and oil is building up to the point of leaking out the bearings into the exhaust.Let me know :)

its under the throttle body near the coil pack

how much white smoke? white smoke usually indicates a headgasket
oil should not be black that quick