1997 Nissan Sentra Turns over but won't crank.

Yes, when you engage the key, to start the car, the engine turns over on its own until you disengage the key.

The first time the car was towed to a mechanic and it worked when it arrived.

The second time a mechanic claimed that a battery cable wasn't attached correctly. Years ago I failed to notice that the ground side was crimped to the frame and bolted to the block. So I simply bolted it to the block when it needed to be replaced.

The third time I was told that it was a compression and spark issue and the mechanic replaced Spark plugs, plug wires, distributer cap, and the plastic head cover.

After each "repair" the car would run fine until you let it sit for a few days. Just recently we towed the car about 150 miles and at the destination it tried to run on its own. The car cranked and turned a few times on its own, but never reached idle speed and died after about a dozen or so rotations.

Yesterday I was attempting confirm that fuel was the missing component and had pulled the air intake from the throttle body. I noticed a lot of oil residue in there. About three years ago my mother had poured some oil into the car and didn't tighten the cap before she drove the car around and it blew oil all over the inside of the engine cavity. She did this about three months before we towed the car 2000 miles and about six before the car started having issues. Could I simply have a sensor that is unable to communicate with the computer?