the 166 flashing 3 times is normal, it does this to inform you that the ECM diagnostic routines are functioning correctly.

Like edward says that code 115 indicates oil pressure low

check the oil pressure sender, they are known to cause issues.
Verify the engine has good oil pressure by installing a pressure guage and taking a manual reading.
You can also remove the connector from the sender, use a vom meter and check the sender, crank over the engine with meter leads on the sender in ohms scale and see if it is switching on and off. By default the sender is open with engine not running and closes when the pressure increases after starting the engine. If the sender does not close when engine is running the DTC code will set.

You can also check voltage on the sensor circuit and make sure it is getting the +5v from terminal 37 of the ecm connector plug.

As far as the grounding on the truck, see if it has a ground strap from the engine to the frame, if it does not make one and temporarily ground the engine directly to the frame and see if that changes anything. If it does then make a ground cable and install it.