00 bravada engine trouble

138k miles, all maintainences, is rough starting, poor idle and dies. Knocking noise at top end until over 1500 rpm then quiet and runs strong. poor acceleration until 1500 even after warm. New plugs and cap and rotor. Found it low on oil after oil change 4 weeks ago. Was filled correctly at oil change. inspectionm and working on it today same things after new parts. removed oil cap while running and is puffing smoke like a train out the oil fill, once it is revved over 1500 it goes away and never smokes again until let off gas. is it blow by at idle only? Is that possible? Could it be just a head gasket or is it definitely a piston problem. Truck rans super and no problem up til about 4 weeks ago. plugs looked a little brownish white and alittle build up but not like oil fouled. Has had intake gaskets once and is still running dex cool. could it be an intake gasket and how do I test this stuff to figure it out if it is not piston rings.

You can try changing the PCV and make sure the line isn't pinched...yours might be clogged up.

The mechanics of past years could tell you alot about your rings and valves with a vacuum gauge...maybe someone here remembers.

It sounds like a scored / broken piston. My guess is something physically broke off the piston or rings and damaged the remaining rings and cylinder wall. Now the pistons does not seal on the cylinder wall, allowing blow by. The loose piston is slapping the wall, causing the noise.
IMO there is no easy fix. The engine must be pulled and at least one cylinder repaired. Frankly, for all that trouble, you may as well rebuild the entire engine.