Car starts but revs for a few seconds with lots of black smoke and then cuts out have disconected exhaust from manifold but no change

Might want to check the exhaust system as it were plugged up You would get this result. Might want to disconnect the exhaust pipe temporarily from the exhaust manifold and see if it will rev up then

Black smoke means rich mix (gas, not diesel right?). Bad coolant temp sensor?

defective catalytic convertor. It may be plugged up.Make sure ALL the exhaust line from intake manifold(at engine block) to final exhaust pipe are not clogged up.You have reverse exhaust pressure going back towards engine manifold.

This seems to be a symptom of a blown head gasket.
White exhaust smoke is an indication of water / coolant in the exhaust. The car running sluggish and using lots of fuel can be caused by compression loss.

Look at the coolant level color and smell (with cold engine - don't open the high pressure reservoir with engine hot as hot coolant may spray you in the face). If you have a blown gasket, the coolant color may become brownish and it may smell bad or the level will get lower even if you have just refilled the cooling system.

What you can also check for is bubbles in the reservoir. With engine cold, remove the reservoir cap. Start the engine. Try to rev the engine (or have someone do it) and observe if there are bubbles in the reservoir tank. If you see lots of bubbles, you have a blown gasket.

seems like you are having turbocharger problems the rattling may be coming from your turbocharger as well as the smoke may be the oil which the turbo its burning as it touches your hot manifold, hope this helps you .

Its the problem of oil.The oil is getting burned out. The vehicle is either burning oil or it is moisture in the gas chamber if you notice white smoke from the exhaust. This is a common possibility.but in some cases this white smoke is not due to oil burning.In that case   White smoke from the exhaust is a symptom of coolant entering the combustion chamber. Normally this is a sign of a blown head gasket or cracked head. And  this is not a common problem must be repaired immediately. You must STOP driving this vehicle or you will cause serious engine damage. you will have to get both this possibilities checked.
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If you have a lot of black smoke then its over fueling getting to much diesel to the cylinder's. Sounds like an injector problem with the slight knocking noise. Che ck the return pipes are not blocked. If the smoke is dark brown then this is a problem with the valves on the intake manifold which control's the intake of exhaust emmisions.

you have leaking fuel injectors they are fouling out plugs.

The oil pressure dropping to zero is common for cherokees. Replace the oil pressure sending unit and it will be good as new. Cracked exhaust manifolds are also common. Usually it will crack where the 2 pipes are welded into 1 pipe before it connects to the front pipe. There are lots of replacement headers out there to choose from. I just ordered one through quadratec.

the oxygen sensor maybe unplugged, it should be in the manifold, sounds like itss running rich, once fault fixed lean sparkplugs and reset ecu