Wtf is wrong with my engine?

[Brock]

https://youtu.be/fyjWFAJJbtY

Truck starter hung up, banged with hammer and it started fine after. Brought it to a mechanic to put a new starter in, afterwards when they pulled it out of the shop.... we left it running overnight because it's cold outside.

In the morning the mechanics shut it off because the fuel light was on and they didnt want it to run out of fuel. Later that day we put a heater on it, waited a bit then started it up..... here is where it gets interesting.

It sounds like its running on 3 cylinders, knocking so bad like a metal barrel filled with mallets. It maxes out the rpm at a certain point. When it was cold, couldn't idle above 900rpm... when warm it would rev/idle up to 1400rpm but once at max idle it would start smoking (and shown in video) then slowly start lowering the idle till it was at 800rpm.

I have not put the computer on it, been busy since with a new job and it's been to cold to start without my coolant heater working. Gonna check it out again tomorrow but I'm seriously at a loss right now.

Thoughts?