smoke and loss power

[melman]

I was traveling down hill for 20 min. had my (jake break) if that's what it's called on ,after at the bottom I switch it off about 4 miles later I started to lose speed to 20 mph smoke came out of the tailpipe bad then I had to go up hill for a short time and I heard clanky noise and gray smoke that smelled bad like something was burning . I parked it and look under the truck and seen small area at the rear end (seal) that had some leaking .checked the gear box had oil in it but was just a little low .not sure what it can be and need to take it to be fixed (tow) but would like to know what to look for. I tried to drive a short distance again after overnight parked and it still would not go over 20mph engine sounds good but a lot of dark smoke even at park . any Idea's

 

[Johnsram]

My first guess would be fouled injector(s) and/or one or more cylinders is not burning the fuel.

 

[melman]

My first guess would be fouled injector(s) and/or one or more cylinders is not burning the fuel.

engine running fine no missing at all .Had the engine light come on after I heard the noise (which it stop right away) and it was the gas cap .no more engine light