Sound like your fuel/air ratio is not correct.
It is when the "correct" air to fuel ratio is used to completely burn the fuel in question in an internal combustion engine. If exactly the correct amount of air is used this is called the stoichiometric mix.
11:3
There is no required a/f ratio on a diesel. It can be as low as 100:1 at no load idle.
13:1
idle air control valve allows correct amount of air that engine needs when idleing to keep air to fuel mixture where it should be
approximately 14.7:1
At idle it can be 50 : 1 or even 100:1.
54:21
The diesel engine can operate at 12:1 to 24:1 at idle.
The ratio is about 15:1 fuel to air.
fuel pressure at idle, air to air after cooled 3406, caterpillar diesel engine