Let me take a shot at explaining this one. When you take your foot off the
throttle you close the air intake into the engine. With very little air/oxygen
getting into the intake manifold, then the fuel that is there (if any) doesn't
burn as efficiently. With the air intake closed off it also
Taking your foot off the pedal does not close off the air supply on a
diesel since there is no throttle plate. It closes off the fuel supply.
The air intake is just a pipe direct to the intake valve. The older
governer style pumps this test may work on since they have a throttle
plate, but
