Helps to understand how it works I feel.
Unlike most diesels, our Toyotas have the accelerator pedal controlling simply a butterfly valve in the air intake manifold. (Most connect the pedal directly to the injector pump.)
The injector pump then varies the fuel quantities according to the amount of vacuum created by a "venturi device" that is situated at that butterfly valve.
When the butterfly is closed, the increased pressure drop across it (and across the venturi device) creates increased air speed (m/sec) through the venturi. And this in turn increases the vacuum applied to the diaphragm.
The injector pump varies the fuel by having its "rack" connected this diaphragm.
High vacuum (foot off accelerator) pulls the rack against spring-pressure (via the diaphragm) towards the "idling-fuel-quantity" end. Low vacuum (foot hard down) allows the spring to move the rack back in the opposite direction towards the "full-power-fuel-quantity" end. So when the diaphragm is torn and you have your foot off the accerator, the butterfly is almost closed starving the engine of air but the injector pump doesn't know your foot is off the accelerator anymore. It thinks your foot is still "hard down" (because the tear tends to equalise the pressures across the diaphragm rather than allowing the vacuum to suck-and-move the diaphragm) so it supplies more fuel.
Understanding this explains all the symtoms.
