Hi rookie,

I dont believe that cleaning the injectors or installing a performance
chip is the solution. While both of these fixes will improve overall
performance, they don't eliminate worn parts like the air flow meter.
You wrote the newer engine had just 57K miles but the AFM could have a
lot more. When you depress the accelerator pedal, two signals command
the Digifant control module to increase RPM. The throttle switch signals
the moment of off-idle and the AFM shows that the air volume entering
the engine is increasing. If either of these signals are impaired the
DCM will be "late" with the needed acceleration enrichment and the
engine will hesitate. Also while the 2.1L needs no timing adjustment,
the basic distributor position must be set correctly. The evaporative
emission valve must closed at idle. Disconnect(temporarily) the evap
PURGE line & plug the opening to the crankcase ventilation to eliminate
this possibility.

As for the leak, pressure test the cooling system(cold then hot) and
determine where the coolant is coming from. Dust the area with talcum
powder if necessary. All WBX motors should be pressured tested during
every major service but few actually are.

Regards,   John