Toyota echo runs poor backfires at air filter,has been serviced,has new fuel filter,worse when cold?

1999 echo ,runs ok but has had this problem for a long time now ,had mechanic look at it,changed fuel filter.seemed better short term but again runs poorly,he suspects could be fuel pump,says backfires due to lack of fuel to injectors i think cause does not have a carby
Toyota echo runs poor backfires at air filter,has been serviced,has new fuel filter,worse when cold?
try changing the sparks , and clean the injection system .