River Plate went joint top with Olimpo on seven points from three matches after an 88th-minute goal from burly striker Mariano Pavone gave them a 1-0 win at Independiente. River have yet to concede a goal.
Estudiantes, still trying to settle into a more attacking style sought by new coach Eduardo Berizzo, were exposed in a quick counter-attack on the hour.
Midfielder Marcelo Carrusca crossed high from the left and Achucarro rose between two defenders to head past a static Agustin Orion in the Estudiantes goal.
Estudiantes, winners of the Apertura championship in the first half of the season, have six points from three matches in the Clausura. Banfield have four after their first win.
Velez Sarsfield, runners-up to Estudiantes in the Apertura in December and among the favourites for the Clausura, have yet to win after being held 1-1 away by 10-man Argentinos Juniors at the Diego Armando Maradona stadium.
The home side, having fallen behind to an early goal by David Ramirez, had midfielder Mauro Bogado sent off on the hour for a second booking but hit back through striker Franco Niell in the 67th minute.
BOCA HELD
Boca Juniors' poor start to the season at their Bombonera ('Chocolate Box') home continued when they were held 0-0 by All Boys without dropped playmaker Juan Roman Riquelme on Saturday.
Coach Julio Cesar Falcioni believed Riquelme's stop-start style in midfield unbalanced his side and sought extra fitness work from the fans' idol while he fielded his favourite 4-4-2 line-up with two holding midfielders.
Falcioni, whose team had lost their opening match 4-1 at home with Riquelme and won 1-0 at Racing Club last weekend while he was out injured, was slated by fans who chanted Riquelme's name throughout the match.
Racing went from the despair of losing their Colombia playmaker Giovanni Moreno for six months with damaged knee ligaments two weeks ago to renewed hope in compatriot Teofilo Gutierrez's two-goal performance on Saturday.
Newly-signed striker Gutierrez scored with a diving header on the stroke of half-time and a close-range volley as Miguel Angel Russo's side came from a goal down to win 2-1 away to San Lorenzo.
