Juan Valdivia, the former mines and energy minister, and two energy officials have already been forced to step down.
The congress has voted to investigate all oil and gas concessions granted since 2006.
It will also examine for irregularities contracts signed with foreign oil companies.
Garcia first served as Peru's president in 1985, leaving the country in economic ruin by the end of his term.
He was elected again in 2006 after championing free-markets and foreign investment.