The transit agency, ATAC, issued a statement Thursday saying that the supplier is bearing the cost of upgrading the engines to accepted standards.
According to the Rome daily Il Messaggero and other local media, the buses were manufactured in 2008 and brought to Rome from Tel Aviv in February after being in service there for eight to 10 years.
Rome is facing a public transport crisis, and the Israeli vehicles were meant to augment an aging fleet while ATAC awaits delivery of newly built buses. The local media said the average age of Rome buses was 12-13 years and many were out of service.
The Italian capital is notoriously underserved by its public transport system. Buses often run late or are overcrowded, and several have caught fire in recent years. Three metro stations in Rome were closed in recent months because of safety issues.