Motiva Enterprises LLC is running fuel production units at reduced rates at the 325,000-barrel-a-day expansion segment of its refinery in Port Arthur, Texas, despite one of the main units being shut down, the company said Wednesday.

The expansion segment's crude distillation unit has been taken down for repairs after workers found extensive corrosion in the relatively new piece of equipment.

Motiva, a joint venture between Royal Dutch Shell PLC (RDSA.LN) and Saudi Arabian Oil Co., officially opened the refinery expansion May 31.

The CDU is the first step in the oil-refining process, separating oil into different cuts that are then processed into different fuels.

Establishing the cause of the expansion segment CDU's problems could take months, Motiva spokeswoman Emily Oberton said. "All expansion units other than the crude unit are fully operational, though some of those units are running at reduced throughput," Ms. Oberton said.

Write to Ben Lefebvre at ben.lefebvre@dowjones.com