CALGARY, Alberta, July 23 (Reuters) - A valve fitting on Enbridge's Inc's 491,000 barrel a day Line 5 oil pipeline failed during a hydrostatic test late on Sunday, making timing for restarting the pipeline uncertain, an Enbridge spokesman said. The pipeline between Superior, Wisconsin, and Sarnia, Ontario, has been shut since Thursday for the test. The valve failed near Bay, City, Michigan, spokesman Graham White said in an email on Monday. "Enbridge crews repaired the valve fitting this morning, and the hydrostatic test was resumed. Line restart has not yet been determined," White said. Enbridge denied a press report that it had experienced a leak of oil and water from Line 5 in Michigan. The incident was "Not a leak, but a valve fitting failure," White said. Line 5, which has been consistently oversubscribed as demand for space exceeds capacity, was carrying out the test in advance of a planned expansion of the line's capacity by 50,000 bpd. In July, Enbridge said shippers got only 74 percent of the space they requested on the line. Enbridge's pipelines carry the bulk of the country's U.S.-bound exports. A source familiar with Enbridge pipeline operations said Line 5 should be up and running normally soon, with providing further detail. Enbridge shares traded down 36 cents in Toronto to CAD $40.72 in morning trading on Monday.