Repairs to Caledonian Isles in Brodick forced the cancellation of two return sailings to Ardrossan on one of CalMac’s busiest routes.
CalMac said a secondary vessel, Isle of Arran, continued to operate on the route.
The operator said later the fault had been fixed and Caledonian Isles would return to service for the 19:20 (7.20pm) sailing from Brodick.
CalMac had tweeted: “Due to an issue with the vessel’s starboard main engine which requires further investigations and repairs, the following sailings have now been cancelled: Depart Brodick - 13:55, 16:40, Depart Ardrossan - 15:20, 18:00.”
The fault is the latest to hit the 29-year-old ferry, which was out of service for more than two weeks at the start of the tourist season in mid-April because of an engine failure.
It should have been replaced by the hugely-delayed Glen Sannox four years ago, but the ferry being built at the Ferguson Marine shipyard in Port Glasgow is not now expected to be completed until between March and May 2023.
Sea trials will follow, so the vessel may not be in service until autumn next year, leaving Arran at heightened risk of further disruption for another summer.
CalMac managing director Robbie Drummond told The Scotsman in July that more vessel breakdowns were likely because its ageing fleet was being worked so hard.
In a stark warning to passengers, he said: "The position is not going to improve – it’s going to get worse because the [average] age of the vessels is going to increase over the next one to two years before we see the new ones coming through."
He predicted sailings may have to be cut to enable more time for ferry maintenance and upgrades.
A CalMac spokesperson said: “MV Caledonian Isles is currently alongside at Brodick and has developed an issue with an air leak.
“Repairs are underway and hope to be completed by this evening.
"Two return sailings have been cancelled but the other vessel continues to operate an Ardrossan-Brodick-Ardrossan service.
"There are also sailings on the Lochranza-Claonaig route on the other side of Arran.”