Johnson was in the Midlands to see how the region was coping with the aftermath of the MG Rover collapse. He said he had been assured in good faith by Rover's new owner Nanjing Automobile that it still planned to build some cars at the Longbridge site.
But he acknowledged that delays to the announcement of the firm's plans were proving damaging.
Johnson said: "With regards to Nanjing we do not know what is going on. I have been talking with Birmingham council leader Mike Whitby who met with Nanjing recently.
"Nanjing told him they were acting in good faith and nothing had changed in relation to their plans to create jobs at Longbridge within 18 months."
But Johnson said he remained sceptical about what Nanjing would do, citing the company's need to secure a partner - a problem which blighted the last days of MG Rover and ensured its ultimate demise.
"It is all becoming a bit of a soap opera really and we should not bank on a happy ending."
Johnson said he was sure that Nanjing had a business plan and the Department of Trade and Industry would help in any way possible.