Federal Reserve Chairman Ben S. Bernanke is giving himself an escape clause from his latest stimulus steps in case the economy finally gains momentum.

He’s made his third round of quantitative easing open-ended — meaning the program doesn’t have a set time frame — and signaled he may adjust its pace if needed. In December, the policy-setting Federal Open Market Committee added outright Treasury purchases to its mortgage-bond buying, saying it would acquire U.S. government debt “initially” at a pace of $45 billion a month on top of $40 billion in home-loan debt.