Mr. Kiser, of the outdoor power equipment group, said some initial tests already indicated that E15 could cause serious problems  including safety issues  with some small engines.

Image The E.P.A. may allow more ethanol in gasoline. Credit... Dan Neville/The New York Times

At Honda, which makes a wide range of engines for products from minivans to power generators, the concern is that the effects of a big increase in an additive like ethanol are unknown, said Edward B. Cohen, vice president for government and industry relations at American Honda. “The impact can be on the emissions system, like the catalytic converter,” he said. “It can be on the various tubes or couplings that are part of the fuel system, and it could affect the performance of the vehicle, particularly cold starting.”

Honda can design engines to run well on new gasoline blends, Mr. Cohen said. The issue is the legacy fleet, whose engines were designed over two decades for varying requirements. There is no single answer, Mr. Cohen said, to the question of how E15 would affect them.The American Petroleum Institute is also concerned, said Robert Greco, the group director of downstream and industry operations. He said more research was needed  probably several years’ worth  before the institute would be convinced that E15 was safe for so many different kinds of engines.

“We think that the current waiver request is premature,” Mr. Greco said. “The science isn’t in yet.”

And Jeremy Martin, a senior scientist at the Union of Concerned Scientists, an environmental advocacy group based in Cambridge, Mass., said there was simply not enough solid information on which to make a decision that would have such a broad impact.

“We shouldn’t just look at a little data and extrapolate,” he said. “There are rules here, and there are procedures. And there is a proper engineering way to come to this determination. One can guess about the most likely outcomes, but that is not sufficient to put all the fleet at risk.”

Wendy Clark, group manager and principal researcher in the fuels performance group at the Energy Department’s National Renewable Energy Laboratory, said a lot of credible organizations were studying E15. But she said it was too early to know for sure how engines would be affected. One question is how many of the studies will be done before Dec. 1, the date by which the E.P.A. is required by law to make its decision.