WASHINGTON — The piece of the government's economic- stimulus plan aimed at bolstering the housing market may not work out as planned.

While Fannie Mae and Freddie Mac can now purchase and guarantee home loans of up to $729,750, they won't be traded in a key secondary market, limiting the prospect for sharply lower rates on "jumbo" mortgages.

To address the worst housing crisis in decades, the $168 billion economic-stimulus package President Bush signed this month included a temporary increase in the cap on mortgages that the government-sponsored companies can purchase or guarantee, from $417,000 to $729,750 in high-cost markets. The change will be in effect through 2008.

In theory, this was supposed to spark investor demand for securities made up of higher- value mortgages backed by Fannie and Freddie, which would have the effect of driving down interest rates on jumbo loans and spurring homebuying and refinancing activity. In practice, the impact is expected to be muted.

That's because mortgages above the conforming loan limit of $417,000 will not be allowed to be blended into packages of other loans traded in the To-Be-Announced secondary market. The rationale is that these larger loans carry greater risks and would thereby push up prices for securities tied to conforming loans, according to Wall Street's biggest trade group, the Securities Industry and Financial Markets Association.