That's because some economists believe the Fed isn't convinced that the U.S. economic slowdown is pronounced enough yet to require more economic stimulus. A slew of recent data that has shown weakness in the economy has been offset by some pockets of strength.
Tuesday was no exception.
The Commerce Department reported that spending by the U.S. consumer was unchanged in June. But personal income edged up 0.5 percent.
"If incomes are rising, but people aren't spending, it tells you that the consumer has some ammunition for more spending during the crucial back-to-school season," said Quincy Krosby, market strategist with Prudential Financial.
There were other positive numbers. The Standard & Poor's/Case-Shiller home price index released Tuesday showed that prices increases in all of the 20 cities it tracks. The Conference Board said Consumer Confidence Index increased to its highest reading since April, and better than economists had forecast.
Investors were also closely watching for the outcome from the European Central Bank's meeting on Thursday to discuss concrete steps to help countries with crippling debt.
It will be the first meeting after ECB President Mario Draghi said last Thursday that the central bank would do "whatever it takes" to preserve the euro, sending markets sharply higher. Over the following days, the leaders of Germany, France and Italy also said they would do all they can to protect the 17-country currency union. The comments raised expectations that the ECB might step in to buy Spanish and perhaps Italian government bonds to lower the borrowing costs for those countries, which have shot up to unsustainably high levels.
Investors were also waiting for the monthly unemployment report on Friday, the most-watched gauge of how healthy the U.S. economy is.
"There's a lot to absorb this week, including two major announcements from two very important central banks and payroll data," Krosby said.
Corporate earnings news did little to inspire investors. Aetna, the health insurance company, reported a 15 percent slump in net income as rising medical costs outweighed a gain in revenue. Its stock fell $1.08, or almost 3 percent, to $36.06.
Archer Daniels Midland, hammered by record corn prices, reported a 25 percent drop in net income. The agriculture company's stock fell $1.40, or 5 percent, to $26.09.
Bond yields ended slightly lower as traders waited for news from the Fed and the ECB. Investors are particularly interested to see whether the Fed will change or extend its bond purchasing program. The yield on the benchmark 10-year Treasury note fell to 1.47 percent from 1.50 percent late Monday. The euro also rose slightly against the dollar, to $1.23 from $1.22.
Among other stocks making big moves:
Coach, the luxury handbag maker, fell $11.2, or 18.6 percent, to $49.33, the biggest loss in the S&P 500 index. The company's revenue came in below analysts' forecasts because of slower sales at North American factory stores as consumers became more cautious about spending.
U.S. Steel gained $1.73, or 9 percent, to $20.65. Higher prices and lower costs for raw materials and energy helped the company beat analysts' forecasts despite challenging economic conditions, particularly in Europe.
Goodyear Tire rose $1.08, or 10.4 percent, to $11.45. The company's income more than doubled in the quarter after lower costs offset a drop in tire sales and beat Wall Street's expectations.
Valero Energy rose $1.42, or 5.4 percent, to $27.50. The second quarter results from the gasoline and petroleum products maker beat Wall Street profit forecasts after the company expanded its fuel-making operation in the United Kingdom, which helped the company increase production.