Q. Why isn’t my dishwasher cleaning my dishes?

A. Most people who open a dishwasher to find plates caked with yesterday’s pasta blame the dishwasher. But the machine is usually not at fault.

“Pre-rinsing dishes is a big mistake,” said John Dries, a mechanical engineer and the owner of Dries Engineering, an appliance design consulting company in Louisville, Ky. “People assume that the dishwasher will perform better if you put in cleaner dishes, and that’s not true. Just scrape. Pre-rinsing with hot water is double bad, because you’re pumping water and electricity down the drain.”

It’s actually triple bad, according to Mike Edwards, a senior dishwasher design engineer at BSH Home Appliances in New Bern, N.C. “Dishwasher detergent aggressively goes after food,” Mr. Edwards said, “and if you don’t have food soil in the unit, it attacks the glasses, and they get cloudy,” a process known as etching that can cause permanent damage.

It’s also important not to use too much detergent, he said.

How much do you need? That depends on how much food soil there is, he said, not how many dishes. “If you have a light load,” he said, “don’t fill the detergent cup all the way.”