So what’s up with direct injection?

First, a primer on how it works: By injecting gas*oline at high pressure directly into the engine’s combustion chamber, direct injection more precisely measures fuel than conventional fuel-injection systems or old-time carburetors.

The result is more complete combustion and cooler cylinder temperatures that enable a higher compression ratio for greater efficiency and power. Engine technology supplier Bosch says that direct injection can return a 15 percent gain in fuel economy while boosting low-end torque as much as 50 percent.

Combining direct injection with other technologies—such as turbocharging—can deliver even greater gains in economy and performance. That in turn enables carmakers to use smaller displacement engines, resulting in an efficiency snowball effect. Many automakers are marketing DI as pro*viding fuel economy that’s almost as good as a hybrid’s, and with better performance—but without the $4,000 hybrid premium.

Although the breakthrough seems like a dream come true, an unwanted side effect has been emerging. Letter writers have complained to Consumer Reports and the National Highway Traffic Safety Administration (NHTSA) that over time DI can lead to clogged fuel systems and engine carbon buildup. The result can be engine hesitation and a loss of power—and the need for expensive repairs.

Some carmakers, including BMW and Kia, have issued technical service bulletins (TSBs) to their dealers recommending that drivers use only name-brand detergent gasoline—without ethanol additives—and that they periodically add a fuel-system cleaner when they refuel. (A TSB is an alert that the automaker sends to dealers to warn about ongoing problems with individual models and how to fix them. It may allow dealers to make repairs at little or no cost to the customer as a goodwill gesture.)

Other automakers have devised an engineering fix that works while the car is oper*ating. It involves modifying the engine to spray a small amount of fuel directly onto the valves to help keep them clean.

It’s important to note that not all cars with direct injection experience long-term problems. But if your engine stumbles more than it used to, or it suddenly lacks power, ask your dealer about it. A fix may be available, and you may not have to pay for it.

This article also appeared in the February 2015 issue of Consumer Reports magazine.