Direct-injection engines improve performance and save fuel, but at a price

It has been touted as the automotive holy grail, the ultimate marriage of better performance and greater fuel economy. It’s the direct-injection (DI) engine, the latest technology designed to squeeze more mpg out of cars. And it has actually been delivering results.

For example, the Mazda3’s combined fuel economy jumped from 28 mpg in 2010 to 32 mpg in 2012 in our testing of the new Skyactiv engine. Other automakers have been using direct injection to add horsepower—the Cadillac CTS, for example, gained 34 hp—without any sacrifice in fuel economy.

Though direct injection is currently seen mostly on luxury vehicles or on mainstream cars as a higher-trim option (priced anywhere from a few hundred dollars to a couple of thousand), it could soon be within the means of more drivers as costs continue to come down.

But those engines are also having reliability problems, something that automakers are trying to keep quiet.

Not surprisingly, a number of readers have asked us about direct injection. Take Anestis Halkidis of North Kingstown, R.I.:

“I have a 2006 VW GLI with the 2.0T FSI engine with direct injection,” he wrote. “At around 80,000 miles, I went to the VW dealer to diagnose a check-engine light. It turns out that the intake valves had to be cleaned due to carbon deposits that were causing drivability issues.”

When his dealer failed to offer free work or any compensation, Halkidis took his car to a local VW shop that cleaned the valves by blasting them with walnut shells. (Yes, walnut shells; it’s a method used by BMW.) The cost to Halkidis was about $400.

So what’s up with direct injection?

First, a primer on how it works: By injecting gas­oline 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.