Dear Car Talk:

I've never had any problems with my 2012 Hyundai Elantra, until now. It has a 1.8-liter engine, automatic transmission and 156,000 miles. At speeds between 35 and 45 mph, it sometimes will hesitate and clunk. I installed new spark plugs and had a motor mount replaced. Also, the idle is running really high. And there's a growl under the hood. My mileage has gone from 33.5 mpg to under 20. I changed out the catalytic converter, replaced the serpentine belt, and cleaned the throttle body. Nothing has helped.

Is the timing too rich? The check engine light is not on, and I don't have any fault codes stored in the computer. Any ideas? -- Bill



This sounds like a real sweetheart, Bill. The most interesting clue you gave me is that the idle is running high.

There's actually no way to adjust the idle on modern cars, like you could by turning a screw on your father's Chevy. It's all computer-controlled now. So if the idle is consistently high, that means something's wrong. The fact that the check engine light has not come on tells me it's not likely to be an engine sensor, or anything in the computerized engine management system. So it's got to be something that's fooling the computer into thinking you're stepping on the gas.

And while it could be a vacuum leak, my first guess would be a bad injector or two. Or four. If one or more of your injectors were sending too much gasoline into the cylinders, that could explain everything. With extra gasoline in the mixture, the downstream oxygen sensor would tell the computer to send in more air, to protect the catalytic converter from being damaged. That would boost up your idle, lower your mileage and -- wait for it -- cause the engine to stumble or hesitate sometimes.

When a cylinder is flooded with too much fuel, that extra fuel can extinguish the spark. That creates an engine misfire, and causes that hesitation you feel. So you'll need to take this car into a mechanic and ask him to look at your injectors. As for the growling noise, I'd look for a missing neighborhood dog, Bill. Whatever's causing the growling (and it could be something like a bad water pump, idler pulley or alternator), I suspect it's unrelated to the other issues.

Replacing injectors is not going to be cheap. But neither is filling up twice as often, and joining the Catalytic Converter of the Month club. So figure it out, and get it fixed.