In today’s (11.8.18) Ask Car Talk the reader’s 2012 Hyundai has a high idle speed and hesitates and generally runs poorly in the 35-45 mph range. Also the mpg’s have gone down considerably. The reader says there are no diagnostic codes. This is Ray’s reply:
"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."
So my question is: If an injector is injecting too much fuel, and the computer has to allow more air in to compensate, which increases the rpm way beyond spec, shouldn’t the computer realize there’s something wrong and flag it with a check engine light and error code? How could this problem not cause a diagnostic code? After all, the computer knows the idle rpm is too high, and knows it is having to allow more air into the engine than the driver’s foot is commanding?