Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Why no diagnostic code for stuck injector?

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?

“The P0507 code means the idle control system has excessively high RPM. The ECU has a specific rpm range the idle is allowed and when it is higher than expected the ECU sets the P0507 code . Once this code is triggered it puts the vehicle into a failsafe mode and the Check Engine Light is illuminated on the dashboard.”

Why the vehicle that Ray is replying to did not trigger a code is hard to answer.


All error codes have a range of allowed error. Maybe the Kia has a wider range before the code is triggered?

The bad injector issue causes a cascade of of smaller problems. A sticking injector cannot be sensed by itself. The injector coil is electrically good - resistance and continuity - but a mechanical stick can’t be identified. EDIT … is hard to identify with software in the ECU.

This is a GREAT example of how you can’t rely totally on the ECU to tell you what to fix. I get code P0XXX and I change the dooble sensor and that fixes the problem. That’s just fine if the ECU can detect the absence or electrical failure of the dooble sensor. It is why running a compression test, or pressure test or vacuum test and engine to determine if the mechanical stuff is OK.

The mechanic needs to understand how the systems work and how they work together! The “Parts Tosser” just changes parts and hopes to heck they hit on the actual problem before they run out of money!

1 Like

hmmm … well another thing that could hike the idle rpm and enrich the mixture (& lower mpgs) is if the engine was cold all the time (faulty thermostat), or the computer thought the engine was cold all the time (faulty CTS).