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This has baffled my mechanics

I ran over a curb about 4 years ago. After that happened, my 2001 MT Hundai Elantra with 124K miles revs up when it’s out of gear, it goes through fuel quickly and I get constant P0121 and P1151 errors. I’ve gotten diagnoses of anything from a plugged catalytic converter to a throttle positioning sensor to a broken electronic control module to a broken exhaust to a vacuum leak. I’ve fixed all those problems except for the vacuum leak.
Here are the things I’ve replaced in the car.
Upper and lower catalytic converter Radiator Intake Manifold Gasket Crankshaft Sensor Electronic Control Module
So far, I’ve had the same problem. Car is sluggish and I get constant P0121 and P1151 errors.
Would any of you guys have an idea what the problem might be? It’s baffling my mechanics.

The too-high idle rpm problem can be a difficult one to diagnose. I had it on my Corolla one time. In my case it was caused by the gadget that up’s the idle rpm when the coolant is cold. In your case, those codes you are getting seem like the computer is confused b/c the position of the gas pedal isn’t properly matching up with the throttle valve position on the throttle body (inside the engine compartment.) That could certainly cause the idle rpm to be too high like that.

Another common cause of high idle rpm is the computer thinks the coolant never warms up, so it keeps the engine in the cold running state. That would usually be a failed engine coolant temp sensor, or a failed engine thermostat which was preventing the coolant from reaching the normal operating temperature.

I forgot to mention that I replaced the thermostat as well too. None of this happened before I ran over the curb.

Do you have a coolant temp gauge on the dash? Is it reading in the middle range after driving for 10 minutes? The reason I seemed to be focused on this aspect is b/c it is consistent with higher than normal gas consumption too, which you say you have, right?

Not unless the temperature is above 50F. It gets really cold up here where I live. I live in East-Central Minnesota, where temperatures seldom go significantly above the freezing mark from Mid-November to about mid-March. But the car worked fine before I ran over the curb. It never had these problems before I ran over the curb.

If you have a vacuum leak, you will get a high idle, lower gas mileage and a loss of power. The P0121 is for the throttle position sensor. The P1151 is the electronic throttle system (ETS) main accelerator position sensor 2 malfunction.

If you have a vacuum leak, that might look like a throttle position error to the computer, so I’d start there.

Edit: a plugged exhaust would look like a vacuum leak, it should be easy to see if you smashed part of your exhaust pipe to the point that it is a major restriction.

So the dash temp guage is behaving the same now as before you ran over the curb then? hmmmm … well it could still be the engine coolant temp sensor used by the computer. On same cars-- like my Corolla – the dash gauge uses a different sensor than the computer uses. Those sensors are not a common failure item, but running over the curb might have caused it’s connector to the wiring harness to come loose. Beyond that …

Checking the throttle positioning system probably requires a Hyundai specific scan too and perhaps a significant amount of diagnostic time. But it should definitely be a doable thing. Not possible to do it over the internet, you’d have have it done at a dealership probably.

Checking the entire engine for vacuum leaks is a common thing mechanics do all the time. And a vacuum leak could cause high idle rpm and fuel consumption. So ask the dealership to check that too, while they check the tps system.

The only other thing I can think of is a possible exhaust system leak. Running over a curb could cause that. A visual inspection of the exhaust system, and ask the dealership to do a fuel trim measurement.

Thanks. I’m going to have my mechanic take a look at my car tomorrow to see if he could confirm and locate the vacuum leak.

I’ll give you an update tomorrow!

Sounds like you are on the right track.

I’ll tell the mechanic to look for an exhaust leak too. I’m thinking a more likely scenario would be an exhaust leak that wasn’t welded properly.

I added an edit.

Is it OK if I screenshot this convo and show it to my mechanic? I promise to blur out the usernames.

You don’t need to blur me, I’m not ashamed of anything I have written so far.

The P0121 code states that there is a problem with the range value of the sensor signal, which is a voltage range between 0 and 5 volts. Make sure that the 5 volts is getting to the sensor, there could be a bad connection somewhere. Also make sure that the connection to ground is good and the sensor wire to the ECU is making good connection. The sensor could just be faulty and needs to be replaced. Make sure the wiring is okay first before replacing the sensor.

The P1151 code shows a wire problem with a sub accelerator position of the sensor. I assume the two codes are related to a single sensor. The sensor wiring needs to be checked out.