Acceleration problem with full throttle

Hello.

I have a 2018 Hyundai Elantra GT GLS (Nu Engine 2.0L, M/T).
It starts normally and accelerates fairly with partial throttle. But when I press full throttle (as when I try to overtake) I get an immediate restriction in acceleration with loud noise and worsening in instant fuel economy.

To make things more clear:

Partial throttle (given certain gear and RPM, say 3rd gear at 2000 RPM):

• Fair acceleration
• Normal exhaust noise
• Fair instant fuel economy

Full throttle (given the same gear and RPM):

• Less acceleration
• Loud exhaust noise
• Markedly worse instant fuel economy

What can cause this problem? How do I investigate this?

PS: I bought the car as a repaired salvage title, so it is not covered by any warranty.

Hey Mustafa W! Trouble codes and scan data is definitely the place to start with this concern. You'll need to get your hands on a scanner with access to live data. Something as simple as a ELM327 dongle and OBD Auto Doctor on your phone will work perfectly.

Since Hyundai typically doesn't use MAF sensors, you'll want to watch the following PIDs during a WOT run in 1st and 2nd gear:

- Engine Speed
- Relative Throttle Position
- Commanded Equivalence Ratio
- Equivalence Ratio B1
- Upstream A/F Sensor
- MAP
- Calculated Load
- Absolute Load

Find somewhere safe to do this, obviously. Having another person to drive or watch data may also be helpful. Post your results and we can help you further.

Thank you for the good approach. My friend has an ELM327 dongle, I'll schedule one day to check those data with him and post the readings here.

Hello.
I've collected all sensor data from my car, and I've chosen certain sensors (as the reply above) for a particular period (one minute) during which I had pressed full throttle (wide-open throttle) as presented in the image below. I've attached the entire file (more than 10 minutes) as well in CSV format.

Thanks for posting the data!

Having a look, we can say that the MAP is functioning correctly, and that there likely aren't any intake restrictions. The ECM seems to be commanding fuel enrichment when it should in response to WOT.

We were missing a couple important PIDs, unfortunately. Commanded Equivalence Ratio is there, but we also needed the upstream air/fuel ratio sensor OR Equivalence Ratio B1 to really tell what's going on with the fuel delivery system. That'll tell us if the engine is getting enough fuel at WOT or not.

Speak of, if you decide to go for another data recording, it'd be better if you did a run from a stop to near redline at wide open. That will stress the engine the most, and also give the data logger a better chance of catching the issue.

Thank you for your reply.
The graph above is a one-minute portion of a 10-minute data recording for those PIDs.
However, I've done another 7-minutes recording with ALL PIDs available through the software (Auto Doctor) just in case I was missing something, but I cannot see neither "Upstream Air/Fuel Ratio" nor "Equivalence Ratio B1". Maybe they are named differently? Here is the full list of all the PIDs available:

- Fuel system status: Fuel system A
- Fuel system status: Fuel system B
- Calculated engine load [%]
- Engine Coolant Temperature [°C]
- Short Term Fuel Trim - Bank 1 [%]
- Long Term Fuel Trim - Bank 1 [%]
- Intake Manifold Absolute Pressure [kPa]
- Engine RPM [RPM]
- Vehicle Speed Sensor [km/h]
- Ignition Timing Advance for #1 Cylinder
- Intake Air Temperature [°C]
- Absolute Throttle Position [%]
- Location of Oxygen Sensors
- Bank 1 - Sensor 2 (O2S): Oxygen Sensor Output Voltage [V]
- Bank 1 - Sensor 2 (O2S): Short Term Fuel Trim [%]
- OBD requirements to which vehicle is certified
- Time Since Engine Start [sec]
- Distance Traveled While MIL is Activated [km]
- Fuel Rail Pressure [kPa]
- Commanded Evaporative Purge [%]
- Fuel Level Input [%]
- Number of warm-ups since DTCs cleared
- Distance traveled since DTCs cleared [km]
- Evap System Vapor Pressure [Pa]
- Barometric Pressure [kPa]
- Bank 1 - Sensor 1 (wide range O2S): Equivalence Ratio (lambda)
- Bank 1 - Sensor 1 (wide range O2S): Oxygen Sensor Current [mA]
- Catalyst Temperature Bank 1, Sensor 1 [°C]
- Control module voltage [V]
- Absolute Load Value [%]
- Fuel/Air Commanded Equivalence Ratio
- Relative Throttle Position [%]
- Ambient Air Temperature [°C]
- Absolute Throttle Position B [%]
- Accelerator Pedal Position D [%]
- Accelerator Pedal Position E [%]
- Commanded Throttle Actuator Control [%]
- Fuel type being utilized
- Long Term Secondary O2 Sensor Fuel Trim - Bank 1 [%]
- Fuel consumption [l/100km]
- Battery voltage [V]
- Engine power [kW]
- Engine torque [Nm]
- Boost pressure [kPa]
- GPS speed [km/h]

I can provide you with graphs for any particular variables/PIDs from the list above, since the data is already available on my computer in a spreadsheet. Just tell me which ones can be useful for the diagnosis.

Nice. These are the ones we're after:

- Bank 1 - Sensor 1 (wide range O2S): Equivalence Ratio (lambda)
- Bank 1 - Sensor 1 (wide range O2S): Oxygen Sensor Current [mA]

Every scanner will name them differently. :silly:

Would you post the raw spreadsheet, please? I find that easier to work with, for whatever reason.

Great!
I'll send you two CSV spreadsheets containing all sensor data, with a total duration of approximately 10 minutes.
I believe I had performed some WOT events during that recording.
If there is any difficulty in opening those CSV files, I can convert them to any format you prefer.

Got it. I see good fuel enrichment from the upstream air/fuel sensor while at WOT. That makes me confident that the engine is getting plenty of fuel under heavy load.

Still no trouble codes stored? If there's no codes, then my first instinct is a restricted catalytic converter. It'd account for your symptoms, noises, lack of codes and the data we're seeing. Restricted converters will often behave the way you're describing, with decent acceleration under part load, but won't flow enough at WOT.

Now you've got options. If the upstream air/fuel ratio sensor is accessible, you can remove it and install a backpressure gauge. The gauge is nothing special, but it's the adapter that goes in place of the sensor that's important. No more than 1 PSI at idle. I can reference some videos if you're interested.

You could also just remove the air/fuel ratio sensor and take it for another drive. :silly: It'll sound like a go kart, but you'll be able to tell if there's a noticeable improvement in performance at WOT. If it's significantly better, then it got better because the catalyst is restricted, and you gave the exhaust another exit. Make sense?

Thanks a lot.
So, my fuel system should be functional with no problems, right?
I remember I've seen code P0420 (Catalytic system efficiency below threshold), it apparently goes well with your diagnosis.

I'm feeling inclined to perform the test of driving without the sensor, but I'm not sure if it is fine to do so given the fact that my sensor is pointed upward within the hood (picture below).



Another concern, won't removing the sensor affect the calculation of air/fuel by the engine?

Mustafa W. wrote: So, my fuel system should be functional with no problems, right?

Exactly. Checking for fuel enrichment at WOT isn't an end-all, be-all fuel system test. BUT, it does tell us that when you're experiencing poor acceleration, it's not because a lack of fuel.

I remember I've seen code P0420 (Catalytic system efficiency below threshold), it apparently goes well with your diagnosis.

:lol: Nice. It's a 2018, so if it truly has a failed converter, then I have to figure something caused it. Converters this new don't plug on their own.

I'm feeling inclined to perform the test of driving without the sensor, but I'm not sure if it is fine to do so given the fact that my sensor is pointed upward within the hood (picture below).

Your concerns are completely justified. If you decide to do this, I'd suggest doing it as briefly as possible. Remove the sensor, do a WOT run, pull over and reinstall the sensor.

Take care with any legal concerns as well. Running without the sensor will be very loud, and almost certain to piss off just about anyone in the area. That's what makes backpressure testing attractive. :lol: Depending on where you live, you can rent one from the local parts store. If not, they're widely available online for cheap.

Another concern, won't removing the sensor affect the calculation of air/fuel by the engine?

Absolutely! :lol: But we're not concerned with the mixture any more. We know the engine computer is in fuel control, at normal loads and at WOT. But at WOT, the engine computer will enrich the mixture regardless if the air/fuel ratio sensor is there or not. If you decide to do this, just make sure the sensor is disconnected electrically.

Thank you a lot for your useful opinions. I'll try to see if I can perform the open-sensor test.
Additionally, there is a local workshop that offers checking and cleaning catalytic convertors using endoscopic camera/tools through the sensor opening. I might allocate some time to visit them and perform checking/cleaning for my cat, and I'll try to update this post with the results.
Again, many thanks for your good efforts.

Mustafa W. wrote: Additionally, there is a local workshop that offers checking and cleaning catalytic convertors using endoscopic camera/tools through the sensor opening. I might allocate some time to visit them and perform checking/cleaning for my cat, and I'll try to update this post with the results.

If you decide to do this, please report back and let us know how it goes?

FWIW, I've never had luck 'cleaning' a catalytic converter. If they've failed to the point that it interferes with engine breathing, then it's not just chemically dead, it's a physical restriction. BUT, if they've got a magic trick to bring it back to life, I'm all for it!

If you decide to do this, please report back and let us know how it goes?

Yesterday I went to the local workshop I mentioned earlier. He opened the first sensor, inserted his endoscopic camera. Surprisingly, there was NO CATALYTIC CONVERTER!!
Yes. The catalytic converter was missing! He suggested that since my car is a salvage title, apparently some looter had "salvaged" the catalytic converter before or during its repair process, leaving an empty exhaust manifold casing!
(Only the inner core is stolen, leaving a hollow external casing that is connected to the remaining exhaust parts as usual).

I'm planning to get one (which will cost a lot, unfortunately), but only after I can resolve the current acceleration problem.
With that unexpected twist, what is the next most likely diagnosis rather than a clogged cat? What is the next step?

If the upstream cat is empty, then it'd possible it broke up and went downstream in the exhaust system. I'm not familiar with this specific car, but the next likely place would be either another downstream cat, a resonator or the muffler itself.

Again, you can test for this by disconnecting the exhaust behind the cat, but ahead of the resonator/muffler. If the power now improves, you can disassemble the exhaust further and find where the first cat ended up.

Thanks for the quick reply.
My primary catalytic converter is located right behind the engine within the exhaust manifold (picture below)

This, in turn, is connected to the rest of the exhaust system (picture below)


I'm not sure if there are any secondary catalytic converters, or those are just mufflers and resonators. But considereing the fact that only 2 oxygen sensors are present within the exhaust manifold; I assume there is only one catalytic convertor.

but the next likely place would be either another downstream cat, a resonator or the muffler itself.

So, you believe there is an exhaust obstruction issue here? right?
I was wondering if the low backpressure (caused by lack of cat core) can itself result in such problem? or is it unrelated? Since I do not really know how do those sensors actually work and affect the engine...

You're assuming that there's low backpressure because the upstream cat is empty, but has anyone done a backpressure test yet? If my theory is correct, and that the upstream cat came apart and went downstream, then it's still very possible to have excessive backpressure, causing your symptoms.

Low backpressure will not cause a lack of power.

The next most likely place for a restriction is part #28600H or #28650D, which is the downstream catalyst and the resonator. Either one can collect the remnants of the upstream catalyst and cause excessive backpressure.

Tyler wrote: but has anyone done a backpressure test yet?

Unfortunately I couldn't get access to any backpressure gauge here, and local mechanics simply don't use it! They rely on "disassemble-and-see" and "change-parts" approaches, which are not really what I'm willing to do yet!

I didn't perform the open-O2-sensor test, since the weather here is really hot, and I'm not ready to risk causing damage to the engine and/or other surrounding parts under the hood due to circulating hot air within the hood compartment.

But I was thinking: is it safe (regarding damage/risk of fire) to detach the exhaust system at the part #28751A and drive the car?
I'm aware of the related health risks (unhealthy fumes and carbon monoxide), and I'm willing to take safety measures accordingly. But my major concern here is whether there are risks of fire and/or damage to car parts due to extremely hot air flowing where it shouldn't??

It'll be reasonably safe for a short period of time. I'm guessing there's heat shields in place along the body where the exhaust runs, so you're not likely to set the carpet on fire. :silly:

Just keep your eyes open for any fuel system components for wiring nearby. Disconnect the exhaust just long enough to see if there's a difference in performance, then shut the engine off and reconnect it. Don't forget to secure the resonator side of the exhaust if the existing hangers won't support it.

Little bit of an old thread and late response lol. This throttle lag you speak of is nothing to do with any broken or missing parts. This is because of the electronic throttle body that gives you a slow response to acceleration. A throttle controller can and will fix this problem but not cheap there around 350$ CAD for me and I noticed a huge difference. In sport and race mode the car accelerates like it should and there is no delay from when I slam the pedal and when the car accelerates.