Acceleration problem on M/T
 

Hello everyone,

I need help regarding a problem I have with my car.
I have bought a used Elantra GT 2018 (Nu engine 2.0L) Manual Transmission about 2 months ago. When I drive with partial throttle it accelerates fairly for that particular throttle, but when I need more acceleration and push full throttle I get less acceleration with marked worsening in Instant Fuel Economy with loud exhaust noise.

Partial throttle (given certain gear and RPM, say 2nd gear@2500RPM):

• 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 could cause this problem? How can I investigate this?

PS: I have no access to any authorized Hyundai service where I live, so I'll have to diagnose this issue on my own.

It's tough trying to diagnose new vehicles without a scan tool.
You could always take the shotgun approach and start replacing things hoping to hit the problem. This can get quite expensive and result in replacing a lot of good parts.
If possible can you have the vehicle checked for any codes in the powertrain computer? If there are any , it could help provide some direction

Since this is a manual transmission I wonder if the clutch is slipping. It should not with a vehicle this new. The clutch pedal should have about 2 cm of play before the you feel the pressure of the clutch. If you have no play it may be at the point were it does not slip with partial throttle and it slips with full throttle. Do you smell anything burnt under full throttle?

The only code I've got was "Catalytic system efficiency below threshold", with the code P0420.
The clutch is working fine. I see no signs of slipping clutch at all. RPM is totally matched with the corresponding speed, but it is difficult to accelerate (raise RPM) with full throttle.

Are you saying you are not able to raise the engine speed even with full throttle?

You received your answer with the converter code. Chances are it may be plugged and require replacement.
If the previous owner ran the vehicle with a misfire problem and ignored it that is a good reason for the converter to be questionable.
One way to confirm the suspicion is to connect a vacuum gauge to an intake vacuum spot and raise the RPM to a high idle or 2000 RPM. If the vacuum keeps dropping the converter is plugged.