Elantra downshifted and added a little oil. What happened?

Engine mechanical system failure, such as throttle opening; Excessive carbon deposition on the back of intake valve and intake pipe.

Engine management system failure, such as incorrect water temperature and intake air temperature signals, line failure, etc.

There are faults in the ignition system, such as excessive carbon deposition or incorrect gap in the spark plug, leakage of high-voltage wire and distributor cover, and damage to the ignition coil.

Faults in the oil supply system, such as excessive colloidal carbon deposition in the fuel injector, lead to the fuel injector not being closed tightly or blocked.

There is also a clutch switch caused by poor contact. Because the engine is controlled by ECU, when shifting gears, refueling and receiving oil, the computer will continuously collect signals from sensors to control the fuel injection quantity of the engine. Once there is something wrong with the signal of the sensor, the computer program will be disordered, resulting in the truck phenomenon.

I suggest you go to a regular 4S shop for inspection. In a regular 4S shop, this kind of failure should not be an incurable disease.