My toyota echo when i accelerate it the rpm going down why is that?

lots of causes. no car told , no maker,model ,year or engine there.
good engine , must be.
but spark misfiring is one cause, heard as violent engine vibration too..
or low fuel pressure. (EFI)? BAD FUEL?
ASSUMES TRANSMISSION DOES NOT SLIP.
A TOYOTA VITZ, CAR, SOLD IN JAPAN? 1999TO 2019
SOLD AS OTHER NAMES

• Toyota Yaris
• Toyota Echo (Oceania and North America, 1999-2005)
• Daihatsu Charade (Europe, 2011-2013)

???????????????? NO COUNTRY OR ENGINE TOLD--------
ANSWERS ARE GENERIC TO THE MAX AND SIMPLE.

DO NOT ADJUST IDLE SPEED! You have an issue with the engine and need to take it to be fixed. Idle speed is set by the manufacturer and should never be adjusted.

If it is not going into overdrive or lockup it could be the fluid or an electrical problem. Some of the shifting is controlled by solenoids now.
You may have to get a transmission shop to evaluate it for you.

Most chances that the clutch is weak

Toyota had a recall on the accelerator pedals/ cables.

on the air cleaner housing

leave it on all the time is better miles per gallon you can turn it off if you are going on a steep hill and you accelerate and down shifts and as soon as you let go a little it shifts back up you can turn it off to keep it at a lower transmission gear its really like down shifting when is off..

change the a/c compressor's bearing or service the compressor.

go to any kragen or autozone parts store, they will run the test for trouble code that turned light on for free, tell them u want an OBD 2 trouble code test, then post the code here.

throttle position sensor or called tps