If your engine is dropping the RPM we would recommend to check the following steps:
1. Check if the side grille cover filter is folded or not. It must be folded up and be open for the air circulation.
2. Check if you air inflating solenoid (located on exhaust pipe) is open and exhaust air can go out. If it's locked the the exhaust air will stuck in the exhaust pipe and reduce the engine power and RPM. Also, you'll see black air is coming from the exhaust.
3. Check if you have water in the air intake system. Check for water in air intake hose and air filter. If you will find the water in the system, you will need to clean and dry it. Also, you will see the white smoke coming out from the exhaust, that means you have moisture in your air intake system.