If your vehicle is showing engine surges or rpms bounce up and down while driving at a steady speed, here are the most likely causes and the parts you'll need to fix each one.
Unmetered air is entering the engine through a cracked hose or failing gasket. This confuses the engine computer, causing it to constantly adjust the fuel mixture and resulting in a surging idle or cruising speed.
The MAF sensor measures the amount of air entering the engine. If it is dirty or failing, it sends erratic signals to the computer, causing the engine to constantly adjust fuel delivery and RPMs.
Carbon buildup can cause the throttle plate to stick, or a worn sensor can send erratic throttle position signals. This makes the engine struggle to maintain a steady airflow, leading to RPM fluctuations.
At steady cruising speeds, the transmission's torque converter locks up for fuel economy. If it rapidly locks and unlocks due to wear, degraded fluid, or low pressure, the RPMs will repeatedly bounce by 200-500 RPM.
O2 sensors monitor exhaust gases to fine-tune the fuel mixture. A 'lazy' or failing sensor can cause the engine computer to swing the fuel mixture too rich and then too lean, creating a surging sensation.
A clogged fuel filter or a weak fuel pump can cause fluctuating fuel pressure. When the engine is under a steady load, these pressure drops cause lean conditions that make the engine surge.
Faulty shift solenoids or internal transmission pressure leaks can cause the transmission to hunt between gears or struggle to maintain fluid pressure, resulting in RPM bounces.