If your vehicle is showing engine surges or rpms fluctuate while driving at a steady speed, here are the most likely causes and the parts you'll need to fix each one.
Unmetered air enters the engine past the MAF sensor, causing the computer to constantly adjust fuel trim, which leads to RPM surges. This is frequently caused by a disconnected, cracked, or misrouted PCV hose after recent under-hood maintenance.
The MAF sensor incorrectly measures incoming air, causing erratic fuel delivery and engine hunting. This is often caused by dirt bypassing an old air filter or excess oil from a freshly cleaned, reusable aftermarket air filter coating the sensor wire.
The transmission's torque converter clutch rapidly engages and disengages, feeling exactly like an engine surge at steady cruising speeds (usually 35-50 mph). This is often due to degraded transmission fluid or a failing shift/lock-up solenoid.
Carbon buildup around the throttle plate or a worn Throttle Position Sensor sends erratic signals to the engine computer, causing the electronic throttle to flutter and RPMs to fluctuate.
The Exhaust Gas Recirculation valve sticks open intermittently due to carbon buildup, introducing exhaust gases into the intake at the wrong time. The engine stumbles and surges to recover its idle/cruising speed.
The VVT solenoid struggles to maintain steady camshaft timing due to sludge buildup or fluctuating oil pressure, causing RPM instability. Using the incorrect engine oil viscosity during a recent oil change is a common trigger for this.
A clogged fuel filter or a failing fuel pressure regulator causes inconsistent fuel delivery. The engine briefly starves for fuel and then surges as pressure catches up.