While the engine runs
RPM slow to drop when you lift off the gas: why
On many modern cars it is normal for the revs to drop slowly when you press the clutch: the control unit does it on purpose to reduce emissions. If they hang for a long time, rise on their own or the idle stays high, there may be a vacuum leak or a fault in the throttle body or the pedal sensor.
Check without hurryWhile the engine runs
Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Usually not urgent, but do not ignore it. If they just drop a little more slowly, it is not urgent. If the engine revs up without you pressing the pedal or the car pushes when you brake, stop in a safe place and switch the engine off: do not drive on until it is checked.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Control unit strategy: a deliberately slow drop for emissions or comfort; not a fault.
- Vacuum leak: a hose or gasket letting in air after the throttle.
- Dirty or out-of-adjustment throttle body: does not return precisely to its rest position.
- Pedal sensor or brake and clutch switches: if the control unit does not know you have lifted off or braked, it holds the revs.
- Crankcase ventilation valve: a split lets in unmeasured air.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: What does the throttle body do and why does a fault trigger limp mode? What are fuel trims and how do you read them?
Car system: Fuel and air
What you can check yourself
Compare cold and warm, and with the A/C on and off.
Make sure the floor mat is not touching the gas pedal.
Check that the brake lights come on when you press the pedal: the control unit uses that switch too.
With the engine idling, listen for an air hiss in the engine bay, keeping your hands away from moving parts.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
Related fault codes
- P0507Idle control system RPM: higher than expected
- P0505Idle control system: malfunction
- P0171Mixture too lean (bank 1)
- P2135Throttle/pedal position sensor/switch A/B voltage: correlation fault
- P0504Brake switch A/B: correlation fault
Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: pedal and throttle position, target and actual idle speed, brake and clutch switch status and fuel trims.
Frequently asked questions
Is it normal for the revs to drop slowly?
On many cars, yes: it helps reduce emissions when you shift. Revs that rise on their own or stay high are not normal.
Does a vacuum leak set a code?
Often, yes: a lean code (P0171) or an idle code, but not always. Live data helps.
Should the throttle body be cleaned?
It can get dirty and affect the idle. After cleaning, many cars need a relearn with a tool.
Keep reading
- Fuel and air: how it works and what fails
- Rough idle: why the engine shakes or hunts when stationary
- Car stalls by itself: causes and what to do
- Slipping clutch: symptoms, causes and what to do
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means