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Hissing Sound After Switching Off The Engine

A short hiss after switching off is often the air conditioning equalising pressure and is normal. A longer hiss with steam or a sweet smell points to coolant escaping.

Normal risk

Usually normal. If there is steam, a sweet smell or the coolant level drops, have it checked soon.

Short answer · Is a hissing sound after turning off the car normal?

When you switch off with the air conditioning on, refrigerant pressure equalises inside the system and hisses for a few seconds. That is normal. Coolant dripping onto a hot exhaust also hisses, usually with steam and a sweet smell. A hiss from the brake servo when pressing the pedal after switch-off is also common.

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Safe checks you can do

  1. Note how long it lastsA few seconds after switch-off with the AC on is normal.
  2. Look and smell under the bonnetSteam or a sweet smell points to a coolant leak.Do not open the radiator cap on a hot engine.
  3. Check the coolant level when coldA dropping level confirms a leak.

Questions that narrow it down

  1. Was the air conditioning on?Pressure equalisation is likely.
  2. Is there steam or a sweet smell?Coolant is escaping.
  3. Did the engine get hot?An overheated system vents coolant.

Possible causes

Listed without ranking. Use the questions above to see which ones fit your car.

  1. Air conditioning equalisingNormal for a few seconds.
  2. Coolant leak on hot partsHiss with steam and a sweet smell.
  3. Pressure cap ventingA worn cap lets coolant escape.
  4. Vacuum leak or brake servoHiss when pressing the brake pedal.

Questions about Hissing Sound After Switching Off The Engine

Why does it only hiss sometimes?

The AC equalising sound depends on how long the system ran and the outside temperature.

Can a radiator cap cause hissing?

Yes. A worn cap that vents too early lets coolant escape as steam.