Hissing or Sizzling Under the Bonnet
See a Mechanic Soon — Urgency 4/5Do not continue driving if you see steam or the temperature gauge is climbing. Pull over, let the car cool, and assess the situation. Driving with a coolant leak can cause the engine to overheat and suffer serious damage. This is general guidance — always get fluid leaks inspected professionally.
This is general guidance. Your situation may differ — when in doubt, don’t drive.
What does it sound like?
A hissing, sizzling, or spitting sound from the engine bay, often after you stop driving. You might also see steam or notice a sweet smell. The noise sounds like water dripping onto a hot surface.
What could be causing it?
It could also be a vacuum hose leak (a hissing sound without the sizzling), a leaking radiator cap, a cracked heater hose, or a failing head gasket allowing coolant to escape. Oil leaking onto hot exhaust components creates a burning smell along with the sizzle.
What should I do right now?
Do not open the bonnet while the engine is hot — let it cool for at least 15 minutes first. Once cool, check your coolant level in the reservoir. If it is low, there is likely a leak. Do not drive far — head straight to a mechanic. If you see steam pouring from under the bonnet, pull over and call for a tow.