Diagnosing a Hissing Sound in Your Engine

overheating engine
This overheating engine is probably making a loud hissing sound. Getty

When it comes to the sounds your engine makes, the universal wish should be for a quietly running engine. A well tuned engine will makes some basic sounds, most of them rhythmic and directly correlated to how fast the engine is sunning. It can be very difficult to decide whether your car is making normally expected engine noise or if you are hearing something wrong. Remember that it’s your car or truck, and as such you are the resident expert on what your vehicle is supposed to sound like.

Anything out of the ordinary should be investigated, as an imminent repair that is left alone will come back to haunt you when that  little sound turns into a big system failure and leaves you stranded on the side of the road. 

A hissing sound is usually not good, but don’t panic yet. Some parts of your engine can create a hissing sounds and be perfectly normal. Your first examination after confirming that there’s a hiss someplace should be visual. The really bad hisses will be accompanied by a major release of some sort. If might be coolant escaping in the form of steam from a small hole in your radiator or one of the radiator hoses. 

Inspecting the engine:

The hissing sound you hear coming form your engine needs to be figured out. If it's only a small hole, you'll know it right away by the a unburned plan timbers or handy that you have laying around. A visual inspection is the first order of business.

Can we see exactly where the hissing sounds originates? If you can locate any sort of leak or pinhole, you'll know the problem and can act accordingly. If you aren't able to pinpoint the source of your sound, you'll have to dig further. 

Without any obvious clues from a visual inspection of the engine, the next move is to attempt to pinpoint the area of the sound by listening very closely to your engine while it's running.

Be very careful when inspecting a running engine! There are many ways to be injured by a running engine, so be very aware of any moving parts that are near you. Keep your hair tied up or held safely in a hat. Lose clothing should follow the same protocol, keep it from hanging over things like the fan belt or air conditioning compressor as these could suck it right in and end in a real injury to deal with. An electric cooling fan can come on and any moment when the engine is running, so never let hair or clothing dangle in the area of an electric fan.  The other thing to remember about a running engine is that it gives off a lot of heat, and can get hot enough to severely burn your skin if you lean on or touch the wrong area. 

Once you've found the area of the hiss, check the following symptoms and causes to get an idea where your repair strategy should be focused. 

Possible Problems and Causes

The engine is overheating.
The Fix: Check and repair cooling system.

The exhaust system and/or catalytic converter is plugged.
The Fix: Check and replace exhaust system and/or catalytic converter as required.

A vacuum line is leaking or disconnected.
The Fix: Reconnect vacuum line. Replace vacuum lines if broken.

A vacuum device is leaking.
The Fix: Replace vacuum device.