How do exhaust fumes build up inside a car?
Points of leakage
Exhaust can leak from any point of weakness inside the engine or exhaust system. Of note, the EGR valve and the DPF can be especially susceptible to blockage and leakage in cars that are used for local journeys in stop-start traffic. This is because under these conditions, the engine and exhaust do not run at a high enough temperature to vapourise the water produced in combustion. Condensation results, forming a focus for corrosion.
Other points of vulnerability include the head gasket, the manifold, the catalytic converter and the muffler. Exhaust can also enter and damage the cooling system, causing further leaks.
Fumes, once escaped, can enter the car through openings that form part of the car design (such as air inlets), or through those that arise due to wear and tear, including broken or loose bulkhead seals, holes in the floor or panels, welded joints that have experienced fatigue, faulty HVAC systems, and defects in the boot seal.
But doesn’t the catalytic converter soak up all the CO?
Catalytic converters cannot remove CO from a leak that occurs upstream from the converter itself. Catalytic converters, furthermore, are designed to work at hot temperatures. Stop-start driving of the kind that compromises the EGR and DPF also prevents the catalytic converter from operating fully.
It is also worth keeping in mind that car exhaust contains many non-catalysed toxins. These include sulphur dioxide, benzene, formaldehyde and particulates.
But if a car passes its emissions test, doesn't this mean it is safe?
An emissions test checks the contents of the fumes that exit the engine, rather than the integrity of the exhaust system itself. Regardless of whether they comply with emissions limits, gases leaking into the passenger cabin will cause harm to health.
Can a garage check the exhaust for leaks?
It is important to have your exhaust inspected regularly. However, a simple garage check on a non-running car cannot detect hidden points of leakage, particularly those that open up only when the engine is under pressure.
Is this just a problem with old or badly-maintained cars?
No. While wear and tear increases the likelihood that a car will develop a fume leak, CO leakage can occur in cars of any age - due, for example, to cracks caused by driving over a poorly-maintained road surface.