Car Exhaust Problems: Causes, Symptoms and Solutions

An exhaust does more than carry gas away from the engine. It controls back pressure, cleans up emissions, silences combustion noise and feeds the sensors that the engine management system relies on to set the fuelling. When part of it fails, the effect shows up somewhere you might not expect, such as at the fuel pump or in the way the car pulls away from a junction.

Here is how to read what your exhaust is telling you, from the tailpipe forwards.

What are the parts of a car exhaust system?

The system runs from the cylinder head to the tailpipe, and each section has a distinct job.

Everything in that list lives underneath the car, exposed to water, grit and five months of road salt a year. Corrosion is the single biggest cause of exhaust problems on British roads.

Why has my exhaust got louder?

A sudden increase in noise almost always means gas is escaping somewhere it should not be, which the trade calls a blow.

The character of the noise narrows it down:

• A deep, chuffing roar that rises with revs points to a hole in a silencer or a split pipe further back
• A hard ticking or tapping when cold that fades as the engine warms is typically a blown manifold gasket or a cracked manifold, because the metal expands and partially closes the gap
• A high pitched hiss suggests a small pinhole or a leaking joint under pressure
• A rattle over bumps with no change in exhaust note is usually a loose heat shield or a broken hanger letting the system knock against the underbody
• A metallic rattle from inside the cat when you tap it means the ceramic honeycomb has broken up

Causes. Corrosion from road salt and internal condensation, failed gaskets, cracked welds at joints, stone or kerb impact damage, and perished rubber mountings that let the system sag and strain a joint.

Solutions. A small hole in a mild steel section can often be cut out and a new section welded in, which is far cheaper than a full replacement. Perished hangers and loose heat shields are quick fixes. A rotten silencer that has failed internally needs replacing, because welding the outer skin does not restore the baffles inside. Manifold gaskets are a labour job rather than a parts one, but leaving one leaking eventually warps the mating face.

Why is my fuel economy getting worse?

Poor economy from an exhaust fault is nearly always a sensor or a restriction, not a hole.

The engine management system decides how much fuel to inject partly based on what the lambda sensors report about oxygen content in the exhaust gas. A sensor that has become lazy with age, or that is coated in contaminants, keeps reporting a lean mixture. The ECU responds by adding fuel that the engine does not need. You get worse economy, higher emissions and often a faint fuel smell, sometimes with no warning light at all.

A partially blocked catalytic converter or DPF causes the same outcome by a different route. The engine has to work harder to push gas out, so it uses more fuel to achieve the same work.

Solutions. A diagnostic scan and live sensor data will show whether the lambda sensors are switching properly. Sensors are usually replaced rather than cleaned. A restricted cat can sometimes be confirmed with a back pressure test before anyone spends money on parts. A DPF that is loaded but not damaged can often be regenerated on the ramp or professionally cleaned, which costs considerably less than a new filter.

Why has my car lost power?

Loss of power linked to the exhaust almost always means restricted flow. The engine cannot exhale, so it cannot inhale properly either.

The classic pattern is a car that feels fine at low revs and then hits a wall, refusing to pull past a certain point. It may also run hot, because trapped heat has nowhere to go. Causes include:

• A collapsed or blocked catalytic converter. The internal substrate breaks up or partially melts and blocks the passage
• A blocked diesel particulate filter. Common on diesels used mainly for short urban journeys, where the filter never reaches the temperature needed to burn off soot
• A crushed pipe, usually from grounding on a speed bump or a kerb
• A faulty EGR valve. The exhaust gas recirculation valve routes a measured amount of exhaust gas back into the intake to reduce combustion temperature and emissions. When it sticks open, the engine hesitates, idles roughly and can produce black smoke. When it clogs shut, emissions rise and the light comes on

Solutions. Diagnosis first, always, because a new catalytic converter fitted to cure a problem caused by a sticking EGR valve is an expensive mistake. EGR valves can frequently be removed and cleaned. Blocked DPFs respond to a forced regeneration or a professional clean. Genuinely destroyed cats need replacing, and it is worth asking why the original failed, since an engine running rich will destroy the replacement too.

What do bad exhaust smells mean?

Each smell points to a fairly specific fault, and none of them should be lived with.

• Rotten eggs or sulphur. Hydrogen sulphide getting through instead of being converted. Usually a failing catalytic converter, sometimes an over rich mixture overwhelming a healthy one
• Petrol. Unburnt fuel reaching the exhaust, which can come from a misfire, a leaking injector or a fuelling fault. A flashing engine light with a fuel smell means stop, because raw fuel destroys catalytic converters quickly
• Sweet and slightly sticky. Coolant burning, which suggests a head gasket or cracked head rather than an exhaust fault as such
• Burning oil. Oil leaking onto hot exhaust components, often from a rocker cover gasket, or oil being burnt in the cylinders
• Exhaust fumes inside the cabin. A leak ahead of or beneath the passenger compartment. Treat this as urgent

Solutions. Smells are diagnostic clues rather than faults in themselves, so the sensible response is a proper inspection rather than an air freshener. Carbon monoxide has no smell, so a car that smells of exhaust inside is a car that may also be delivering something you cannot detect.

What does the colour of exhaust smoke mean?

Colour identifies which fluid is being burnt, which narrows the fault down considerably.

A puff of blue smoke on start up that then clears usually points to valve stem seals letting oil seep past overnight. Blue smoke that continues under acceleration is more likely to be rings or a turbo.

Solutions. Smoke is a symptom of an engine fault reaching the exhaust rather than an exhaust fault. Diagnosis involves a compression or leak down test for internal wear, a pressure test for coolant loss, and a look at the air intake and fuelling side for black smoke. It is not a job to guess at.

Is it safe to drive with an exhaust leak?

No, not beyond the shortest necessary journey. The risk is carbon monoxide, which is colourless and odourless, and a leak ahead of the cabin can push it up through the bulkhead or the floor.

There is a fire risk too. Exhaust gas leaving a joint under the car is extremely hot and is aimed at underbody insulation, wiring looms and fuel lines that were never designed to sit in a jet of it. If you have to move the car, open the windows, keep the journey short and keep the heater on fresh air rather than recirculate.

Will an exhaust fault fail the MOT?

Yes, on several separate counts. The exhaust is checked for security, leaks, noise and emissions.

• Leaks. Any leak that makes a noticeable noise, or any part of the system that is insecure, is a failure
• Missing components. A catalytic converter that was fitted as standard and is now missing is a failure. The same applies to a diesel particulate filter, which testers check for visually
• Noise. A system that is significantly louder than standard fails
• Emissions. Petrol cars are tested for carbon monoxide, hydrocarbons and lambda value. Diesels are tested for smoke opacity, and visible smoke of any colour during the test is an automatic failure

Modifications matter here. Fitting a straight through system or removing a cat may sound appealing but it makes the car illegal for road use, not merely non compliant at test time.

How do you make an exhaust last longer?

Heat and dryness preserve an exhaust. Cold, damp and salt destroy it.

• Take a longer run regularly. Fifteen to twenty minutes at motorway speed once a fortnight drives the condensation out of the system and, on a diesel, lets the DPF regenerate
• Rinse the underside in winter. Road salt is the main reason British exhausts rot. A jet wash under the car after a salted week makes a genuine difference
• Fix small leaks quickly. A pinhole becomes a rotten section within a few months, and once a section is gone, adjacent joints are usually seized
• Do not ignore the engine light. Misfires and rich running kill catalytic converters, which is the single most expensive item in the system
• Mind speed bumps and kerbs. Grounding the centre section crushes pipe and cracks welds. Approach at an angle where the road allows
• Check the hangers. Cheap rubber mounts that have perished let the system hang wrong, which loads the joints and cracks them

What can you fix yourself?

Heat shields, rubber hangers and clamps are reasonable DIY jobs if you have axle stands and the sense to use them. Almost nothing else is.

Exhaust work involves seized fasteners, welding, working under a raised vehicle and dealing with components that carry emissions responsibilities. A silencer bandage or exhaust putty on a small hole will buy a week or two, but it is a temporary fix rather than a repair and it will not pass an MOT. Anything involving a catalytic converter, DPF, lambda sensor or manifold is a job for a garage with a ramp and a diagnostic tool.

While the exhaust itself is mechanical work, tailpipe soot and heat marking on rear bumpers is something we deal with regularly, along with the underbody corrosion that often accompanies an ageing exhaust. Our body shop is in Radcliffe and we also work mobile across Greater Manchester and the surrounding counties. Details are on our vehicle body repairs page, and you can request a free quote through our quick quote form.