A persistent petrol smell is not a quirk to put up with, especially if it is strongest inside the cabin, under the bonnet or immediately after you park. Petrol vapour is flammable. A smell can come from something comparatively simple, such as a filler cap that is not sealing, but it can also point to a leak or a fault in the fuel or emissions system.

The sensible rule is simple: if the smell is strong, new, getting worse, or you can see liquid fuel, do not keep driving to see whether it clears. Switch off somewhere safe, keep cigarettes, flames and sparks well away, and arrange professional help.

First decide whether you should stop driving

Stop and do not restart the car if you can see a wet patch or drips that smell of petrol, see fuel around the engine bay or filler area, notice smoke, or the smell is powerful inside the car. Do not investigate with a lighter, do not run the engine with the bonnet up in a confined space, and do not touch a suspected leak. Move people away from the vehicle and call your breakdown provider or a garage for advice. If there is smoke, a fire or an immediate danger, move to safety and call 999.

A faint smell for a few moments after filling up is different from a smell that follows you home. Even then, it is worth checking that the cap is fitted correctly before assuming the car is fine.

The safe checks you can make yourself

Only make these checks with the engine off, the car outside in the open air and no ignition sources nearby. They are observations, not a reason to dismantle anything.

1. Check the fuel-filler cap and flap

Make sure the cap is the correct one, turns fully and is not cross-threaded. Look for a damaged seal, split tether or obvious damage around the filler neck. If the smell began straight after filling up, fuel may also have spilled on the bodywork or around the filler area. Wipe a small spill from painted surfaces with water and a clean cloth, but do not hose fuel into a drain.

A missing or poor-fitting cap can allow vapour to escape and may also trigger an engine warning light on cars that monitor the evaporative-emissions system. Replacing a cap is inexpensive compared with guessing at a more serious fault, but buy the right part for the registration or VIN rather than a universal cap that merely appears to fit.

2. Look underneath without crawling under the car

From a safe standing position, look for fresh wetness, a spreading puddle or a sharp petrol odour around the rear of the car and beneath the engine bay. Do not slide underneath, even if the car is on level ground. A leak can come from fuel lines, connections, the tank area or the filler neck, and the source is not always directly above the patch you can see.

If you find anything suspicious, recovery is a better choice than driving to a workshop. Petrol evaporates quickly, so the absence of a puddle does not prove there is no fault.

3. Note when the smell appears

This is useful information for the technician:

  • Only after filling up: think cap, filler neck, overfilling or a vapour-control issue.
  • Only with the engine running or at idle: an under-bonnet fuel connection, injector seal, hose or fuelling fault becomes more plausible.
  • Mostly inside the cabin: do not dismiss it as an outside smell. Check whether it is worse with the heater or ventilation on and tell the garage.
  • Only on bends, hills or with a full tank: mention that too, as movement of fuel can reveal a fault around the tank or filler assembly.
  • Alongside an engine warning light, rough running or poor starting: stop using the car if the smell is strong and have it assessed promptly.

Do not try to diagnose an injector, fuel rail or pressurised fuel line yourself. Modern petrol injection systems can operate at high pressure, and a small-looking problem may not be small once the engine is running.

Common causes a garage will investigate

A garage will normally start with a visual inspection and then follow the evidence. The likely areas include the filler cap and neck, tank and lines, hose clips and connections, fuel filter housing where fitted, injectors and seals, and the evaporative-emissions system. On many petrol cars, that system stores fuel vapour and feeds it back to the engine to be burned rather than releasing it to the air. A fault there can cause a smell and sometimes an engine warning light without leaving a visible puddle.

A smell can also be caused by fuel spilled during refuelling, an exhaust leak drawing fumes towards the cabin, or an engine running poorly enough to produce a fuel-rich exhaust smell. Those possibilities still need checking, because the fix depends on the cause.

The DVSA MOT inspection manual makes clear that visible induction or exhaust leaks that could affect emissions are a major defect. More importantly, do not rely on a current MOT as a reason to ignore a new petrol smell. An MOT is a test on the day, not a guarantee that a developing fault cannot appear afterwards.

What to tell the garage or recovery operator

A precise description can reduce diagnostic time. Say whether the odour is inside, outside or both; whether the tank was recently filled; whether there is a warning light; whether the car runs normally; and whether you have seen a drip or wet area. Give the registration, fuel type and any recent work, including a fuel filter, injector, tank, exhaust or body repair.

If a garage says it cannot reproduce the smell, ask whether it has inspected the filler cap and neck, visible lines and connections, fuel-vapour system and injector area. You are not telling them how to repair the car. You are making sure the symptom has not been reduced to a vague note on the job card.

Can you drive it to the garage?

There is no useful one-size-fits-all answer. A brief, faint smell that is clearly from a splash at the pump and disappears after the cap is secured is not the same as a strong cabin smell or visible fuel. But when the source is unknown, caution is cheap. Ask the garage or recovery service whether they want the car driven or transported.

Do not attempt a long journey, carry fuel containers in the cabin, or park the car in an attached garage while the smell is unexplained. If the problem is accompanied by smoke, visible leakage or a strong odour, do not drive it.

The bottom line

A petrol smell is a symptom, not a diagnosis. Check the cap, look for obvious evidence from a safe distance and make a note of when it happens. Then get a persistent smell investigated rather than masking it with air freshener or waiting for the next service. It may be a small seal or a spill, but the downside of getting a fuel-system problem wrong is too large to gamble on.