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28 September 2023 What problems can a loose filler cap on your diesel fuel tank cause?
Is the fuel tank filler cap actually important for my diesel engine? Can I still drive with a loose filler cap? Hopefully this guide will answer your questions.
What does the fuel tank filler cap do?
The primary function of the fuel filler cap is to seal off the fuel system, preventing any contaminants from getting in and fuel fumes from getting out. Nowadays, most caps come with their own locking mechanism which also helps to prevent fuel theft, a crime that has become more common amid the energy crisis.
The design of car fuel tanks and fuel tank caps may vary depending on the size, age, make and model of the vehicle. Many vehicles made before 1985 featured vented tank caps. These components were designed with small vents or holes in them to allow air to enter the tank when fuel is consumed and prevent a vacuum from forming. They also helped regulate pressure in the tank by allowing some air to escape.
These were later replaced by airtight non-vented caps that addressed the problem of fuel evaporation, reducing overall EVAP emissions. They are carefully designed to prevent vacuums inside the tank. Components such as the purge valve help regulate the air pressure and control emissions.
FEBI BILSTEIN Fuel cap
not lockable, Plastic, black, with support strap
Article number: 45550
£ 17,69
price incl. 20% VAT, excl. delivery costs
-10%
Article number: 6906-16-0240470P
compared to
RRP
£ 58,72
£ 52,61
price incl. 20% VAT, excl. delivery costs
Article number: B18783
£ 16,14
price incl. 20% VAT, excl. delivery costs
-11%
Article number: 6906-16-0400470P
compared to
RRP
£ 68,58
£ 60,41
price incl. 20% VAT, excl. delivery costs
Article number: PWP-TY-006
£ 57,54
price incl. 20% VAT, excl. delivery costs
Why is it bad to drive with a loose filler cap on your diesel tank?
The first issue caused by a loose filler is both an economical and environmental one. Although diesel has a much slower evaporation rate than petrol, fuel evaporation will still occur while driving if the system isn’t properly sealed. The emitted fuel vapours will affect both the air quality and your car’s fuel economy as the evaporated diesel is wasted before it reaches the combustion chambers.
Secondly, it leaves an opening for moisture, dirt, debris and other contaminants to get into the fuel supply system. This can eventually cause problems with your engine and lead to rust, a loss of power, a spluttering motor, and rough idling.
As mentioned before, this component also plays a role in maintaining pressure within the tank and car fuel lines. When it is loose or missing, pressure may be able to escape without being regulated.
Lastly, while spillages caused by bad tank filler caps are very rare, the possibility of a diesel spill cannot be ruled out, especially if the tank is too full. These spills pose a significant road safety threat as the oily substance reduces traction on the roads and is also known to damage road surfaces.
FAST Fuel cap
with key, black
Article number: FT94647
£ 10,34
price incl. 20% VAT, excl. delivery costs
VALEO Fuel cap
with key, black, with breather valve
Article number: 247519
£ 21,11
price incl. 20% VAT, excl. delivery costs
FAST Filler Pipe, fuel tank
Article number: FT61824
£ 7,96
price incl. 20% VAT, excl. delivery costs
-11%
Article number: 6906-00-8116528P
compared to
RRP
£ 67,07
£ 59,16
price incl. 20% VAT, excl. delivery costs
-11%
Article number: 6906-19-0390470P
compared to
RRP
£ 62,15
£ 55,05
price incl. 20% VAT, excl. delivery costs
Symptoms of a bad diesel fuel tank cap
Over time the fuel cap can become worn, losing its tightness and sometimes even dropping off while you are driving. Thankfully it isn’t a difficult or expensive part to replace. Knowing how to spot the signs early will prevent potentially costlier faults from occurring.
- It won’t fit or tighten properly
- You can smell fuel from outside the vehicle - This is due to the vapours that are released from the fuel delivery system
- Increased fuel consumption - Caused by evaporation and diesel contamination
- The check engine light is illuminated - The ECU may detect performance issues and trigger the check engine light on the dashboard
If you’re unable to tighten the cap or the tank seals fail, it is recommended to get this fixed as soon as possible. You can find compatible car parts available online by simply entering relevant keywords, e.g. “Mercedes fuel tank”, or specific data, such as the OEM part number or your vehicle’s identification number.
If the symptoms are still there after replacing the components, it is likely caused by another fault somewhere along the fuel delivery or combustion system. Fixing it will require a proper vehicle diagnosis.
