Gas Caps and Filler Assemblies

What a fuel cap does

A fuel cap seals the filler neck to keep fuel and vapors in and debris out. On modern vehicles, it is part of the evaporative emissions system, and a loose or damaged cap can trigger a check engine light. Older vehicles may use a vented cap that allows air in as fuel is drawn out.

Listings include locking caps, non-locking caps, brushed aluminum caps and drop-in replacements for factory parts.

Cap types

• Locking caps use a key to deter fuel theft and tampering.
• Non-locking caps are the simplest replacements.
• Billet or aluminum caps add a styled look for custom builds.
• Filler assemblies replace the neck or door when corroded or damaged.

Choosing the right cap

Match the cap to the vehicle and fuel system. Thread style, diameter, venting and whether the system is vented or sealed all matter, and a wrong cap can cause pressure problems or leaks. Confirm fitment for your exact year, make and model and check the manufacturer's specifications.

Check whether a locking cap comes with keys and whether the lock is keyed alike with other locks. Gaskets wear out, so check the seal if a cap is old but otherwise intact.

Care and installation

Installation is a simple twist-on. Turn it until it clicks or seats fully, as the vehicle's instructions specify. A cap that is not fully tightened is a common cause of evaporative emissions codes.

Keep the filler neck clean, and inspect the cap's rubber seal for cracks. Never use a cap that does not suit your fuel type or vehicle. Handle fuel carefully, away from ignition sources, and replace a damaged filler neck promptly, since leaks present a fire hazard.

Further checks and care

Questions to ask before you choose

Decide whether theft protection matters for where you park, and whether you prefer a factory look or a styled cap. Check that your vehicle's fuel door or filler design allows a larger billet cap to clear it. If a warning light came on after a fill-up, inspect the old cap's seal and threads first. Keep a spare key in a safe place if you choose a locking cap.

Measuring and confirming fitment

Caps are matched by thread type, neck diameter and venting. Compare the old cap with the listing, and check for a tether or a lanyard attachment. Vehicles with a capless filler system do not use a cap at all. For diesel and gasoline applications, ensure the cap is made for the correct fuel type, since vapor handling differs. Note the vehicle's model year range, because filler neck designs changed over time.

Materials and maintenance

Caps are made from plastic, stamped steel or billet aluminum, with a rubber or fluoroelastomer gasket. Locking caps include a key cylinder, which should be lightly lubricated with a graphite or dry lubricant, not oil. Wipe the neck and the gasket before closing the cap, because dirt can prevent a proper seal. Replace cracked or flattened gaskets and any cap that no longer clicks or tightens securely.