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Why This Filler Neck Still Matters in 2026

If you own a 1999–2004 Ford F-150 or F-250 with the 4.6L or 5.4L SOHC Triton V8, the fuel tank filler neck is one of the most failure-prone parts in the entire evaporative emissions chain — and one of the easiest to ignore. Two decades of road salt, moisture, and debris behind the rear wheel well take their toll on every factory neck out there.

The stakes are higher now. Tightened EPA evaporative emission standards and OBD-II readiness checks flag even marginal EVAP leaks. A corroded or cracked neck can be the difference between a clean pass and a rejection at your state inspection station.

The Direct-Fit Fix

The fix is the direct-fit replacement carrying OEM cross-reference 577-928 — the Fuel Tank Filler Neck For 99-04 Ford F-150 F-250 4.6L 5.4L GAS SOHC 577-928 at Koeep. It restores factory EVAP integrity on these 10th-generation F-Series trucks and fits all cab and bed configurations on the SOHC gas V8.

577-928 Spec Sheet

What the Filler Neck Actually Does

On these trucks the neck is far more than a pipe. It is an integral part of Ford's Enhanced EVAP system, which the EPA holds to a sealed tolerance of 0.020 inches — roughly 0.5 mm of effective leak diameter.

When the onboard diagnostic runs its EVAP monitor — during steady cruising or after a cold-start drive cycle — it pressurizes or vacuums the entire fuel vapor containment system. One rust pinhole anywhere on that path trips a DTC and lights the check engine light.

Four Ways These Necks Fail

• Rust-through at the lower bend. The lowest point, where condensation collects, perforates first.
• Hose degradation at the tank junction. The rubber connector to the tank hardens and cracks, opening a second leak path.
• Vent tube corrosion. The smaller vapor line beside the main tube corrodes just as fast.
• Cap seal failure. The cam-on cap's O-ring flattens, compounding neck leaks with a poor seal.

DTC Codes That Point at the Neck

Many states now query Mode $06 data during enhanced OBD-II plug-in inspections. Replacing the neck preemptively with the Koeep 577-928 replacement unit removes that variable entirely.

Will It Fit Your Truck?

The 577-928 covers four combinations, all model years 1999–2004:

• Ford F-150 — 4.6L SOHC V8 (gas)
• Ford F-150 — 5.4L SOHC V8 (gas)
• Ford F-250 Light Duty — 4.6L SOHC V8 (gas)
• Ford F-250 Light Duty — 5.4L SOHC V8 (gas)

The 2004 F-150 Heritage Edition is covered too — it is mechanically identical to the 2003 F-150 (10th generation), not the all-new 11th-generation 2004 truck, and keeps the cam-on neck.

Three exclusions: no fit on the 4.2L OHV V6, the 5.4L BI-FUEL (CNG/gasoline) variant, or Super Duty F-250/F-350 platforms. Verify cam-on vs. threaded before you order.

Running a newer Super Duty? Our 10PCS fuel injector set (5C3Z-9F593-AA) fits the 2005–2010 F-250/F-350 Super Duty 6.8L. And if your F-150 runs rough while you chase that EVAP code, test the MAP sensor DV2A9F479AC for the Ford F-150 and Transit 150/250/350.

Installation: Steps and Standards

A manageable DIY job with basic hand tools and jack stands. Long-term EVAP compliance lives in the details:

1. Depressurize the fuel system. Pull the fuel pump relay, run the engine until it stalls, then disconnect the negative battery terminal.
2. Get access. Remove the rear driver-side wheel and inner fender liner; lowering the spare tire adds clearance on some trucks.
3. Label and disconnect the vent and vapor lines. The vapor vent line runs parallel to the main tube and joins the charcoal canister line forward of the tank.
4. Replace the tank-junction hose at the same time. Never reuse a 20+ year-old rubber coupler.
5. Torque the clamps. Stainless fuel-grade worm-gear clamps at 25–35 in-lbs are the 2026 industry recommendation.
6. Smoke test before clearing codes. Confirm zero leakage at the new connections with a shop or consumer smoke machine.

Hoses, clamps, and every other part in this system live in our Air & Fuel Delivery collection. Need something we do not stock? Send a request for quote and we will help you track it down.

The 2026 Emissions Landscape

Tier 3 evaporative emission standards are fully phased in across all 50 states as of the 2026 model year. They mainly target new vehicle certification, but states are adopting stricter EVAP integrity thresholds for vehicles model year 1996 and newer.

A 1999–2004 F-150 that squeaked through a tailpipe-only test a decade ago now faces a full OBD-II EVAP monitor check. Any DTC in the P0440–P0457 range gets flagged.

Treat the swap as a compliance investment, not just a repair. A fresh neck extends your 10th-generation F-Series well into the 2026–2030 window — the 577-928 unit from Koeep is built for exactly that job.

One More Project Idea

Not every job lives under the truck. If you also daily a sedan, see how a 5D carbon fiber center console sticker transforms a Ford Fusion interior in our step-by-step guide.

Frequently Asked Questions

Ready to fix the EVAP leak and pass your 2026 inspection?

Shop the 577-928 Filler Neck at Koeep