Ultimate OBS (GMT400) LS Swap Fuel System Guide — 1988–1998 Chevy/GMC

C1500/K1500 • Tahoe/Yukon • Suburban • 2-Door Blazer

Planning an OBS LS swap in your 1988–1998 GMT400 Chevy/GMC? This step-by-step guide delivers a clean, repeatable path to a leak-free LS fuel system on GMT400 trucks — covering the correct OBS sending-unit adapters, in-tank vs. inline pumps, and both return-less and return-style layouts with AN-6 PTFE hose.

Table of Contents

• OBS/GMT400 vs. Squarebody: What’s Different for an LS swap
• Choose Your OBS LS swap Layout: Return-less or Return-Style
• GMT400 LS swap Fuel Pumps (In-Tank & Inline) & Flow Guidance
• AN Hose Sizing, Lengths & Routing for OBS/GMT400 LS swaps
• OBS LS swap Parts Lists (by layout)
• Step-by-Step Installation — OBS/GMT400
• Best Practices, Filtration & Safety — LS fuel systems
• FAQ — OBS / GMT400 / LS swap

OBS/GMT400 vs. Squarebody: What’s Different for an LS swap

• EFI tank & baffles (GMT400): Reuse the factory EFI-ready tank on an OBS LS swap.
• Sender outlets are metric: Feed = M16×1.5, Return = M14×1.5. Convert both to AN-6 with:
  OBS/GMT400 M14 & M16 → AN-6 Sender Adapter Fittings
• Frame space (OBS): Ideal for a rear-mounted filter/regulator in a return-less GMT400 LS swap.

Choose Your OBS LS swap Layout: Return-less or Return-Style

Option A — Return-Less (Most OBS/GMT400 builds)

One AN-6 feed to the rail. Pressure is controlled near the tank with a Corvette-style ~58 PSI filter/regulator; excess fuel returns at the rear. Cleaner OBS LS swap engine bay, short return line.

Return-Less LS Fuel Line Kit (Great for OBS/GMT400)

Option B — Return-Style (Dual-Line Rails)

AN-6 feed and AN-6 return run to the engine bay. Use a ≤10 µm EFI filter on the pressure side; the rail/regulator manages return. Preferred for adjustable/boosted LS fuel systems.

Return-Style LS Fuel Line Kit (OBS/GMT400 Ready)    Factory-Style Inline EFI Filter (≤10 µm)

GMT400 LS swap Fuel Pumps (In-Tank & Inline) & Flow Guidance

In-tank is preferred on OBS/GMT400 LS swaps (cooler, quieter, reliable). Inline can work if mounted low with a good pre-filter.

*Gasoline, NA. Boost/E85 need more flow; verify injector/BSFC.

Shop Fuel Pumps for OBS/GMT400 LS Swaps

AN Hose Sizing, Lengths & Routing for OBS/GMT400 LS swaps

• Size: AN-6 PTFE for feed (and return if used) suits most GMT400 LS swaps to ~600 HP.
• Hose type: PTFE core handles modern fuels/ethanol; zero vapor smell.
• Lengths (typical OBS): Return-less ~18–20 ft; Return-style ~25–30 ft total (measure your truck).
• Routing: Along frame, away from heat/sharp edges; cushioned clamps every ~12–18″.

PTFE Hose & Fitting Bundles (AN-6)    Extra PTFE Hose Ends

OBS LS swap Parts Lists (by layout)

Return-Less (Rear-Mounted Filter/Regulator)

1. OBS Sender Adapters: M14×1.5 & M16×1.5 → AN-6 (OBS/GMT400)
2. Fuel Pump (in-tank or inline): Browse Pumps
3. Hose & Fittings: PTFE Bundles & PTFE Ends (AN-6)
4. Kit Option: Return-Less LS Fuel Line Kit (OBS/GMT400)
5. Tools: AN Hose Install Tools

Return-Style (Dual-Line Rails)

1. OBS Sender Adapters: M14×1.5 & M16×1.5 → AN-6 (OBS/GMT400)
2. Fuel Pump: In-Tank & Inline Options
3. Inline EFI Filter (≤10 µm): Factory-Style Inline Filter (AN-6)  •  Replacement Element
4. Hose & Fittings: PTFE Bundles & PTFE Ends (AN-6)
5. Kit Option: Return-Style LS Fuel Line Kit (OBS/GMT400)
6. Tools: AN Tools

Step-by-Step Installation — OBS/GMT400

1) Prep & Sender Setup

1. Disconnect battery; safely relieve fuel pressure. Work ventilated; keep a class B extinguisher at hand.
2. Drop the tank if needed; remove the sender; inspect tank/strainer.
3. Install OBS/GMT400 adapters:
   Feed: M16×1.5 → AN-6  •  Return: M14×1.5 → AN-6
   Get the OBS/GMT400 Sender Adapter Kit
4. Lightly oil O-rings; snug to seal (do not over-torque).

2) Choose & Fit the Pump (GMT400)

• In-Tank: 190/255/340 LPH based on HP; confirm wiring/connector; install new seal.
• Inline: Mount low/near tank; add coarse pre-filter; isolate from heat; expect more noise.

Shop Pumps for OBS/GMT400 LS Swaps

3) Plumb the System — OBS LS swap

Return-Less Layout (Rear-Mounted Filter/Reg)

1. Sender feed (AN-6) → filter/regulator IN (rear frame)
2. Filter/regulator OUT → engine rail feed (AN-6)
3. Filter/regulator RETURN → sender return (AN-6)

Return-Style Layout (Dual Lines)

1. Sender feed (AN-6) → ≤10 µm inline filter → rail feed
2. Rail return (AN-6) → sender return

4) Finalize & Test the LS fuel system

• Secure lines every ~12–18″; avoid sharp bends (use 45°/90° ends); keep away from heat.
• Prime (key-on cycles) and leak-check before first start; verify ~58 PSI at the rail.
• Re-check after first heat cycle; snug fittings as needed.

Best Practices, Filtration & Safety — LS fuel systems

• Filter spec: ≤10 µm on the pressure side for EFI injectors.
• PTFE cutting tip (OBS installers): Wrap the cut with tape before shears/cutoff to control fray; clean/deburr the liner.
• Vent correctly: Don’t cap the tank vent. For custom vents: Rollover Valve (AN-6/-8/-10)
• Adapters you’ll likely need: AN Adapter Fittings  •  Hardline-to-AN Adapters
• Tools: Aluminum AN wrenches & soft vise jaws: AN Hose Install Tools

FAQ — OBS / GMT400 / LS swap

Do I need a new tank for an OBS LS swap?
No — the GMT400 tank is EFI-ready with baffles. Clean/inspect; replace only if damaged.

What hose size is right for a GMT400 LS swap?
AN-6 PTFE feed (and return if used) fits most builds to ~600 HP; upsize only for extreme power/E85.

Return-less vs. return-style for an OBS LS fuel system?
Return-less (rear regulator) keeps the bay clean and mirrors OEM. Return-style is ideal for rails with built-in return or adjustable/boosted setups.

Target fuel pressure on LS swaps?
Most Gen III/IV target ~58 PSI. Verify at first start with a gauge.

Build your OBS/GMT400 LS swap fuel system once — and build it right.

Shop Return-Less LS Kit (OBS/GMT400)    Shop Return-Style LS Kit (OBS/GMT400)    Get OBS/GMT400 Sender Adapters

Need help choosing parts for your OBS LS swap? Visit the AN Hose 101 hub or our AN/PTFE installation instructions. We’re happy to help dial in your exact setup.