Engine Swap Fuel System Modifications - 350 and 454 V8 Swaps - Square Body Trucks

Overview

When performing engine swaps in 1973-1987 GM Square Body trucks, the fuel system often requires significant modifications to properly support the new powerplant. Whether upgrading from a straight-six to a V8, swapping between different V8 configurations, or installing a modern fuel-injected engine, understanding fuel system requirements is critical for reliable operation and performance.

This guide covers fuel system considerations for 350 Small Block and 454 Big Block V8 swaps in C10, C20, K10, K20, K5 Blazer, and Suburban models. Key areas include fuel pump selection, line routing, tank modifications, and carburetor/fuel injection compatibility.

Why This Matters:
- Inadequate fuel delivery causes poor performance, hesitation, and potential engine damage
- Incorrect fuel pressure can damage carburetors or fuel injection systems
- Proper line routing prevents vapor lock and maintains consistent fuel flow
- Safety considerations prevent fuel leaks and fire hazards

Common Issues

Fuel Pump Inadequacy

• Mechanical pump limitations: Stock mechanical pumps may not flow sufficient volume for higher-performance engines
• Pressure mismatch: Carbureted engines typically require 4-7 PSI, while TBI systems need 9-13 PSI
• Vapor lock: Inadequate fuel flow or poor line routing causes fuel vaporization in hot conditions

Fuel Line Problems

• Insufficient diameter: Stock 5/16" lines may restrict flow for high-performance applications
• Poor routing: Lines too close to exhaust components cause vapor lock
• Deteriorated rubber sections: Old fuel hoses crack and leak, creating safety hazards

Tank and Sender Issues

• Incompatible fuel level senders: Different resistance ranges between carbureted and fuel-injected applications
• Inadequate pickup design: Stock pickups may not support high-flow fuel pumps
• Venting problems: Improper tank venting causes fuel starvation or tank collapse

Parts & Materials Needed

Fuel Pumps

Mechanical Pumps (Carbureted Applications)

• Standard Flow: AC Delco EP158 (350 SBC), EP159 (454 BBC)
• High Performance: Carter M4891 (110 GPH), Edelbrock 1712 (130 GPH)

Electric Pumps

• Low Pressure (Carb): Holley 12-812-1 (97 GPH @ 7 PSI)
• High Pressure (TBI): AC Delco EP381 (in-tank), Walbro GSS342 (255 LPH)
• External Universal: Carter P4070 (72 GPH @ 6 PSI)

Fuel Lines and Fittings

• 3/8" Steel Tubing: NAPA FLT 3838 (25ft roll) for main feed lines
• 5/16" Steel Tubing: NAPA FLT 3136 (25ft roll) for return lines
• Rubber Fuel Hose: Gates 27034 (3/8" x 3ft), Gates 27032 (5/16" x 3ft)
• Clamps: Ideal 6708 series stainless steel clamps
• Fittings: Russell 640360 (3/8" to 3/8" NPT), Holley 26-35 (fuel log kit)

Filters and Regulators

• Inline Filter: WIX 33040 (3/8" connections), AC Delco GF652
• Fuel Pressure Regulator: Holley 12-803 (4.5-9 PSI), Edelbrock 1723 (1-4 PSI)
• Return System Kit: Aeromotive 13109 (complete returnless to return conversion)

Tools Required

• Tubing bender and cutter
• Flaring tool (double flare for steel lines)
• Fuel pressure gauge (0-15 PSI)
• Safety glasses and fire extinguisher
• Jack stands and wheel chocks

Step-by-Step Guide

Phase 1: Assessment and Planning

1. Determine Fuel Requirements

Engine Type          Flow Rate (GPH)    Pressure (PSI)
Stock 350 SBC        60-80              4-7
Performance 350      80-120             4-7
Stock 454 BBC        80-100             4-7
Performance 454      120-180            4-7
TBI 350              40-60              9-13

2. Evaluate Existing System

• Test current fuel pump pressure with mechanical gauge
• Inspect all fuel lines for deterioration or damage
• Check tank condition and pickup tube integrity
• Verify electrical connections for electric pump applications

Phase 2: Fuel Pump Installation

Mechanical Pump Installation (Carbureted)

1. Remove old pump - Clean mounting surface on engine block
2. Install new gasket - Use Fel-Pro 70691 or equivalent
3. Mount pump - Torque bolts to 25 ft-lbs in cross pattern
4. Connect lines - Inlet from tank, outlet to carburetor
5. Prime system - Crank engine briefly to fill pump

Electric Pump Installation

1. Choose location - Mount near tank, below fuel level when possible
2. Install mounting bracket - Use vibration-isolating mounts
3. Wire pump - 12V switched power through relay (Bosch 75A relay recommended)
4. Install pressure regulator - Mount in engine bay with return line to tank
5. Add inline filter - Between pump and regulator

Phase 3: Fuel Line Modification

Upgrading Line Size

1. Remove stock 5/16" lines - Support vehicle safely, drain fuel system
2. Route new 3/8" feed line - Follow original path, maintain 6" clearance from exhaust
3. Install return line - 5/16" from regulator to tank (fuel-injected applications)
4. Secure lines - Use proper clips every 24" maximum
5. Pressure test - 10 PSI for 30 minutes, check for leaks

Vapor Lock Prevention

• Insulate lines near exhaust components with DEI 010101 heat sleeve
• Route lines along frame rails away from engine heat
• Install heat shield between lines and exhaust if clearance is minimal

Phase 4: Tank and Sender Modifications

Return Line Addition (TBI Applications)

1. Install return fitting - Weld 1/4" NPT bung in tank top
2. Add pickup tube - Extend to tank bottom, 1" from floor
3. Modify filler neck - Install rollover valve if required

Fuel Level Sender Compatibility

• Carbureted applications: 0-30 ohm or 0-90 ohm (check gauge)
• TBI applications: 0-90 ohm standard
• Universal sender: Classic Instruments SN58 (adjustable resistance)

Year/Model Differences

1973-1980 Models

• Mechanical pumps standard on all V8 applications
• 5/16" fuel lines adequate for stock engines
• Vented fuel caps - no vapor recovery system
• Steel tanks in most applications (some plastic in later years)

Specific Considerations:
- 1973-1974 models may have larger fuel lines (3/8") on 454 applications
- K-series 4WD models have frame-mounted tanks requiring different pump mounting
- Early C20/K20 models use dual tanks with manual selector valve

1981-1987 Models

• Electric pumps introduced on some applications (primarily California models)
• Computer-controlled carburetors (CCC) require specific fuel pressure ranges
• Vapor recovery systems complicate tank modifications
• Plastic tanks more common, especially in later years

TBI System Introduction:
- 1987 model year: First factory TBI on 350 engines
- Fuel pressure: 9-13 PSI required
- Return fuel system: Mandatory for proper operation
- In-tank pump: Preferred installation method

Model-Specific Differences

C10/C20 (2WD)

• Standard cab-mounted tank location
• Easier fuel pump access compared to 4WD models
• Single tank configuration standard

K10/K20 (4WD)

• Frame-mounted tank behind transfer case
• Limited access for pump installation
• Skid plate protection may interfere with modifications

K5 Blazer/Suburban

• Rear-mounted tank under cargo area
• Longer fuel lines required
• Multiple tank options in some Suburban models

Tips & Best Practices

Fuel System Sizing

• Oversize fuel pumps by 20-30% for performance applications
• Use 3/8" lines for any engine producing over 300 HP
• Install bypass regulator even with mechanical pumps for consistent pressure

Installation Best Practices

• Always use proper fuel hose - never use vacuum hose or hydraulic hose
• Double-flare all steel connections for leak-proof joints
• Support fuel lines every 18-24 inches to prevent vibration damage
• Install shut-off valve at tank for maintenance access

Safety Considerations

• Disconnect battery before any fuel system work
• Keep fire extinguisher readily available
• Work in well-ventilated area - fuel vapors are heavier than air
• Use proper grounding when welding tank modifications

Performance Optimization

• Install fuel pressure gauge for monitoring and tuning
• Use braided stainless lines in engine bay for durability
• Consider fuel cooling for high-performance applications
• Maintain proper filter service intervals - replace every 12,000 miles

Troubleshooting

Low Fuel Pressure/Flow

Symptoms:
- Engine hesitation under acceleration
- Stalling during hard cornering
- Poor high-RPM performance

Diagnostic Steps:
1. Test fuel pressure at carburetor/TBI unit
2. Check filter condition - replace if restricted
3. Verify pump operation - listen for pump noise, check current draw
4. Inspect fuel lines for kinks or restrictions

Solutions:
- Replace fuel filter (most common cause)
- Upgrade to higher-capacity fuel pump
- Increase fuel line diameter
- Repair or replace damaged lines

High Fuel Pressure

Symptoms:
- Rich running condition
- Black exhaust smoke
- Carburetor flooding
- Hard starting when hot

Diagnostic Steps:
1. Measure pressure with mechanical gauge
2. Check regulator operation (if equipped)
3. Verify pump specifications against requirements

Solutions:
- Install or adjust fuel pressure regulator
- Replace pump with correct pressure rating
- Add return line system to reduce pressure

Vapor Lock Issues

Symptoms:
- Stalling in hot weather
- Difficulty restarting when hot
- Loss of power during extended driving

Diagnostic Steps:
1. Check fuel line routing near exhaust components
2. Measure underhood temperatures at fuel lines
3. Test for adequate fuel flow when hot

Solutions:
- Reroute fuel lines away from heat sources
- Install heat shielding on fuel lines
- Add electric fuel pump to maintain positive pressure
- Install vapor separator tank

Electrical Issues (Electric Pump Applications)

Symptoms:
- Pump not running
- Intermittent operation
- Blown fuses

Diagnostic Steps:
1. Check fuse and relay operation
2. Test voltage at pump connector
3. Measure pump current draw - should be 4-6 amps typically
4. Verify ground connections are clean and tight

Solutions:
- Replace faulty relay (most common)
- Repair wiring connections
- Clean ground connections
- Replace pump if current draw is excessive