Fuel System Overhaul and Maintenance - Square Body Trucks

Overview

The fuel system in 1973-1987 GM Square Body trucks underwent significant changes during the production run, transitioning from carbureted systems to throttle body injection (TBI) and incorporating various emissions control components. Understanding these systems is crucial for proper maintenance, troubleshooting, and restoration of C10, C20, K10, K20, K5 Blazer, and Suburban models.

The fuel system consists of the fuel tank, sending unit, fuel lines, fuel pump (mechanical or electric), fuel filter, and carburetor or fuel injection system. Proper fuel system maintenance ensures reliable operation, optimal fuel economy, and prevents costly engine damage from contaminated fuel or lean/rich running conditions.

Common Issues

Fuel Tank Problems

• Rust and corrosion in steel tanks, particularly around the filler neck and bottom seams
• Fuel tank strap failure due to corrosion, causing tank to sag or drop
• Sending unit failure resulting in inaccurate fuel gauge readings
• Fuel pickup sock clogging from debris and varnish buildup

Fuel Pump Issues

• Mechanical pump diaphragm failure (carbureted models) causing fuel starvation
• Electric pump failure (1981+ TBI models) often due to running tank dry
• Fuel pump relay problems on TBI-equipped trucks

Fuel Line Deterioration

• Rubber fuel lines cracking and developing leaks, especially near heat sources
• Steel line corrosion particularly problematic in salt-belt regions
• Vapor lock issues in hot weather with improperly routed lines

Carburetor/TBI Problems

• Carburetor flooding or lean running due to worn components
• TBI injector clogging from fuel varnish and deposits
• Fuel pressure regulator failure on TBI systems

Parts & Materials Needed

Basic Fuel System Service

• Fuel filter: AC Delco GF61 (carbureted), GF652 (TBI)
• Fuel pump: Mechanical - AC Delco 40972 (small block), 41072 (big block)
• Electric fuel pump: AC Delco EP381 (in-tank TBI)
• Fuel tank sending unit: Spectra Premium FG34A (19-gallon), FG34B (20-gallon)

Fuel Tank Restoration

• Fuel tank: Spectra Premium GM34A (short bed), GM34B (long bed)
• Tank straps: Dorman 578-034 (pair)
• Filler neck: Spectra Premium FN34A
• Tank sealer: POR-15 Fuel Tank Sealer Kit or Caswell Plating Tank Tone

Fuel Lines and Hardware

• 3/8" fuel line (supply): Gates 27006 or equivalent SAE J30R9 rated
• 5/16" fuel line (return on TBI): Gates 27005
• Fuel line clamps: Oetiker stepless clamps or original GM tower clamps
• Quick-disconnect fittings: AC Delco 25163353 (3/8"), 25163352 (5/16")

Tools Required

• Fuel line disconnect tools
• Fuel tank strap wrench or large adjustable wrench
• Fuel pressure gauge (0-15 PSI)
• Safety glasses and nitrile gloves
• Fire extinguisher

Step-by-Step Guide

Fuel System Inspection and Service

1. Safety Preparation

⚠️ WARNING: Always disconnect battery, ensure adequate ventilation, and have fire suppression equipment available when working with fuel systems.

1. Park vehicle on level ground and engage parking brake
2. Disconnect negative battery terminal
3. Relieve fuel pressure (TBI models): Remove fuel pump fuse, start engine until it stalls
4. Allow engine to cool completely

2. Fuel Filter Replacement

Carbureted Models (1973-1980):
1. Locate filter in carburetor inlet fitting or along frame rail
2. Use two wrenches to prevent carburetor damage when removing inlet fitting
3. Replace filter with flow direction arrow pointing toward carburetor
4. Torque inlet fitting to 18-22 ft-lbs

TBI Models (1981-1987):
1. Locate filter along driver-side frame rail
2. Clean fittings before removal to prevent contamination
3. Use fuel line disconnect tools to remove quick-connect fittings
4. Install new filter with arrow pointing toward engine
5. Ensure quick-connect fittings click securely into place

3. Fuel Pump Testing and Replacement

Mechanical Pump Testing (Carbureted):
1. Disconnect fuel line at carburetor
2. Crank engine while observing fuel flow - should produce strong, steady stream
3. Test fuel pressure: Connect gauge to pump outlet - should read 4-6 PSI at idle

Mechanical Pump Replacement:
1. Remove fuel lines and mark for proper reconnection
2. Remove two mounting bolts from engine block
3. Clean gasket surfaces thoroughly
4. Install new pump with gasket, torque bolts to 25-30 ft-lbs
5. Reconnect fuel lines and test for leaks

Electric Pump Testing (TBI):
1. Connect fuel pressure gauge to test port on fuel rail
2. Turn ignition to "ON" (don't start) - pressure should read 9-13 PSI
3. Start engine - pressure should remain constant
4. Turn off ignition - pressure should hold for several minutes

4. Fuel Tank Service

Tank Removal:
1. Drain fuel tank completely using approved pump and container
2. Disconnect filler neck, vent lines, and electrical connections
3. Support tank with floor jack and wood block
4. Remove tank strap bolts and lower tank carefully
5. Remove sending unit using special wrench or brass punch and hammer

Tank Inspection and Cleaning:
1. Inspect for rust, dents, and leaks
2. Clean exterior with degreaser and wire brush
3. For internal cleaning, use tank cleaning solution or professional service
4. Apply fuel tank sealer if internal rust is present

Sending Unit Service:
1. Replace fuel pickup sock and O-ring seal
2. Test sending unit resistance: Empty = 90 ohms, Full = 0 ohms
3. Bend float arm if necessary to achieve proper resistance values

Fuel Line Replacement

Steel Line Replacement

1. Support vehicle securely on jack stands
2. Remove damaged section using tubing cutter
3. Install new steel line using proper bending techniques to avoid kinks
4. Use double-flare fittings for all connections
5. Secure lines every 24" with proper clamps

Rubber Line Service

1. Replace all rubber lines every 10 years or when cracking is visible
2. Use only SAE J30R9 rated fuel injection hose for TBI systems
3. Install clamps 1/4" from hose ends
4. Route lines away from exhaust components with minimum 4" clearance

Year/Model Differences

1973-1980: Carbureted Systems

• Fuel pump: Engine-driven mechanical pump
• Fuel pressure: 4-6 PSI
• Fuel lines: Single supply line (3/8")
• Tank capacity: 16-gallon (short bed), 20-gallon (long bed), 25-gallon (Suburban)
• Emissions: Basic evaporative controls, charcoal canister

1981-1987: Throttle Body Injection (TBI)

• Fuel pump: Electric in-tank pump
• Fuel pressure: 9-13 PSI
• Fuel lines: Supply (3/8") and return (5/16") lines
• Tank capacity: 16-gallon (short bed), 20-gallon (long bed), 25-gallon (Suburban)
• Emissions: Computer-controlled system with oxygen sensor feedback

Model-Specific Variations

K-Series (4WD) Models:
- Transfer case may require fuel line rerouting
- Skid plates can complicate tank removal
- Additional ground clearance considerations for line routing

K5 Blazer/Suburban:
- Rear-mounted spare tire affects tank access on some models
- Longer wheelbase Suburbans use 25-gallon tanks
- Side-mount spare tire models have different filler neck routing

Tips & Best Practices

Fuel Quality Management

• Use quality fuel from high-turnover stations
• Add fuel stabilizer for vehicles stored over 30 days
• Replace fuel filter annually or every 12,000 miles
• Keep tank at least 1/4 full to prevent condensation and pump overheating

Installation Best Practices

• Always use proper fuel-rated components - never substitute automotive hose
• Double-check all connections before starting engine
• Prime fuel system properly after service to prevent air locks
• Test fuel pressure after any fuel system work

Preventive Maintenance

• Inspect fuel lines annually for cracks, chafing, and proper routing
• Clean tank periodically especially if vehicle sits for extended periods
• Replace fuel pump every 100,000 miles as preventive maintenance
• Service sending unit when fuel gauge becomes erratic

Common Mistakes to Avoid

• Using incorrect fuel line materials (vacuum hose, heater hose)
• Over-torquing fuel line fittings causing damage
• Failing to relieve fuel pressure before service
• Installing fuel filter backwards
• Reusing old fuel line clamps

Troubleshooting

Engine Won't Start - Fuel Related

Symptoms: Engine cranks but won't start, no fuel smell
Diagnosis:
1. Check for fuel in tank (don't rely on gauge)
2. Listen for fuel pump operation (TBI models)
3. Check fuel pressure at rail or carburetor
4. Inspect fuel filter for clogging

Solutions:
- Replace clogged fuel filter
- Replace failed fuel pump
- Clean fuel tank and lines if contaminated

Poor Fuel Economy

Symptoms: Significantly reduced MPG, rich exhaust smell
Diagnosis:
1. Check fuel pressure - high pressure causes rich running
2. Inspect for external fuel leaks
3. Test fuel pressure regulator (TBI models)
4. Check carburetor float level (carbureted models)

Solutions:
- Replace fuel pressure regulator
- Adjust carburetor float level
- Repair fuel leaks
- Service or rebuild carburetor

Fuel Gauge Inaccurate

Symptoms: Gauge reads empty when tank has fuel, or doesn't move
Diagnosis:
1. Test sending unit resistance with multimeter
2. Check wiring continuity from tank to gauge
3. Test gauge with known good sending unit

Solutions:
- Replace sending unit
- Repair wiring connections
- Replace instrument cluster gauge

Fuel Starvation at High RPM

Symptoms: Engine cuts out under acceleration, runs fine at idle
Diagnosis:
1. Check fuel pump pressure under load
2. Inspect fuel pickup sock for clogging
3. Test fuel pump volume (should flow 1 pint in 30 seconds)

Solutions:
- Replace fuel pump
- Clean or replace pickup sock
- Clear fuel line restrictions

Vapor Lock Issues

Symptoms: Engine stalls in hot weather, hard starting when hot
Diagnosis:
1. Check fuel line routing near exhaust components
2. Verify proper heat shielding
3. Test fuel pressure when hot vs. cold

Solutions:
- Reroute fuel lines away from heat sources
- Install heat shields on fuel lines
- Replace mechanical pump with electric (carbureted models)
- Add fuel line insulation