Overview
The cooling system in 1973-1987 GM Square Body trucks equipped with the 350 V8 is a critical component that requires careful attention, especially as these vehicles age and are often subjected to more demanding use than originally designed for. Whether you're dealing with an original small block 350, a later TBI 350, or have swapped in a performance 350, upgrading the cooling system ensures reliable operation and protects your investment.
This comprehensive guide covers radiator upgrades, fan conversions, thermostat modifications, and supporting cooling system components specifically for C10, C20, K10, K20, K5 Blazer, and Suburban models. Modern upgrades can significantly improve cooling efficiency while maintaining the classic appearance and functionality these trucks are known for.
Common Issues
Square Body trucks commonly experience several cooling-related problems that upgrades can address:
Original Radiator Limitations
- 2-row brass/copper radiators in most 1973-1980 trucks are marginal for modern driving conditions
- Single-row radiators in some base model trucks are inadequate for sustained highway driving
- Clogged cooling passages from decades of use and poor maintenance
- End tank failures where plastic meets metal in later aluminum radiators
Inadequate Airflow
- Mechanical fan limitations - original 4-blade fans move insufficient air at idle
- Clutch fan failures - thermal clutches often stick or fail completely
- Shroud problems - missing, cracked, or improperly fitted shrouds reduce efficiency
Thermostat and Flow Issues
- Incorrect thermostat ratings - many trucks have wrong temperature thermostats installed
- Bypass flow problems - restricted coolant flow during warm-up
- Water pump inefficiency - original pumps may have reduced flow capacity
Parts & Materials Needed
Core Upgrade Components
Radiator Options:
- Budget: 3-row brass/copper radiator - LMC Truck #40-8068 (1973-1980)
- Premium: Aluminum radiator - Champion Cooling CC162 or Griffin 1-25221-X
- Performance: Dual-pass aluminum - US Radiator USRA2582 for high-output engines
Electric Fan Conversion:
- Single 16" fan: Flex-a-lite 116 (2800 CFM) for stock applications
- Dual fan setup: Derale 16925 (3200 CFM combined) for performance builds
- Heavy-duty single: SPAL 30102120 (2650 CFM, pusher configuration)
Fan Control Systems:
- Basic thermostat switch: Davies Craig 0444 (185°F on, 176°F off)
- Adjustable controller: Flex-a-lite 31147 with manual override
- ECM-controlled: Painless Performance 30116 for TBI applications
Supporting Components
Thermostats:
- Standard duty: 180°F Stant 13478 for carbureted engines
- Heavy duty: 160°F Robert Shaw 333-160 for performance applications
- TBI engines: 195°F AC Delco 131-158 (required for proper fuel system operation)
Water Pumps:
- High-flow replacement: Edelbrock 8851 (35% more flow than stock)
- Performance: Stewart Components 13435 (billet aluminum)
- OEM replacement: AC Delco 252-713 for stock applications
Additional Components:
- Coolant recovery tank: LMC Truck #40-8130 (pressurized system)
- Radiator cap: 16 PSI Stant 10230 for upgraded systems
- Coolant hoses: Gates 21404 (upper), Gates 21401 (lower) for 350 V8
Tools Required
- Socket set (10mm-19mm, 3/8"-3/4")
- Drain pan (minimum 4-gallon capacity)
- Wire crimping tools and electrical connectors
- Multimeter for electrical testing
- Torque wrench (20-100 ft-lbs range)
Step-by-Step Guide
Phase 1: Radiator Upgrade
1. System Preparation
- Park on level ground, engine cold
- Remove radiator cap and drain plug
- Drain coolant completely (approximately 3.5 gallons)
- Remove upper and lower radiator hoses
2. Original Radiator Removal
- Disconnect transmission cooler lines (automatic transmission models)
- Remove fan shroud mounting bolts (4x 10mm bolts)
- Remove radiator mounting bolts (2x upper, 1973-1980 use 7/16" bolts)
- Lift radiator straight up and out
3. New Radiator Installation
- Install new radiator with supplied mounting hardware
- Torque specifications: Upper mounts to 15 ft-lbs, lower cushions hand-tight
- Connect transmission lines using new O-rings
- Install new radiator cap rated for upgraded system pressure
Phase 2: Electric Fan Conversion
1. Mechanical Fan Removal
Fan clutch removal torque: 36 ft-lbs (reverse thread on most applications)
Water pump pulley bolts: 20 ft-lbs
Fan spacer (if equipped): varies by year
2. Electric Fan Mounting
- Pusher configuration: Mount fan on front of radiator using supplied brackets
- Puller configuration: Mount between radiator and engine (preferred for most applications)
- Ensure minimum 1" clearance between fan blades and radiator core
3. Wiring Installation
- Route power wire through firewall using existing grommet
- Install 30-amp relay in fuse box or auxiliary location
- Connect temperature sender to radiator (1/8" NPT fitting)
- Wire gauge requirements: 12 AWG minimum for fan power, 16 AWG for control circuits
Phase 3: System Integration
1. Thermostat Replacement
- Remove thermostat housing (2x 12mm bolts, 25 ft-lbs torque)
- Clean mating surfaces thoroughly
- Install new thermostat with spring toward engine
- Use new gasket with RTV sealant on both sides
2. Water Pump Upgrade (if applicable)
- Remove all accessories and brackets
- Bolt torque sequence: 30 ft-lbs in cross pattern
- Apply thread sealant to bolts entering water passages
- Install new gaskets without sealant
3. System Fill and Testing
- Fill with 50/50 coolant mixture (approximately 3.5-4 gallons total)
- Burp system by running engine with radiator cap off until thermostat opens
- Test fan operation at 185°F (adjustable controllers may vary)
- Check for leaks under full operating pressure
Year/Model Differences
1973-1980 Models
Radiator Specifications:
- C10/K10: 2-row standard, 22" wide core
- C20/K20: 3-row available, 22" wide core
- K5 Blazer: 2-row, 20" wide core (unique size)
- Suburban: 3-row standard, 22" wide core
Mounting Differences:
- Upper radiator mounts use 7/16" bolts with rubber isolators
- Fan shroud mounts directly to radiator with metal clips
- Mechanical fan uses 4-blade design with thermal clutch
1981-1987 Models
Radiator Specifications:
- C10/K10: Aluminum radiator available, plastic end tanks
- C20/K20: Heavy-duty 3-row standard on diesel, optional on gas
- K5 Blazer: Continues 20" wide core through 1987
- Suburban: Larger capacity radiator on 3/4-ton models
System Changes:
- Pressurized coolant recovery system standard
- Improved fan clutch design with better thermal response
- TBI engines (1987) require 195°F thermostat for proper operation
Model-Specific Considerations
4WD vs 2WD:
- K-series trucks have different fan clearance requirements
- Transfer case adds heat load requiring upgraded cooling
- Skid plates may interfere with electric fan mounting
Air Conditioning:
- A/C equipped trucks have auxiliary cooling fans on some models
- Condenser placement affects airflow through radiator
- Additional electrical load must be considered for fan upgrades
Tips & Best Practices
Installation Best Practices
Radiator Selection:
- Aluminum radiators provide 40% better heat transfer than brass/copper
- Core thickness matters more than row count - 2" thick dual-pass outperforms 4-row traditional
- End tank design - welded aluminum tanks are more reliable than plastic
Electric Fan Optimization:
- Shrouding is critical - fan should cover 70% of radiator core area
- Puller configuration is 15% more efficient than pusher
- Variable speed controllers reduce electrical load and noise
Coolant System Maintenance:
- Use distilled water only for mixing coolant
- Flush system every 3 years or 30,000 miles minimum
- Pressure test annually to catch small leaks before they become major problems
Performance Tuning
Temperature Management:
- 160°F thermostat for performance applications (carbureted only)
- 180°F thermostat for street driving balance
- 195°F thermostat mandatory for TBI fuel injection systems
Flow Optimization:
- High-flow water pump can reduce coolant temperature 10-15°F
- Remove thermostat bypass restriction on performance builds
- Radiator placement - seal gaps around radiator to force air through core
Common Mistakes to Avoid
Electrical Issues:
- Insufficient wire gauge causes voltage drop and fan performance loss
- Poor ground connections lead to intermittent operation
- Inadequate relay sizing results in contact failure
Installation Errors:
- Backwards thermostat prevents proper coolant flow
- Over-tightening radiator mounts causes stress cracks
- Missing fan shroud reduces cooling efficiency by up to 30%
Troubleshooting
Overheating Diagnosis
Temperature Gauge Reading High:
-
Verify actual temperature with infrared thermometer
- Normal operating range: 180-210°F
- Gauge accuracy varies significantly in older trucks -
Check coolant level and pressure
- System should hold 16 PSI pressure test
- Low coolant level indicates leak or inadequate fill -
Test thermostat operation
- Remove and test in hot water with thermometer
- Should open within 5°F of rated temperature
Electric Fan Not Operating:
-
Check fuse and relay
- 30-amp fuse should not be blown
- Relay should click when temperature switch activates -
Test temperature switch
- Switch should close circuit at approximately 185°F
- Use multimeter to verify continuity when hot -
Verify fan motor operation
- Apply 12V directly to fan motor
- Motor should draw 12-18 amps maximum
Performance Issues
Insufficient Cooling at Highway Speed:
- Radiator capacity inadequate - upgrade to larger core
- Airflow restriction - check for debris, damaged shrouding
- Internal engine issues - compression test may reveal problems
Overcooling in Cold Weather:
- Thermostat stuck open or incorrect temperature rating
- Fan running continuously - check temperature switch calibration
- Missing radiator blocking - install winter front if needed
Coolant Loss Without Visible Leaks:
- Pressure cap failure - test cap hold pressure
- Internal combustion leak - perform combustion gas test
- Overflow system problems - check recovery tank operation
System Optimization
Fine-Tuning Temperature Control:
- Adjust fan switch temperature for local climate
- Consider dual-stage fan control for better modulation
- Monitor coolant temperature with aftermarket gauge for accuracy
Addressing Persistent Issues:
- Flow testing can reveal internal restrictions
- Infrared imaging shows hot spots indicating problems
- Professional diagnosis recommended for complex cooling issues
This comprehensive cooling system upgrade approach will significantly improve the reliability and performance of your Square Body truck's 350 V8, ensuring many more years of dependable service while maintaining the classic character these trucks are known for.