1986 C10 Thermostat Manual

The 1986 Chevrolet C10 thermostat is a crucial engine cooling system component that regulates coolant flow between the engine and radiator. This manual covers thermostat specifications, installation procedures, troubleshooting, maintenance, and replacement guidelines for the 1986 C10 pickup truck models with various engine options including the 4.3L V6, 5.0L V8, and 5.7L V8 engines.

Thermostat Specifications

The 1986 C10 uses a wax pellet type thermostat with specific temperature ratings and dimensions.

Specification	Details
Opening Temperature	195°F (90.5°C)
Fully Open Temperature	212°F (100°C)
Diameter	1.75 inches (44.5mm)
Housing Material	Brass or composite
Gasket Type	Rubber O-ring or paper gasket
Coolant Capacity	14-16 quarts (system total)
Pressure Rating	15-18 PSI radiator cap
Engine Compatibility	4.3L V6, 5.0L V8, 5.7L V8
OE Part Number	GM 12520436
Aftermarket Equivalent	Stant 13548, Motorcraft RT1139

Tools Required

Basic hand tools needed for thermostat replacement.

1. Socket set (3/8" drive) with 8mm, 10mm, 13mm sockets
2. Ratchet and extensions
3. Screwdrivers (flat and Phillips)
4. Drain pan (2 gallon capacity)
5. Pliers (slip-joint and needle-nose)
6. Torque wrench (0-25 ft-lbs)
7. Gasket scraper or putty knife
8. Funnel for coolant refill

WARNING! Always allow engine to cool completely before beginning work to prevent burns.

Removal Procedure

Step-by-step instructions for removing the old thermostat.

1. Park vehicle on level surface and engage parking brake.
2. Allow engine to cool completely (overnight recommended).
3. Place drain pan under radiator drain petcock.
4. Open radiator drain valve and drain approximately 2 gallons of coolant.
5. Locate thermostat housing at front of engine where upper radiator hose connects.
6. Remove upper radiator hose from thermostat housing.
7. Remove two housing bolts (typically 10mm or 13mm).
8. Lift off housing to expose thermostat.
9. Remove old thermostat and clean mating surfaces thoroughly.
10. Inspect housing for cracks or corrosion and replace if necessary.

CAUTION! Do not force housing if stuck - tap gently with rubber mallet.

Installation Steps

Proper installation ensures optimal cooling system performance.

1. Clean thermostat housing and engine block mating surfaces.
2. Install new gasket or O-ring (apply light film of RTV if specified).
3. Position new thermostat with spring side toward engine block.
4. Ensure thermostat jiggle pin is at 12 o'clock position.
5. Reinstall housing and tighten bolts finger-tight.
6. Torque housing bolts to 18-22 ft-lbs in criss-cross pattern.
7. Reconnect upper radiator hose and tighten clamp.
8. Close radiator drain valve securely.

Important: Do not overtighten housing bolts as housing may crack.

Coolant Refill Procedure

Proper coolant filling prevents air pockets and overheating.

• Coolant Type: GM-approved ethylene glycol (green conventional)
• Mix Ratio: 50/50 coolant to distilled water
• Total System Capacity: 14-16 quarts (refill approximately 2 gallons)
• Bleeding Procedure: Required to remove air from system

1. Fill radiator through filler neck until full.
2. Start engine with radiator cap off.
3. Allow engine to reach operating temperature (thermostat opens).
4. Add coolant as level drops when thermostat opens.
5. Install radiator cap when coolant circulation is steady.
6. Check for leaks at thermostat housing and connections.

Testing and Verification

Verify proper thermostat operation after installation.

Operation Check: Front panel: Temperature gauge, heater controls. Remote: Temperature settings, diagnostic codes.

Temperature: Verify thermostat opens at 195°F; check with infrared thermometer. Pressure: System holds 15 PSI; check for leaks. Performance: Heater output hot; temperature stable at 195-210°F.

Common Issues

Typical thermostat-related problems and symptoms.

1. Stuck Open: Engine runs cool, poor heater performance, increased fuel consumption.
2. Stuck Closed: Engine overheats, coolant boiling, temperature warning light.
3. Leaking Housing: Coolant loss, white residue around housing, low coolant level.
4. Intermittent Operation: Fluctuating temperature gauge, inconsistent heater output.
5. Wrong Temperature Rating: Incorrect thermostat causes poor engine performance.

Tip: Always replace thermostat with correct temperature rating for your engine.

Maintenance Schedule

Recommended thermostat and cooling system maintenance.

• Thermostat Replacement: Every 60,000 miles or 5 years
• Coolant Change: Every 30,000 miles or 2 years
• System Inspection: Every oil change
• Pressure Test: Annually or if overheating occurs
• Hose Replacement: Every 4 years or 50,000 miles

WARNING! Never open hot cooling system - risk of severe burns.

Parts and Compatibility

Genuine GM and aftermarket replacement parts information.

Part Description	Part Number	Compatibility
Genuine GM Thermostat	12520436	All 1986 C10 engines
Stant Replacement	13548	4.3L, 5.0L, 5.7L V8
Motorcraft Equivalent	RT1139	All engines
Housing Gasket	14081451	Standard housing
O-ring Seal	14081452	Composite housing
Radiator Cap	15036029	15 PSI rating
Coolant	12346500	GM green conventional

Safety Precautions

Essential safety measures when working with cooling system.

Coolant Handling: Ethylene glycol is toxic; wear gloves and eye protection. Hot Components: Never work on hot engine; severe burn risk. Pressure Release: Always relieve system pressure before opening. Environmental: Dispose of old coolant properly at recycling center.

CAUTION! Coolant is poisonous to pets and children - clean spills immediately.

Troubleshooting Guide

Symptom	Possible Cause	Corrective Action
Engine overheats	Stuck closed thermostat	Replace thermostat; check coolant level
Poor heater output	Stuck open thermostat	Replace thermostat; bleed air from system
Coolant leak at housing	Damaged gasket or overtightened	Replace gasket; proper torque housing bolts
Temperature fluctuation	Weak thermostat spring	Replace thermostat; check coolant mixture
Coolant loss	Cracked housing or bad seal	Inspect housing; replace if damaged

Diagnostic Tip: Test thermostat in hot water - should open at 195°F.

Technical Support: 1-800-222-1020; www.gm.com