Chevrolet 250 Inline-6 Engine - Square Body Trucks

Overview

The Chevrolet 250 cubic inch inline-6 engine was the standard powerplant in many Square Body trucks from 1973-1987, serving as the base engine option across C10, C20, K10, K20, K5 Blazer, and Suburban models. This robust, simple engine earned a reputation for reliability and longevity, making it a favorite among fleet operators and budget-conscious buyers.

The 250 I6 produced approximately 105-130 horsepower and 185-205 lb-ft of torque depending on the year and emissions equipment. While not powerful by modern standards, this engine's simplicity, fuel economy, and durability made it an excellent workhorse for light-duty applications.

Understanding this engine is crucial for Square Body owners as it represents one of the most common powerplants in these trucks, and many examples remain in service today due to their inherent reliability.

Common Issues

Carburetor Problems

• Rochester 1-barrel carburetor flooding - Common on 1973-1979 models
• Accelerator pump diaphragm failure - Causes hesitation during acceleration
• Float adjustment issues - Leads to rich/lean running conditions

Cooling System Issues

• Radiator clogging - Especially common in trucks used for heavy work
• Water pump bearing failure - Typically occurs every 80,000-100,000 miles
• Thermostat housing cracking - Cast iron housing prone to thermal stress cracks

Ignition System Problems

• Points-based ignition wear (1973-1974) - Requires regular adjustment
• HEI distributor cap carbon tracking (1975+) - Causes misfiring in wet conditions
• Timing chain stretch - Common after 150,000+ miles

Oil System Issues

• Rear main seal leakage - Rope seal design prone to leaking
• Oil pump drive failure - Can cause catastrophic engine damage
• Valve cover gasket leaks - Simple fix but very common

Parts & Materials Needed

Basic Maintenance Parts

• Oil Filter: AC Delco PF25 or equivalent
• Air Filter: AC Delco A49C (1973-1979), A1000C (1980-1987)
• Fuel Filter: AC Delco GF61 (inline), GF481 (carburetor mount)
• Spark Plugs: AC Delco R44T (standard), R44TX (extended life)

Common Replacement Parts

• Water Pump: GM part #14011089 or Gates 42030
• Thermostat: 180°F or 195°F depending on application
• Radiator Hoses: Gates 21398 (upper), 21399 (lower) - verify year/model
• Timing Chain Set: Cloyes 9-3100A
• Carburetor Rebuild Kit: Rochester 1ME - varies by year

Performance Upgrade Parts

• Headers: Hooker 5204 or Hedman 68570
• Cold Air Intake: K&N 57-3501 (custom fabrication required)
• Electronic Ignition: Pertronix Ignitor kit for points-equipped models

Tools Required

• Standard metric and SAE socket sets
• Torque wrench (0-150 ft-lbs)
• Timing light
• Compression gauge
• Carburetor adjustment tool kit

Step-by-Step Guide

Basic Tune-Up Procedure

1. Spark Plug Replacement

1. Remove air cleaner assembly
2. Disconnect spark plug wires (mark cylinder locations)
3. Remove plugs with 5/8" spark plug socket
4. Gap new plugs to 0.035"
5. Install plugs with anti-seize compound
6. Torque to 15 ft-lbs

2. Point Adjustment (1973-1974)

1. Remove distributor cap and rotor
2. Rotate engine until points are fully open
3. Adjust point gap to 0.019" using feeler gauge
4. Check dwell angle: should be 31-34 degrees

3. Timing Adjustment

1. Connect timing light to #1 cylinder
2. Start engine and warm to operating temperature
3. Disconnect and plug vacuum advance line
4. Adjust timing to 8° BTDC (1973-1979) or 12° BTDC (1980-1987)
5. Reconnect vacuum advance

4. Carburetor Adjustment

1. Adjust idle mixture screws for highest RPM
2. Set idle speed to 600 RPM (manual) or 500 RPM (automatic) in Drive
3. Readjust mixture for smoothest idle
4. Check accelerator pump operation

Timing Chain Replacement

Preparation

1. Drain cooling system and remove radiator
2. Remove fan, water pump pulley, and harmonic balancer
3. Remove timing cover (requires oil pan to be loosened)

Chain Replacement

1. Align timing marks on both sprockets
2. Remove camshaft sprocket bolts
3. Remove chain and both sprockets as assembly
4. Install new sprockets with timing marks aligned
5. Install new chain with 0.5" maximum deflection
6. Torque cam sprocket bolts to 20 ft-lbs

Year/Model Differences

1973-1979 Models

• Carburetor: Rochester 1ME single-barrel
• Compression Ratio: 8.25:1
• Ignition: Points (1973-1974), HEI (1975+)
• Emissions: Basic PCV and EGR systems
• Power Output: 105-110 HP

1980-1987 Models

• Carburetor: Rochester 1ME with feedback control (1980-1986), TBI (1987 in some applications)
• Compression Ratio: 8.3:1
• Ignition: HEI with EST (Electronic Spark Timing)
• Emissions: Computer-controlled systems, catalytic converter
• Power Output: 115-130 HP

Model-Specific Applications

• C10/K10: Standard application, most common configuration
• C20/K20: Heavy-duty cooling system, higher-capacity oil pump
• K5 Blazer: Shorter exhaust system, different air cleaner configuration
• Suburban: Longer fuel lines, dual fuel tanks on some models

Tips & Best Practices

Maintenance Best Practices

• Change oil every 3,000 miles with conventional oil or 5,000 miles with synthetic
• Use 10W-30 oil in most climates; 5W-30 in extreme cold
• Replace timing chain at 150,000 miles preventatively
• Flush cooling system every 30,000 miles to prevent clogging

Performance Improvements

• Install dual exhaust for 10-15 HP gain and better sound
• Advance timing 2-4 degrees if using premium fuel
• Install electric cooling fan to reduce parasitic power loss
• Consider Clifford intake manifold for significant power gains

Common Mistakes to Avoid

• Over-tightening spark plugs - Can strip aluminum heads on later models
• Ignoring timing chain noise - Chain failure can cause valve damage
• Using wrong thermostat - Affects emissions and performance
• Neglecting PCV system - Causes oil leaks and poor performance

Cost-Effective Upgrades

• HEI conversion for points-equipped engines (~$150)
• Electronic carburetor replacement with Weber 32/36 (~$300)
• Header installation for improved exhaust flow (~$200-400)

Troubleshooting

Engine Won't Start

Symptoms: No start, cranks normally
Diagnosis:
1. Check for spark at plugs
2. Verify fuel delivery to carburetor
3. Check compression (should be 140-160 PSI)
4. Verify timing marks alignment

Solutions:
- Replace ignition components if no spark
- Clean/rebuild carburetor if no fuel delivery
- Check valve adjustment if low compression

Poor Idle Quality

Symptoms: Rough idle, stalling, irregular RPM
Diagnosis:
1. Check vacuum leaks with propane torch method
2. Test carburetor mixture adjustment
3. Verify ignition timing
4. Check PCV valve operation

Solutions:
- Replace vacuum hoses and gaskets
- Adjust or rebuild carburetor
- Reset timing to specification
- Replace PCV valve and clean system

Overheating Issues

Symptoms: High coolant temperature, coolant loss
Diagnosis:
1. Check coolant level and condition
2. Test radiator cap pressure rating
3. Verify thermostat opening temperature
4. Check water pump operation

Solutions:
- Flush cooling system and replace coolant
- Install new radiator cap (15 PSI rating)
- Replace thermostat with correct temperature rating
- Replace water pump if bearing noise present

Oil Consumption

Symptoms: Blue smoke, frequent oil additions
Diagnosis:
1. Check valve stem seals (smoke on startup)
2. Test compression and leak-down
3. Inspect PCV system operation
4. Check rear main seal for external leaks

Solutions:
- Replace valve stem seals
- Rebuild engine if compression low
- Replace PCV valve and clean breather system
- Install rear main seal (consider conversion to lip seal)

Power Loss

Symptoms: Reduced acceleration, poor highway performance
Diagnosis:
1. Check ignition timing and advance mechanisms
2. Test carburetor accelerator pump
3. Verify exhaust system restriction
4. Check air filter condition

Solutions:
- Adjust timing and rebuild distributor advance
- Rebuild carburetor with emphasis on accelerator pump
- Replace restricted exhaust components
- Install new air filter and check housing for restrictions