Overview
The Chevrolet 305 cubic inch (5.0L) small block V8 was one of the most common engine options available in Square Body trucks from 1976-1987. Introduced as GM's response to fuel economy concerns following the oil crisis, the 305 served as a middle ground between the smaller 267ci V8 and larger displacement 350ci engines. While often criticized for lackluster performance, the 305 was reliable, fuel-efficient, and adequate for light-duty truck applications.
The 305 shared the same external dimensions as other small block Chevys, making it easily swappable with 283, 327, 350, and 400ci engines. However, its smaller bore size (3.736" vs 4.000" on the 350) limited its performance potential. Understanding the 305's characteristics, common issues, and upgrade paths is essential for Square Body owners looking to maintain, repair, or improve their trucks.
Key 305 Specifications:
- Displacement: 305 cubic inches (5.0L)
- Bore x Stroke: 3.736" x 3.480"
- Compression Ratio: 8.6:1 to 9.5:1 (depending on year)
- Horsepower: 130-165 HP (depending on year and configuration)
- Torque: 245-270 lb-ft
Common Issues
Carburetor Problems (1976-1986)
The Rochester Quadrajet and later Computer Command Control (CCC) systems are prone to:
- Vacuum leaks at carburetor base gaskets and vacuum lines
- Accelerator pump diaphragm failure causing hesitation
- Float level issues leading to flooding or lean conditions
- CCC solenoid failures (1981-1986) causing poor idle and stalling
TBI Fuel Injection Issues (1987)
The 1987 Throttle Body Injection system commonly experiences:
- Fuel injector clogging from ethanol fuel
- Pressure regulator failure causing fuel pressure issues
- IAC valve carbon buildup leading to erratic idle
- MAP sensor failures affecting fuel delivery
Engine Internal Problems
- Camshaft wear due to flat-tappet cam design and modern oil formulations
- Intake manifold gasket leaks (especially lower intake on Vortec heads)
- Timing chain stretch after 100,000+ miles
- Oil pump drive shaft wear causing low oil pressure
Cooling System Issues
- Radiator inadequacy for heavy loads or hot climates
- Water pump bearing failure around 80,000-100,000 miles
- Thermostat housing corrosion on aluminum intakes
Parts & Materials Needed
Maintenance Parts
Oil Change:
- Oil Filter: AC Delco PF52 or Fram PH8A
- Oil: 5 quarts 10W-30 conventional or synthetic blend
- Drain plug gasket: GM 12474618
Tune-Up Components:
- Spark Plugs: AC Delco R45TS (1976-1986) or AC Delco R43CTS (1987 TBI)
- Distributor Cap: AC Delco D328A (HEI)
- Rotor: AC Delco D465
- Spark Plug Wires: AC Delco 9748UU
Carburetor Service:
- Quadrajet Rebuild Kit: Edelbrock 1477 or Rochester 17080213
- Carburetor Base Gasket: Fel-Pro 60539
- Air Filter: AC Delco A867C
Common Replacement Parts
Timing Components:
- Timing Chain Set: Cloyes 9-3100 (standard) or Comp Cams 3100 (performance)
- Timing Cover Gasket: Fel-Pro TCS45994
Gaskets & Seals:
- Intake Manifold Gasket Set: Fel-Pro MS95018
- Head Gasket Set: Fel-Pro HS9333PT
- Oil Pan Gasket: Fel-Pro OS30609R
Water Pump:
- AC Delco 251-713 (standard duty)
- Edelbrock 8851 (high-flow performance)
Tools Required
- Standard metric and SAE socket sets
- Torque wrench (ft-lb and in-lb)
- Timing light
- Vacuum gauge
- Fuel pressure gauge (1987 TBI)
- Compression tester
Step-by-Step Guide
Basic Tune-Up Procedure
Step 1: Spark Plug Replacement
1. Remove air cleaner assembly
2. Label and disconnect spark plug wires
3. Remove old plugs with 5/8" spark plug socket
4. Gap new plugs to 0.045" (carbureted) or 0.035" (TBI)
5. Install new plugs, torque to 22 ft-lbs
6. Apply dielectric grease to plug boots before reconnecting
Step 2: Distributor Service
1. Remove distributor cap (two spring clips)
2. Mark rotor position relative to distributor body
3. Remove rotor (pull straight up on HEI)
4. Install new rotor, ensuring proper seating
5. Install new cap, check for cracks or carbon tracking
Step 3: Air/Fuel Filter Service
1. Replace air filter element in housing
2. For carbureted engines: Replace fuel filter at carburetor inlet
3. For 1987 TBI: Replace in-line fuel filter near fuel tank
Quadrajet Carburetor Adjustment
Float Level Adjustment:
1. Remove air horn with float assembly
2. Hold air horn upside down
3. Measure float level: 7/32" from air horn gasket surface to float
4. Bend float arm tang to adjust
Idle Mixture Adjustment (Pre-1981):
1. Warm engine to operating temperature
2. Turn mixture screws clockwise until seated, then back out 3 turns
3. Adjust screws equally for highest vacuum reading
4. Set idle speed to specification with idle speed screw
1987 TBI System Service
Fuel Pressure Check:
1. Connect fuel pressure gauge to test port on fuel rail
2. Key on, engine off: 9-13 PSI
3. Engine running: 9-13 PSI
4. Engine off, pressure should hold above 6 PSI for 5 minutes
IAC Valve Cleaning:
1. Remove IAC valve from throttle body (two bolts)
2. Clean carbon deposits with carburetor cleaner
3. Check pintle for damage
4. Install with new O-ring, torque to 13 ft-lbs
Year/Model Differences
1976-1980: Early 305 Engines
- Compression Ratio: 8.6:1
- Horsepower: 130-140 HP
- Carburetor: Rochester M2MC 2-barrel or M4MC Quadrajet
- Ignition: HEI distributor with mechanical and vacuum advance
- Availability: C10, C20, K10 (not available in K20 or heavy-duty applications)
1981-1986: Computer Controlled Carburetion (CCC)
- Compression Ratio: 9.5:1 (1981-1982), 9.0:1 (1983-1986)
- Horsepower: 150-165 HP
- Carburetor: Rochester E4ME Quadrajet with electronic controls
- Ignition: HEI with EST (Electronic Spark Timing)
- Emissions: Extensive vacuum and electronic controls
- Availability: All models including K20 and Suburban
1987: Throttle Body Injection
- Compression Ratio: 9.0:1
- Horsepower: 165 HP
- Fuel System: Single-point TBI with electronic controls
- Engine Management: ECM-controlled fuel and ignition
- Availability: All Square Body models (final year)
Model-Specific Applications
C10/K10 (1/2-ton):
- Standard duty cooling system
- 3.08:1 or 3.42:1 rear axle ratios common
- Manual or automatic transmission options
C20/K20 (3/4-ton):
- Heavy-duty cooling system (larger radiator)
- Lower axle ratios (2.73:1, 3.08:1) for payload capacity
- Typically automatic transmission only
K5 Blazer:
- Standard duty cooling with optional heavy-duty
- Transfer case standard (NP203, NP208, NP241)
- Shorter exhaust system routing
Suburban:
- Heavy-duty cooling system standard
- Longer wheelbase affects driveshaft length
- Rear A/C option affects underhood routing
Tips & Best Practices
Performance Improvements
Budget-Friendly Upgrades:
- Dual exhaust system: Improves breathing and sound
- K&N air filter: Increases airflow while remaining reusable
- Recurved distributor: Optimize timing curve for better performance
- Electric fan conversion: Frees up horsepower and improves cooling
Moderate Performance Build:
- Edelbrock Performer intake: Edelbrock 2101 with matching carburetor
- Headers: Hooker or Hedman long-tube headers
- Camshaft upgrade: Comp Cams XE262H for mild performance increase
- HEI upgrade: Performance module and coil
Reliability Modifications
Oil System Improvements:
- Use ZDDP additive or diesel oil for flat-tappet cam protection
- Install oil pressure gauge to monitor pump condition
- Consider high-volume oil pump for high-mileage engines
Cooling System Upgrades:
- Aluminum radiator (Griffin or Champion)
- 180°F thermostat for better performance
- Electric fans with proper CFM rating
- Coolant recovery tank upgrade
Fuel System Maintenance
Carbureted Engines:
- Use ethanol-resistant fuel lines and gaskets
- Install inline fuel filter before mechanical pump
- Keep carburetor properly adjusted for best economy
1987 TBI System:
- Use Top Tier gasoline to prevent injector clogging
- Replace fuel filter every 30,000 miles
- Clean IAC valve annually
Troubleshooting
Poor Performance/Lack of Power
Symptoms: Sluggish acceleration, poor fuel economy, lack of power
Diagnosis Steps:
1. Compression test: Should be 140-170 PSI, within 20% between cylinders
2. Vacuum test: Should maintain 17-21" Hg at idle
3. Fuel pressure test: 5-9 PSI (mechanical pump), 9-13 PSI (TBI)
4. Timing check: Base timing should be 6-8° BTDC
Common Causes:
- Worn camshaft lobes or lifters
- Clogged catalytic converter
- Incorrect carburetor jetting
- Vacuum leaks at intake manifold
Engine Overheating
Symptoms: Temperature gauge reading high, coolant loss, steam from engine
Diagnosis Steps:
1. Check coolant level when engine is cold
2. Pressure test cooling system (should hold 15 PSI)
3. Test thermostat operation (opens at rated temperature)
4. Verify fan operation and shroud placement
Common Causes:
- Failed water pump (bearing noise, coolant leak from weep hole)
- Clogged radiator (external debris or internal scale)
- Bad thermostat (stuck closed or wrong temperature rating)
- Blown head gasket (combustion gases in coolant)
Hard Starting/No Start
Symptoms: Extended cranking required, intermittent starting issues
Diagnosis Steps:
1. Check fuel delivery at carburetor or TBI unit
2. Verify spark at all cylinders
3. Test starter draw and cranking RPM
4. Check engine timing
Carbureted Engine Causes:
- Fuel pump failure (mechanical pump diaphragm)
- Choke not operating properly
- Flooded engine from leaking float/needle valve
- Vapor lock in hot weather
1987 TBI Causes:
- Fuel pump failure (electric pump in tank)
- Clogged fuel injectors
- Failed fuel pressure regulator
- ECM or sensor problems
Rough Idle/Stalling
Symptoms: Engine shakes at idle, stalls when coming to stop
Diagnosis Steps:
1. Check idle speed (should be 550-650 RPM in drive)
2. Test for vacuum leaks with propane torch method
3. Verify PCV system operation
4. Check EGR valve function (1981+)
Common Causes:
- Vacuum leaks at intake manifold or carburetor base
- Dirty or failed IAC valve (1987 TBI)
- Incorrect idle mixture adjustment
- Carbon buildup in throttle body or intake ports
Oil Pressure Problems
Symptoms: Low oil pressure warning, gauge reading low
Diagnosis Steps:
1. Verify with mechanical gauge at oil pressure sending unit
2. Check oil level and condition
3. Remove oil pan and inspect pickup screen
4. Check oil pump drive shaft engagement
Common Causes:
- Worn oil pump (high mileage engines)
- Clogged oil pickup screen
- Worn main or rod bearings
- Oil pump drive shaft wear or breakage
Prevention:
- Regular oil changes with proper viscosity
- Use ZDDP additive for flat-tappet cam protection
- Monitor oil pressure gauge regularly
- Address leaks promptly to prevent low oil level