Overview
The 6.2L Detroit Diesel (RPO LH6) was General Motors' first serious attempt at a light-duty diesel engine, available in Square Body trucks from 1982-1987. This naturally aspirated V8 diesel was designed to compete with Ford's 6.9L International diesel and provide better fuel economy than gasoline engines during the fuel crisis era.
Key Specifications:
- Displacement: 379 cubic inches (6.2L)
- Configuration: 90-degree V8, naturally aspirated
- Bore x Stroke: 3.98" x 3.80"
- Compression Ratio: 21.5:1
- Power Output: 130-135 HP @ 3,600 RPM
- Torque: 240-250 lb-ft @ 2,000 RPM
- Block Material: Cast iron with 4-bolt main caps
The 6.2L was available in C10, C20, C30, K10, K20, K30, K5 Blazer, and Suburban models from 1982-1987, making it a significant option for Square Body enthusiasts seeking diesel power and fuel economy.
Common Issues
Injection Pump Problems
The Roosa Master DB2 injection pump (Stanadyne) is the most common failure point:
- Symptoms: Hard starting, rough idle, loss of power, black smoke
- Typical lifespan: 100,000-150,000 miles
- Root causes: Contaminated fuel, water intrusion, internal wear
Glow Plug System Failures
The glow plug system is critical for cold starting:
- Controller failures: The glow plug controller (located on firewall) commonly fails
- Glow plug burnout: Individual plugs fail, causing hard starting
- Relay issues: Glow plug relay in fuse box fails frequently
Harmonic Balancer Deterioration
- Rubber separation: The rubber damper separates from the hub
- Symptoms: Vibration, timing chain noise, potential crankshaft damage
- Critical replacement: Must use diesel-specific balancer (different from gas engines)
Fuel System Contamination
- Water intrusion: Poor fuel tank venting allows moisture buildup
- Algae growth: Biological growth in fuel tanks
- Filter clogging: Requires frequent fuel filter changes
Parts & Materials Needed
Essential Maintenance Parts
Fuel Filter (Primary): AC Delco TP1015 or WIX 33166
Fuel Filter (Secondary): AC Delco TP3018 or WIX 33040
Oil Filter: AC Delco PF454 or equivalent
Glow Plugs: AC Delco 60G (set of 8)
Glow Plug Controller: AC Delco 212-310
Air Filter: AC Delco A1137C
Injection Pump Rebuild/Replacement
Remanufactured DB2 Pump: Stanadyne DE503 (C10/K10)
Remanufactured DB2 Pump: Stanadyne DE504 (C20/K20/Suburban)
Injection Lines: Complete set, GM part 12552094
Harmonic Balancer
Diesel Harmonic Balancer: GM 14088765 or Dorman 594-126
Balancer Bolt: GM 14084598 (Grade 10.9)
Tools Required
- Injection pump timing tools: Kent-Moore J-33075 timing meter
- Glow plug removal tools: Deep well sockets, penetrating oil
- Fuel pressure gauge: 0-15 PSI range
- Multimeter: For electrical diagnosis
Step-by-Step Guide
Injection Pump Timing Procedure
WARNING: Incorrect injection pump timing can cause severe engine damage.
-
Preparation:
- Warm engine to operating temperature
- Remove air cleaner assembly
- Clean injection pump area thoroughly -
Static Timing Setup:
- Remove injection pump top cover
- Install timing probe in pump housing
- Rotate engine to TDC compression stroke cylinder #1
- Align timing marks on harmonic balancer -
Dynamic Timing (Preferred Method):
- Connect Kent-Moore J-33075 timing meter to injection pump
- Start engine and warm to 175°F+
- Check timing at 1000 RPM
- Specification: 3.5° ± 1° BTDC -
Timing Adjustment:
- Loosen injection pump mounting nuts
- Rotate pump clockwise to advance timing
- Rotate pump counterclockwise to retard timing
- Tighten mounting nuts to 35 ft-lbs
Glow Plug System Diagnosis
-
Controller Testing:
- Key ON, engine cold (below 100°F)
- Check voltage at glow plug bus bar
- Should read 12V for 15-60 seconds depending on temperature
- No voltage = failed controller or relay -
Individual Glow Plug Testing:
- Remove glow plug connector
- Check continuity between terminal and ground
- Specification: 0.8-1.2 ohms resistance
- Infinite resistance = failed glow plug -
Replacement Procedure:
- CAUTION: Use penetrating oil, glow plugs break easily
- Remove valve covers if necessary for access
- Apply anti-seize compound to threads
- Torque specification: 15 ft-lbs
Year/Model Differences
1982-1984 (Early Production)
- Engine Code: LH6
- Injection pump: Stanadyne DB2-4299 (mechanical governor)
- Glow plug system: 6-volt glow plugs with voltage dropping resistor
- Power output: 130 HP / 240 lb-ft
1985-1987 (Late Production)
- Engine Code: LH6 (same designation)
- Injection pump: Stanadyne DB2-4911 (improved design)
- Glow plug system: 12-volt glow plugs (no resistor)
- Power output: 135 HP / 250 lb-ft (slight improvement)
- Improved: Better cold-start characteristics, more reliable electronics
Model-Specific Applications
C10/K10 (1/2-ton):
- Standard with TH400 automatic transmission
- Manual transmission option: SM465 4-speed
C20/K20 (3/4-ton):
- Heavier-duty cooling system
- Standard with TH400 or SM465
- Available with NP205 transfer case (4WD)
Suburban/K5 Blazer:
- Same engine specifications as pickup trucks
- Standard TH400 transmission
- Suburban available in 1/2-ton and 3/4-ton configurations
Tips & Best Practices
Fuel Quality Management
- Use only high-quality diesel fuel from high-turnover stations
- Add fuel biocide twice yearly to prevent algae growth
- Replace fuel filters every 10,000 miles or sooner in dusty conditions
- Drain fuel tank water separator monthly during wet seasons
Cold Weather Operation
- Block heater installation highly recommended for temperatures below 20°F
- Glow plug cycle: Always wait for glow plug light to cycle before starting
- Winter fuel additives: Use anti-gel additives below 10°F
- Idle warm-up: Allow 3-5 minutes warm-up before driving in cold weather
Maintenance Schedule
Every 3,000 miles: Oil and filter change (15W-40 diesel oil)
Every 10,000 miles: Fuel filter replacement
Every 15,000 miles: Air filter replacement
Every 30,000 miles: Glow plug inspection
Every 60,000 miles: Injection pump service
Every 100,000 miles: Injection pump rebuild/replacement
Performance Modifications
- Turbocharger kits: Banks Sidewinder system (discontinued but available used)
- Exhaust upgrades: 3" diameter exhaust system improves performance
- Air intake improvements: K&N air filter or cold air intake
- Injection pump modifications: Increase fuel delivery (professional installation required)
Troubleshooting
Hard Starting Issues
Cold Engine (Below 80°F):
1. Check glow plug operation (12V at plugs for 15+ seconds)
2. Test individual glow plugs for continuity
3. Verify glow plug controller function
4. Check battery voltage (minimum 12.6V)
Warm Engine (Above 80°F):
1. Check fuel supply pressure (5-8 PSI at injection pump)
2. Inspect fuel filters for contamination
3. Test injection pump timing
4. Check for air in fuel system
Loss of Power Symptoms
Gradual Power Loss:
- Clogged fuel filters (most common)
- Dirty air filter
- Injection pump wear
- Low compression (valve/ring wear)
Sudden Power Loss:
- Fuel supply interruption
- Injection pump failure
- Boost leak (if turbocharged)
- Exhaust restriction
Excessive Smoke Diagnosis
Black Smoke (Rich Condition):
- Over-fueling injection pump
- Clogged air filter
- Incorrect injection timing (too advanced)
- Turbocharger failure (if equipped)
White/Blue Smoke:
- Injection timing too retarded
- Low compression
- Glow plug malfunction
- Fuel contamination (water/gasoline)
Electrical System Issues
Glow Plug System Diagnosis:
Step 1: Check fuse #3 in fuse block (20A)
Step 2: Test glow plug relay operation
Step 3: Measure voltage at controller (Pink wire = 12V key ON)
Step 4: Check controller ground (Black wire continuity)
Step 5: Test output voltage at glow plug bus bar
Charging System:
- Alternator: 100-amp unit standard (higher output recommended)
- Belt tension: 1/2" deflection at longest span
- Charging rate: 13.8-14.4V at 1500 RPM
Professional Service Recommendations
Injection Pump Work:
- Timing adjustment requires specialized equipment
- Pump rebuilds should be performed by diesel specialists
- Recommended shops: Stanadyne authorized service centers
Major Engine Work:
- Compression testing requires diesel-specific equipment
- Head gasket replacement requires specialized procedures
- Torque specifications: Head bolts 85 ft-lbs (final step)
The 6.2L Detroit Diesel, while not the most powerful option, provides excellent fuel economy and reasonable reliability when properly maintained. Understanding its unique requirements and common failure points is essential for successful Square Body diesel ownership.