Does the 3.9L Cummins Engine Use Glow Plugs

[MikePhua]

The Design Philosophy Behind the 3.9L Cummins
The 3.9L Cummins engine, also known as the 4BT (Four-cylinder B-series Turbocharged), was developed in the mid-1980s as a compact, rugged diesel powerplant for industrial, agricultural, and light commercial use. Derived from the larger 5.9L 6BT inline-six, the 4BT shares many components and design principles, including a cast iron block, direct injection, and high compression ratios. Its simplicity and reliability made it a favorite for retrofits, skid steers, generators, and delivery trucks.
Cummins produced tens of thousands of 4BT engines globally, with widespread adoption in North America, South Africa, and Southeast Asia. Its reputation for cold-start reliability and mechanical durability remains strong, even decades after its introduction.
Glow Plugs vs. Grid Heaters
Unlike many small diesel engines that rely on glow plugs to preheat the combustion chamber, the 3.9L Cummins does not use glow plugs. Instead, it employs an intake air heating system—commonly referred to as a grid heater. This electrically powered heating element is mounted in the intake manifold and warms incoming air before it enters the cylinders.
Key differences:

• Glow Plugs
  Installed directly into the cylinder head
  Heat the combustion chamber locally
  Common in European and Japanese diesel engines
  
• Grid Heaters
  Located in the intake manifold
  Heat the entire air charge entering the engine
  Used in Cummins B-series engines and some heavy-duty platforms
  

The grid heater activates during cold starts, typically below 40°F (4°C), and remains on for a few seconds to a minute depending on ambient temperature and ECM programming. It draws significant current—often 100 amps or more—requiring robust battery and alternator support.
Cold Start Behavior and Recommendations
The 3.9L Cummins is known for its ability to start in cold weather without glow plugs, thanks to:

• High compression ratio (17.5:1 or higher)
  
• Direct injection system
  
• Robust starter motor torque
  
• Intake air heating via grid heater
  

However, in extreme cold (below -10°C), starting may be difficult without supplemental heating. Operators often use:

• Block heaters: Installed in the freeze plug port, warming coolant and engine block
  
• Battery warmers: Maintain cranking voltage
  
• Fuel heaters: Prevent gelling in diesel lines
  
• Minimal ether injection: Used cautiously to avoid pre-ignition damage
  

A contractor in upstate New York reported that his 4BT-powered Case backhoe struggled to start in winter until he installed a 1250-watt block heater. After 30 minutes of preheating, the engine started reliably even at -15°C.
Common Misconceptions and Field Confusion
Many operators unfamiliar with Cummins engines assume glow plugs are standard in all diesels. This leads to confusion when troubleshooting cold-start issues. In reality:

• The absence of glow plugs is intentional and not a design flaw
  
• Grid heaters are more effective in multi-cylinder heating
  
• Glow plug-style diagnostics do not apply to the 3.9L Cummins
  

Some older machines may have disabled or bypassed grid heaters due to wiring faults or battery limitations. In such cases, cold starts become difficult, and operators may resort to ether or prolonged cranking—both of which can damage the engine if misused.
Maintenance Tips for Reliable Cold Starts
To ensure consistent performance:

• Test grid heater resistance annually (typical range: 0.1–0.3 ohms)
  
• Inspect relay and solenoid for corrosion or sticking
  
• Use heavy-duty batteries rated for cold cranking amps above 800
  
• Clean intake manifold to prevent carbon buildup around the heater
  
• Replace worn starter motors with OEM-rated torque output
  
• Monitor fuel filter condition and water separator function
  

Installing a voltmeter or ammeter in the cab can help monitor grid heater activation and battery draw during startup.
Conclusion
The 3.9L Cummins engine does not use glow plugs, relying instead on a grid heater system and high compression to achieve reliable cold starts. While this design differs from many small diesel engines, it reflects Cummins’ philosophy of simplicity, durability, and centralized air heating. With proper maintenance and supplemental heating in extreme conditions, the 4BT remains a dependable powerplant—starting strong even when the temperature drops and the diesel thickens.