Restarting a Caterpillar D4H After Running Out of Fuel

[MikePhua]

Running a diesel bulldozer out of fuel is a frustrating but common problem, especially on older machines that work long hours in remote areas. The Caterpillar D4H, a widely respected mid‑sized dozer, is no exception. When the fuel tank runs dry, air enters the fuel system, preventing the injection pump from delivering fuel to the engine.
Restarting the machine requires a methodical bleeding process, an understanding of the D4H’s fuel system, and awareness of the common pitfalls that can prolong downtime.
This article provides a detailed, narrative‑style explanation of why the D4H becomes difficult to restart after losing fuel, how to bleed the system properly, and what operators can do to prevent the issue in the future.

Background of the Caterpillar D4H
Caterpillar introduced the D4H in the 1980s as part of its H‑series dozers, which featured:

• Improved power‑shift transmissions
  
• Better operator visibility
  
• More efficient cooling systems
  
• Stronger undercarriage components
  
• A reliable Caterpillar 3204 or 3304 diesel engine (depending on year)
  

The D4H became a global success, with thousands sold across construction, forestry, agriculture, and land‑clearing industries. Its combination of power, maneuverability, and durability made it a favorite among contractors and owner‑operators.

Why Running Out of Fuel Causes Problems
Diesel engines rely on constant, pressurized fuel flow. When the tank runs dry:

• Air enters the fuel lines
  
• The injection pump loses prime
  
• Injectors receive no fuel
  
• The engine cannot fire
  

Unlike gasoline engines, diesel systems cannot self‑purge air. They must be manually bled.
Terminology Note: Airlock 
A condition where trapped air prevents fuel from reaching the injection pump or injectors, causing the engine to stall or fail to start.

Understanding the D4H Fuel System
The D4H uses a mechanical fuel system consisting of:

• Fuel tank
  
• Lift pump (hand‑priming pump on many models)
  
• Fuel filters (primary and secondary)
  
• Injection pump
  
• High‑pressure injector lines
  
• Injectors
  

When air enters any part of this system, the engine will not start until the air is removed.

Symptoms After Running Out of Fuel
Operators typically report:

• Engine cranks but does not fire
  
• No smoke from exhaust (indicating no fuel delivery)
  
• Weak or no fuel flow at injector lines
  
• Hand primer feels soft or ineffective
  
• Engine may start briefly and die again
  

These symptoms confirm that the system has lost prime.

Bleeding the Fuel System on a D4H
Restarting the machine requires a step‑by‑step bleeding process. The exact steps vary slightly by engine model, but the general procedure is consistent.

1. Refill the Fuel Tank Completely
Adding only a small amount of fuel may not be enough to push air out of the lines. Filling the tank helps gravity feed the system.

2. Use the Hand Primer Pump
Most D4H models include a hand‑priming pump mounted near the fuel filters.
Steps:

• Unlock or unscrew the primer (if it has a locking collar)
  
• Pump until resistance increases
  
• Continue pumping until fuel flows without bubbles
  

If the primer never firms up, there may be:

• A suction leak
  
• A clogged filter
  
• A damaged primer pump
  

3. Bleed the Fuel Filters
Each filter housing has a bleed screw.
Procedure:

• Loosen the bleed screw
  
• Pump the primer until fuel flows steadily
  
• Tighten the screw
  

Repeat for both primary and secondary filters.

4. Bleed the Injection Pump
The injection pump has one or more bleed screws.
Steps:

• Loosen the screw
  
• Pump until bubble‑free fuel emerges
  
• Tighten the screw
  

This step is essential—many operators skip it and struggle to restart the engine.

5. Crack the Injector Lines
If the engine still will not start:

• Loosen the injector line nuts at the injectors
  
• Crank the engine
  
• Watch for strong spurts of fuel
  
• Tighten the nuts once fuel flows cleanly
  

Terminology Note: Cracking Injector Lines 
Loosening high‑pressure fuel lines to allow trapped air to escape during cranking.

6. Attempt to Start the Engine
Once fuel reaches the injectors:

• Crank in short bursts
  
• Avoid overheating the starter
  
• Use ether only if absolutely necessary and only in minimal amounts
  

The engine should begin to fire as remaining air clears.

Common Problems During Bleeding
Several issues can complicate the process.

Weak or Failed Hand Primer
Older primers often leak internally, preventing proper fuel flow.

Clogged Filters
Running out of fuel can stir up sediment, clogging filters instantly.

Suction Leaks
Loose clamps or cracked hoses allow air to re‑enter the system.

Worn Lift Pump
A weak lift pump cannot supply enough fuel to purge air.

Low Battery Voltage
Extended cranking drains the battery, reducing starter speed and fuel pressure.

Real‑World Case Studies
Case 1: Primer Pump Failure 
A contractor ran a D4H out of fuel and could not build pressure with the primer. Replacing the primer pump allowed the system to bleed properly, and the machine started immediately.
Case 2: Sediment Clogging Filters 
A farmer refueled after running dry but still could not start the dozer. Both filters were packed with debris. After replacing them and bleeding the system, the engine fired normally.
Case 3: Air Leak at Suction Line 
A forestry operator found that the machine would start but die after a few minutes. A cracked suction hose was allowing air to enter. Replacing the hose solved the issue.
Case 4: Injector Lines Needed Bleeding 
A municipality’s D4H would crank endlessly. Only after cracking the injector lines did fuel reach the injectors. The engine started within seconds.

Preventing Future Fuel‑Related Problems
To avoid running out of fuel and the resulting downtime:

• Keep the tank above one‑quarter full
  
• Replace filters regularly
  
• Inspect suction hoses annually
  
• Clean the fuel tank periodically
  
• Use high‑quality diesel
  
• Train operators to monitor fuel levels
  
• Install a fuel gauge if the original is unreliable
  

Anecdotes and Industry Stories
A veteran operator once joked, “A D4H will push dirt all day, but run it out of fuel and it’ll make you earn your paycheck.”
Another mechanic recalled a machine that took three hours to restart because the owner didn’t know about the bleed screws on the injection pump.
A rental company shared that fuel‑related no‑start calls were among the most common service requests for older dozers.

Why the D4H Remains Popular
Even decades after production ended, the D4H remains valued because:

• It is simple and rebuildable
  
• It has strong pushing power
  
• It is easy to maintain
  
• It has excellent aftermarket support
  
• It is built with heavy steel rather than lightweight components
  

Many D4Hs continue working daily, proving the durability of Caterpillar’s engineering.

Conclusion
Running a Caterpillar D4H out of fuel is inconvenient, but with a clear understanding of the fuel system and a methodical bleeding process, the machine can be restarted reliably.
By bleeding the filters, injection pump, and injector lines—and ensuring the primer, hoses, and lift pump are functioning—operators can restore fuel flow and return the dozer to work.
With proper maintenance and attention to fuel levels, the D4H will continue delivering dependable performance for years, maintaining its reputation as one of Caterpillar’s most trusted mid‑sized dozers.