Komatsu D21P‑6 Hard Starting Issues

[MikePhua]

The Komatsu D21P‑6 is one of the most recognizable compact crawler dozers ever produced, valued for its maneuverability, low operating cost, and reliability. Yet even the most dependable machines can develop hard‑starting problems as they age. This article explores the common causes behind difficult starting on the D21P‑6, explains the underlying mechanical principles, and provides practical solutions. It also includes historical context, terminology explanations, and real‑world stories from the field to help owners better understand and maintain this classic machine.

Background of the Komatsu D21 Series
Komatsu introduced the D21 series in the 1980s as a compact alternative to larger crawler tractors. The D21P‑6, part of the sixth generation, became especially popular in Asia, North America, and Europe due to its:

• Low ground pressure design
  
• Hydrostatic steering
  
• Fuel‑efficient diesel engine
  
• Compact footprint suitable for forestry, agriculture, and construction
  

Komatsu, founded in 1921 in Japan, has long been one of the world’s largest construction equipment manufacturers. By the time the D21P‑6 was released, Komatsu had already sold hundreds of thousands of small dozers globally. The D21 series alone is estimated to have exceeded 40,000 units across all variants, making it one of the most widely used compact dozers in history.

Understanding Hard‑Starting Symptoms
Owners of aging D21P‑6 machines often report similar symptoms:

• Long cranking time before the engine fires
  
• White smoke during cranking
  
• Engine starts only with ether or pre‑heating
  
• Engine runs well once started
  
• Fuel system loses prime after sitting overnight
  

These symptoms point toward issues in fuel delivery, compression, or cold‑start assistance.

Terminology Explained
To better understand the causes, here are key terms:

• Fuel priming: The process of filling fuel lines with diesel so the injection pump can deliver fuel properly.
  
• Glow plugs / intake heaters: Devices that warm the intake air to help cold starting.
  
• Compression ratio: The ratio of cylinder volume at bottom vs. top of piston travel; diesel engines rely on high compression to ignite fuel.
  
• Lift pump: A small mechanical or electric pump that supplies fuel to the injection pump.
  
• Return line: A line that sends unused fuel back to the tank; leaks here can cause air intrusion.
  

Common Causes of Hard Starting
The D21P‑6 is known for being reliable, but several age‑related issues can make starting difficult.
Fuel System Air Intrusion
Air leaks are the number one cause of hard starting on older Komatsu dozers. Even a pinhole leak can cause the system to lose prime.
Typical leak points include:

• Cracked rubber fuel lines
  
• Loose hose clamps
  
• Worn lift pump check valves
  
• Leaking banjo fittings
  
• Fuel filter seals
  
• Return line fittings
  

A small air leak may not drip fuel outward but will allow air to enter when the machine sits.
Weak Glow Plug or Intake Heater System
Many D21P‑6 units rely on an intake heater rather than individual glow plugs. If the heater element is weak or the relay fails, cold starting becomes difficult.
Symptoms include:

• No clicking sound from the relay
  
• No voltage at the heater terminal
  
• Slow heating time
  

Low Cranking Speed
Diesel engines require high cranking speed to build compression heat. A weak battery or starter can make starting nearly impossible.
Common causes:

• Old battery
  
• Corroded battery cables
  
• Worn starter brushes
  
• Poor engine ground connection
  

Low Compression
Although Komatsu engines are durable, decades of use can reduce compression due to:

• Worn piston rings
  
• Cylinder glazing
  
• Valve leakage
  

Low compression reduces the heat needed for diesel ignition, especially in cold weather.

A Real‑World Story: The Overnight Mystery
A contractor in British Columbia owned a D21P‑6 that started perfectly during the day but refused to start each morning. After replacing filters, batteries, and even the starter, the problem persisted.
Eventually, a mechanic discovered a tiny crack in the fuel return line hidden under the tank. Overnight, air entered the system, causing the fuel to drain back. Once the line was replaced, the machine started instantly every morning.
This story highlights how small leaks can cause big headaches.

Diagnostic Steps
Owners can follow a systematic approach to identify the cause:

• Check fuel lines for cracks or loose clamps
  
• Inspect the lift pump for weak output
  
• Test the intake heater for proper voltage
  
• Measure cranking RPM with a tachometer
  
• Perform a compression test if other steps fail
  
• Check for fuel draining back into the tank overnight
  
• Install a clear line temporarily to observe air bubbles
  

These steps help isolate the root cause without unnecessary parts replacement.

Practical Solutions
Once the cause is identified, the following solutions are effective:

• Replace all rubber fuel lines with modern diesel‑rated hose
  
• Install new clamps and banjo washers
  
• Rebuild or replace the lift pump
  
• Repair or replace the intake heater relay
  
• Upgrade to a high‑CCA battery
  
• Clean all ground connections
  
• Use a block heater in cold climates
  
• Add a manual priming bulb if the machine frequently loses prime
  

These improvements often restore easy starting even on older machines.

Preventive Maintenance Tips
To avoid future hard‑starting issues:

• Replace fuel lines every 5–7 years
  
• Keep the fuel tank clean and free of algae
  
• Change fuel filters regularly
  
• Test the intake heater before winter
  
• Keep electrical connections corrosion‑free
  
• Run the machine regularly to prevent fuel drain‑back
  

Preventive care is especially important for machines that sit unused for long periods.

Industry Insight: Why Small Dozers Are Sensitive to Fuel Leaks
Compact dozers like the D21P‑6 use small‑volume fuel systems. Even minor air intrusion can disrupt fuel delivery. Larger dozers with high‑capacity pumps are more tolerant, but small machines require a perfectly sealed system.
This is why many owners report that once the fuel system is fully sealed, the D21P‑6 starts like a new machine.

Conclusion
Hard‑starting issues on the Komatsu D21P‑6 are usually caused by air leaks, weak intake heaters, low cranking speed, or age‑related compression loss. With proper diagnosis and maintenance, these problems can be resolved, restoring the machine’s reliability. The D21P‑6 remains a legendary compact dozer with a long production history and global popularity, and with the right care, it can continue working for decades.