Mustang MTL25 Hard‑to‑Start Issue

[MikePhua]

Machine Background
The Mustang MTL25 is a tracked compact loader powered by a Yanmar 4TNV106T diesel engine.  According to its specs, it has a standard-flow hydraulic rate of 23–24 GPM and a relief pressure around 2,988 psi.

Problem Description
On some MTL25 machines, particularly in cold mornings, operators need to crank the engine for around 45 seconds before it starts. During that time, the exhaust emits heavy white smoke, which clears once the engine finally fires. After running, the machine restarts more easily — but if the engine cools down for an hour or more, the same hard-start behavior returns.
They’ve already tried:

• Replacing the electric fuel lift pump
  
• Cleaning all fuel filters from tank to engine
  

These fixes helped somewhat, but the core issue persists.

Likely Causes

1. Fuel Leak‑Back or Injector Pressure Loss
   • If the injection pump leaks internally (back to the tank or return lines), fuel pressure could drop after shutdown, starving the injectors on the next start.
     
   • This causes long cranking times as the system repressurizes.
     
2. Preheat / Glow Plug System
   • The machine has a preheat (“grid heater”) system: users mention turning the key counterclockwise for ~15 seconds before cranking.
     
   • Despite this, white smoke during cranking suggests insufficient heat at startup — this could mean glow plugs or grid heater performance is marginal.
     
3. Fuel Filter / Seal Problems
   • Some suggest checking fuel filter bowl O-rings, which, when compromised, can allow fuel to leak or air to enter the system.
     
   • Air in the fuel system or loss of prime can also prolong starting.
     

Diagnostic Steps

• Check Preheat Voltage
  • Use a voltmeter to confirm that the glow plugs or grid heater are getting proper voltage during the preheat phase.
    
  • If there is voltage but no effect, consider replacing the glow plugs or grid heater element.
    
• Test Fuel Pressure
  • After shutdown, install a pressure gauge on the injection pump’s delivery side to see if pressure holds or bleeds off.
    
  • If pressure drops, the injection pump might need rebuilding or internal seals need replacing.
    
• Inspect Fuel Filters and Seals
  • Replace all fuel filters again (primary, secondary) and inspect or replace O-rings in filter housings.
    
  • Bleed the system thoroughly to eliminate air.
    
• Check Fuel Return Lines
  • Inspect return lines from injectors or pump to the tank for leaks or loose connections.
    
  • Ensure return flow is correct and not causing back‑pressure or siphoning.
    

Possible Solutions

• Rebuild or Replace Injection Pump: If the pump is leaking internally, a rebuild may solve the issue.
  
• Replace Glow Plugs / Grid Heater: Upgrade or renew heating elements to ensure proper start temperature.
  
• Improve Fuel System Seals: Use high‑quality O-rings on fuel filters and ensure all connections are tight.
  
• Install a One-Way Check Valve: Adding a check valve on the fuel delivery line may help maintain prime/pressure between shutdowns.
  

Real‑World Insight
One experienced user suggested that losing fuel pressure when the engine is off is the most likely root cause. They recommended testing for pressure bleed-down rather than assuming it’s just a glow-plug issue — especially because white smoke indicates a lot of fuel but insufficient ignition heat.

Take‑Home Recommendations

• Focus on fuel delivery system diagnostics (pump pressure, return leak, filter seals) first.
  
• Confirm the preheat system is functioning correctly (voltage, element condition).
  
• After any repair, make sure to prime and bleed the fuel system thoroughly.
  
• Keep a maintenance log for starts, fuel system changes, and parts replaced — recurring symptoms often trace back to patterns over time.