Reviving a Stuck CAV Injector Pump on the Massey Ferguson 135

[MikePhua]

The MF135 and Its Agricultural Legacy
The Massey Ferguson 135 is one of the most iconic utility tractors ever built. Introduced in the mid-1960s and produced into the late 1970s, the MF135 was powered by either a Perkins AD3.152 three-cylinder diesel engine or a Continental gasoline variant. With over 400,000 units sold globally, it became a staple on farms from North America to Africa, prized for its simplicity, reliability, and ease of repair.
The Perkins diesel version, in particular, earned a reputation for cold-start reliability and fuel efficiency. However, like many mechanical injection systems of its era, the MF135’s CAV DPA (Distributor-type Pump Assembly) injector pump can suffer from internal sticking, especially after long periods of inactivity or exposure to contaminated fuel.
Symptoms of a Stuck Injector Pump
When the CAV pump fails to deliver fuel to the injectors despite confirmed flow to the inlet, the issue often lies within the pump’s internal metering valve or plungers. Common signs include:

• Engine cranks normally but fails to start
  
• Fuel reaches the pump inlet and bleeder screws, but not the injector lines
  
• Manual shutoff lever feels stiff or unresponsive
  
• No fuel spurts from injector ports during cranking
  

These symptoms suggest that the internal fuel metering valve or plungers are seized due to varnish, corrosion, or debris.
Initial Diagnostics and Bleeding Procedure
Before assuming pump failure, basic checks should be performed:

• Verify fuel flow from tank to filter and pump inlet
  
• Bleed the pump using the 5/16-inch screw near the nameplate
  
• Crank the engine with the throttle wide open and shutoff lever in the run position
  
• Loosen injector lines and observe for fuel spurts
  

If fuel shoots from the bleeder screw but not from injector ports, the issue is internal to the pump.
Freeing a Stuck Metering Valve
The metering valve regulates fuel delivery based on throttle input. When stuck, it blocks fuel from reaching the plungers. To address this:

• Remove the top cover of the CAV pump carefully
  
• Clean visible components with diesel and lint-free cloths
  
• Exercise the shutoff rod and throttle linkage to check for free movement
  
• Tap gently with a screwdriver handle to loosen sticky parts
  
• Reassemble with a new top cover gasket and O-rings
  

Mini gasket kits are available online and typically cost under $15. These include seals for the top cover and shutoff rod.
Risks and Precautions
Removing the top cover exposes delicate components, including springs and linkages. Key precautions include:

• Photograph the assembly before disassembly
  
• Note the position of the governor spring and throttle linkage
  
• Avoid forcing parts—use solvent and patience
  
• Do not crank the engine with the cover removed
  

One farmer in North Carolina attempted a top cover cleaning without documenting spring positions. After reassembly, the shutoff rod jammed, and the throttle failed to respond. He had to reopen the pump and realign the linkage, losing a day of work.
When to Consider Full Pump Removal
If cleaning the top cover does not restore fuel delivery, deeper issues may be present:

• Stuck plungers due to varnish or rust
  
• Internal corrosion from water-contaminated fuel
  
• Broken governor components
  

In such cases, the pump must be removed and rebuilt. Removal involves:

• Disconnecting fuel lines and throttle linkage
  
• Removing mounting bolts and timing gear cover
  
• Marking timing positions to preserve injection timing
  
• Sending the pump to a qualified rebuilder
  

Rebuilds typically cost $250–$400 depending on region and parts availability. Some operators opt for exchange units from reputable suppliers.
Preventive Measures for Long-Term Reliability
To avoid future injector pump issues:

• Use clean, water-free diesel and drain water separators regularly
  
• Add fuel stabilizer during off-season storage
  
• Start and run the engine monthly to keep internals lubricated
  
• Replace fuel filters annually
  
• Keep the tank full to reduce condensation
  

For tractors stored in humid environments, consider installing a desiccant breather on the fuel tank to reduce moisture ingress.
CAV Pump Design and Historical Context
The CAV DPA pump was developed by Lucas CAV in the 1950s and became standard on many British and European diesel engines. Its rotary distributor design offered compact packaging and reliable performance. By the 1970s, it was used on Perkins, Leyland, and Ford engines worldwide.
Despite its durability, the pump’s internal tolerances are tight, and varnish buildup can easily seize moving parts. Regular use and clean fuel are the best defense.
Conclusion
A stuck CAV injector pump on the MF135 is a solvable problem with patience, methodical diagnostics, and careful cleaning. Whether freeing a metering valve or preparing for a full rebuild, understanding the pump’s function and vulnerabilities is key. In the world of vintage tractors, the MF135 remains a workhorse—and its heart, the CAV pump, deserves the same care and respect as the soil it helps cultivate.