The Mitsubishi 4M40 engine was originally designed for the Mitsubishi Pajero SUV, with an output of 150 PS. It is an automotive-type diesel engine, featuring a timing chain drive, single overhead camshaft (SOHC), and shim-adjusted valve clearance. The original fuel injection pump was a Delphi (U.S.) distributor-type pump, with a pre-combustion chamber and single-hole injectors, representing the 1970s–1980s level of technology.
It is known for high fuel consumption and poor cold-start performance, but runs smoothly and quietly (especially the versions with twin balance shafts).
The 4M40 is a 2.8L low-displacement diesel engine, available in both naturally aspirated and low-boost turbocharged variants (with different power ratings).
Besides automotive applications, it has also been widely used as a power source for construction machinery, such as CAT and Sumitomo excavators in the 5–8 ton class.

Cold-Weather Starting Characteristics

When the machine is in good condition, this engine is designed to start normally at ambient temperatures as low as –20°C, provided that the battery, glow plugs, starter motor, cylinder compression, and fuel system are all in proper working order.
Any fault in these subsystems—especially the electrical preheating system—can severely affect cold starting.

Many operators complain that the 4M40 is difficult to start in cold weather, but this is usually due to poor machine condition, not a design flaw. Even a Rolls-Royce will stall if neglected! In our company’s other department (Mercedes-Benz service), similar issues are often seen with vehicles being towed in.

Complaining doesn’t fix machines—problem-solving does. The 4M40 is already an obsolete model, and blaming it changes nothing.

About the Timing Chain “Problem”

Many users criticize the 4M40 for its timing chain system, but this is a misunderstanding of design intent.
The 4M40 was built for automotive applications, where noise level is a key design indicator.
While spur gear drives are more durable, they produce more noise. Therefore, chain drives are used to reduce noise.
Mercedes-Benz and Volkswagen engines also use timing chains, which still require periodic replacement—it’s not a defect.

Versions Used in China

Due to cost factors, two versions of the 4M40 are commonly seen in China:

1. Imported or “gray market” 4M40 — equipped with dual balance shafts, providing smoother operation and longer chain life.

2. Locally assembled or cost-reduced versions (e.g., 307D) — without balance shafts. These engines have rougher operation and are often used in low-speed working conditions, causing severe vibration. As a result, timing chain inspection or replacement is often needed after about 2000 hours of operation.
   This is a typical problem when a high-speed automotive engine is operated at low speed for long periods.

This model is no longer in production, so users must adapt to its characteristics rather than complain.

Cold Start Troubleshooting Recommendations

During discussions in 2010 with Mitsubishi experts investigating 4M40 complaints in mainland China, no official response was provided regarding these design issues.

For users facing cold-start problems, it is strongly recommended to consult an authorized dealer or service agent for accurate diagnostics.
Street repair shops might occasionally solve the issue by luck, but authorized dealers have trained technicians and proper diagnostic tools.

For users with technical knowledge, preliminary inspection can be done as follows:

1. Check the electrical system:
   Ensure all fuses are intact, wiring is correctly connected, and glow plugs show no visible damage.
   Open the oil filler cap and check for excessive blow-by under both hot and cold conditions (this engine uses a closed crankcase ventilation system, so no external vent tube is visible).
   If the engine is hard to start in both hot and cold conditions, the issue lies elsewhere and is not specific to cold-starting.

2. Check glow plug heating:
   Turn the ignition ON, then lightly touch each glow plug to feel if it warms up (⚠️ Do not grab directly—risk of burns).
   If some are hot and others not, replace the faulty glow plugs and retest.
   (Note: Improper removal may cause plugs to break inside the cylinder head—use qualified technicians.)

3. If all glow plugs fail to heat:
   The issue likely lies in the glow control module (microcontroller board)—seek professional service.
   Attempting to repair it without understanding the circuit can cause further damage.

4. If glow plugs work but white smoke appears during cranking:
   This usually indicates low cylinder compression, which is common in high-hour engines or those with wear on piston rings or valves.