The 4d56 engine, a 2.5-liter turbocharged diesel engine, is renowned for its reliability and durability. Found in various Mitsubishi vehicles, including the Delica and Pajero, this engine can benefit from proper tuning to optimize performance and fuel efficiency. This guide delves into the intricacies of tuning the 4D56, focusing on key adjustments and considerations.
Fine-tuning the 4D56: Fuel and Boost Adjustments
Initial adjustments begin with setting the idle. With the engine running, adjust the full load screw (S11) until the idle reaches approximately 750 rpm. Secure the locknut while the engine runs. This screw influences baseline fueling; screwing it in increases fueling. Be cautious not to over-tighten, as this could cause the engine to run away, meaning it struggles to return to idle. If the engine exhibits this behavior, back off the full load screw and utilize the idle speed screw for adjustment.
After setting the idle, road testing and fine-tuning are necessary. Two primary scenarios require attention: full load (e.g., hill climbing) and acceleration from light load or idle.
Full Load Tuning
Under full load and maximum boost, an air/fuel ratio gauge should ideally read a steady 20:1. A slightly richer mixture (17-20:1) can produce more power but also elevates exhaust gas temperatures (EGTs). Without a gauge, monitor the exhaust smoke and EGTs. Black smoke and high EGTs (exceeding 1200°F or 650°C) indicate excessive fueling. Adjust the boost compensator diaphragm (S8) to reduce fueling under boost. Rotate the diaphragm to move the offset cone away from the engine’s front. Repeat this process until EGTs are within acceptable range. If boost levels are increased later, this adjustment needs revisiting.
Adjusting the diaphragm might affect idle speed. If so, compensate by adjusting the full load screw (S11). If the diaphragm reaches its maximum adjustment (180 degrees) and the issue persists, two solutions exist: tighten the diaphragm spring seat (S7) by two turns or, if that fails, lengthen the idle screw (S4) by approximately 5mm and restart the tuning process from step five.
Light Load and Acceleration Tuning
During acceleration, a brief air/fuel ratio of 16:1 is desirable initially, quickly transitioning to a steady 20:1 once boost reaches around 8 psi. If tuning by smoke and EGTs, a brief puff of black smoke upon hard acceleration is acceptable, particularly from idle, as it aids turbo spool-up. Excessive smoke necessitates tightening the star wheel/spring seat (S7), increasing spring pressure on the diaphragm and limiting fuel injection until sufficient boost is present. A significantly loose compensator spring can cause irregular puffs of black smoke at idle due to diaphragm instability. Remedy this by tightening the spring seat.
After achieving satisfactory driving performance, readjust all cables, linkages, and AC idle-up mechanisms to factory specifications. Reinstall the throttle position sensor if removed. Subsequently, use the idle speed adjuster for minor idle speed adjustments.
Further Considerations for the 4D56
The stock TD04-09B turbo can reliably handle 14 psi of boost. If EGTs are high at 12 psi, increasing boost to 14 psi can lower EGTs without sacrificing power. However, this increases intake air temperature, requiring careful monitoring of timing advance. Consider an intercooler to mitigate this issue. Upgraded turbos coupled with intercoolers might allow for boost levels up to 17 psi, beyond which gains diminish and risks increase.
Conclusion: Achieving Optimal Performance
Tuning the 4D56 involves a delicate balance. By meticulously adjusting fuel delivery and boost pressure, significant performance improvements can be achieved. However, caution is paramount. Always prioritize safe tuning practices and monitor EGTs to avoid engine damage. This guide provides a foundational understanding of 4D56 tuning, empowering owners to unlock the full potential of their engines while ensuring longevity.
