The Mercedes-Benz M104 engine, a 6-cylinder DOHC powerplant, was a significant step forward for the German manufacturer, powering a range of models in the 1990s. However, not all M104 engines are created equal. Significant differences exist between the earlier and later iterations, primarily concerning their fuel injection and ignition systems. This article delves into these variations, highlighting the strengths and weaknesses of each.
Early M104 (3.0 Liter): CIS-E Fuel Injection System
The initial version of the M104 engine, the 3.0-liter variant (M104.990), found in models like the 1990-1992 300CE, retained the CIS-E fuel injection system. This system, also utilized in the preceding M103 engines, is characterized by its blend of mechanical injection with electronic control. It employs a distributor, a single coil wire, and six spark plug wires, representing a more traditional approach to fuel and ignition management. Essentially, CIS-E is a mechanically based system with electronic enhancements for finer control.
Later M104 (3.2 Liter): HFM-SFI and Advanced Technology
In 1993, Mercedes-Benz introduced the 3.2-liter M104 engine, marking a technological leap. This updated engine adopted the HFM (Hot-Film Mass Air Flow sensor) fuel injection system. HFM-SFI is a fully electronic system that integrates electronic ignition and sequential fuel injection into a single module. A key change is the elimination of the distributor in favor of individual ignition coils mounted directly on the spark plugs. Each coil pack serves two spark plugs simultaneously, one directly and the other via a short high-tension lead. This setup results in three coil wires and three high-tension lead wires.
The HFM system also introduced electronically controlled idle speed, removing manual adjustment. Furthermore, it incorporates adaptive technology to compensate for engine wear and unmeasured air intake, maintaining optimal driveability as the engine ages. Another significant advancement with HFM-SFI is its ability to selectively retard ignition timing only in cylinders experiencing knocking, unlike the earlier EZL system which retarded timing across the entire engine. This precise control maximizes power output by keeping the ignition timing as advanced as possible.
Adding to the improvements, the 3.2-liter M104 engine features variable valve timing on the intake camshaft. This innovation broadens and flattens the torque curve, enabling the engine to develop horsepower at lower RPMs. The result is more accessible and noticeable power, enhancing the driving experience.
Potential Issues and Running Costs
While the 3.2-liter M104 engine offered performance and technological advantages, it also brought potential maintenance concerns. Early M104 engines, particularly the later 3.2L versions, were known to have head gasket problems. Additionally, Mercedes-Benz models from 1993 to 1995, equipped with the M104, are notorious for suffering from deteriorating engine wiring harnesses. Finally, the M104 engine incorporates an electronic throttle actuator, which is a common failure point and can be expensive to replace.
Interestingly, the 1992 300E models, equipped with the earlier 3.0L M104, avoid some of these issues. They do not suffer from the biodegradable engine wiring harness problem, nor do they utilize a problematic electronic throttle actuator. Instead, they employ a more reliable idle control valve system.
Conclusion
The Mercedes-Benz M104 engine evolved significantly during its production run. The transition from the 3.0-liter CIS-E version to the 3.2-liter HFM-SFI variant brought substantial technological advancements, resulting in improved performance and efficiency. However, potential issues such as head gasket failures, wiring harness degradation, and throttle actuator problems can make the later M104 engines more expensive to maintain. Understanding these differences is crucial for Mercedes-Benz enthusiasts and owners when considering or maintaining vehicles equipped with the M104 engine.
