Introduction to STIHL MS 500i Electronic Fuel Injection (EFI) Architecture
The STIHL MS 500i represents the world's first mass-production chainsaw incorporating Electronic Fuel Injection (EFI), dispensing entirely with conventional carburetors. In this 79.2 cc two-stroke engine (6.8 HP), fuel metering is calculated real-time by an engine control unit that monitors crankcase pressure impulse profiles and intake temperatures, yielding hyper-fast acceleration from 0 to 100 km/h chain speed in just 0.25 seconds.
Crankcase Impulse Dynamics and Pressure/Temperature Sensing
In a two-stroke EFI engine, the crankcase functions as a displacement scavenging pump. Crankcase absolute pressure fluctuates dynamically from **-0.4 bar** (during piston upward stroke) to **+0.7 bar** (during piston downward stroke). The STIHL MS 500i utilizes a combination Pressure/Temperature sensor mounted directly in the crankcase wall:
- Injection Timing Strategy: The control module triggers the solenoid injector into the intake manifold once per revolution based on the exact phase angle of the crankcase pressure pulse wave.
- Generator Flywheel Power Supply: Operating completely battery-less, a high-output flywheel generator produces immediate electrical power to pressurize the fuel system (up to 4.5 bar) on the very first pull of the recoil starter rope.
- Crankcase Hermetic Integrity: Any vacuum leak across the crankshaft radial seals or cylinder base gasket dilutes the intake charge, causing catastrophic lean-burn piston seizure.
Diagnostic Protocol via STIHL MDG 1 / Husqvarna CST Diagnostics
Servicing STIHL MS 500i and Husqvarna AutoTune (550 XP, 572 XP) engines requires dedicated diagnostic hardware (STIHL MDG 1 or Husqvarna Common Service Tool):
- Crankcase Pressure/Vacuum Leak Test: Seal the intake and exhaust ports using rubber blanking plates. Apply +0.5 bar pressure and -0.5 bar vacuum using a pressure pump; the crankcase must hold steady pressure for at least 20 seconds.
- Solenoid Fuel Injector Test: Measure the injector internal solenoid coil resistance. Standard specification is 1.8 Ω to 2.2 Ω. Execute an automated flow test via the MDG 1 software.
- Spark Plug Specification: Require the exclusive use of resistor-type spark plugs (NGK CMR6H) to prevent electromagnetic interference (EMI) from corrupting ECU calculations.
Technical Specifications and Fastener Torques
- Cylinder to Crankcase Fasteners (M5 Torx): 10 Nm (applied with medium threadlocker).
- Flywheel Retaining Nut: 28 Nm.
- Fuel Pump / Generator Module Screws: 6 Nm.