1 Vehicle Details:
• 1998 Mercedes E240
• Engine No M112
2 The Problems
Engine does not start!
3 Investigation with Scan Tool
No current faults logged in system (ME2-SFI), however a condition (live data) was stored: Safety fuel shut off is ON.
4 Investigation with PicoScope
Somewhere deep in the Mercedes Manuals I found a hint that a SAFETY Fuel Shut OFF is initiated by the ECU when there is something wrong with the throttle body. I then went ahead and carried out some checks. Firstly, to check the pedal position potentiometer is working correctly; both sensors can be seen below standard blue and emergency limp home sensor red.
Next, I checked the throttle body end stop switches. Both seem to be correct (S1 & S2) for CTP (Closed Throttle Position) and WOT (Wide Open Throttle). One operates at 4.2 V the other at 0.8 V. I am not sure if the values are correct but I can tell it is switching at least.
The throttle valve position potentiometer output remains constant at 4.2 V (blue), with no change during re-programming or at ignition ON. I can hear the throttle motor working but the position is frozen at almost maximum. The Motor voltage (12 V green) is switched off at the moment I want to start the car or push the gas pedal. This is due to a safety fuel shut off being activated by the ECU. Red is showing the GND to body GND.
After removing and inspecting the throttle body it was clear the throttle valve had heavy carbon deposits, causing the throttle valve flap to be stuck in the open position. The ECU then switched to a safety mode “fuel shut off”.
After cleaning and re-programming (adaption reset) the car started without problems and the idle speed was fine.
Case Study Information
Detail: Andy Cape - Cape Auto Diagnostics
Author: Mike Valentine
Technical Author @ Pico Technology UK