Bit of an essay to follow but want to get all the relevant info down:
Car: ’92 LHD 300 CE-24 cabrio model 124.061, engine M104.980 (CIS-E injection system + EZL ignition system) with 70k miles on clock
Problem: For 2+ years have suffered from the warm engine erratic idle / rough at low revs problem, so often reported/discussed in the various Merc forums – i.e. car runs perfectly from cold ‘til engine reaches normal working temp (approx 85 deg reached after 7mins or so of driving). Once you then slow down to idle the hesitations start. In the last few months the problem has worsened such that on 3 occasions the car has actually stalled, and then refused to restart until its been left for 30mins or so (presume something needs to cool down). If relevant, each of these occasions has been on the car’s first outing after having been not driven for 4+ days.
Tried remedies: Replaced plugs, leads, lead ends, dizzy cap, OVP, fuel filter, and crankshaft position sensor, plus cleaned the butterfly flap in throttle body …. all without success.
Latest thoughts: Main problem is that nothing ever shows up in the diagnostic codes for either the MAS, EZL or CIS-E controller checks. Based on that I’d started to think it was time to go the expensive route of replacing the controllers one-by-one, but then recently happen to read 2 forum threads that have got me questioning the operation of my oxygen sensor: 1)
“how do you know your oxygen sensor is BAD???”, where similar problems are happening to a similar car; 2)
“E220 Hesitates and shudders while accelerating” on
http://forums.mercedesclub.org.uk where someone states
“the lambda (O2) sensor only generates a fault code if it completely fails, not if it continues to work all be it impaired.”. If this 2nd post is correct, then it might be time to look more closely at the O2 sensor….
Today’s investigations: Having read up various posts regarding the operation and testing of the O2 sensor it seems the thing to do is check out the mV values sent from the sensor to the CIS-E control unit using a multi-meter; this being possible either directly off the sensor, or at one of the pins of the CIS-E control unit, or about half way between the two where the cables have a connection in the black plastic box under the floor of the front passenger’s foot-well. As this last place seems to be the most recommended and the easiest to access, that’s where I went today (see attachment).
Test method: Drove the car for 10mins or so from cold so engine temp got up to 85 deg plus erratic idle started. Stopped but left engine idling; disconnected the circular connector of the single wire – black wire with female connector on sensor side / thicker green wire with male connector on chassis harness side; left alone the companion two wire connector that I think is for the sensor’s heater; put one probe of multi-meter into the female connector for the sensor’s signal and other probe onto a nearby screw-head to earth it.
Results: - Engine idling @ 600rpm: steady value of 0.70V +/- 0.02V read
- Up revs to 1500: steady value of 0.9V +/- 0.05V
- Let revs drop back to 600rpm: steady 0.70V +/- 0.02V returns
Turn off engine, leave for 10 mins, restart – engine temp still 85 deg:
- Engine idling @ 600rpm: reading initially rises from 0.0 to 0.20V
but then drops back to a steady 0.08V +/- 0.01A (i.e. 1/10th
of previous idle value!)
- Up revs to 1500: steady value of 0.9V +/- 0.05V
- Let revs drop back to 600rpm: drops right back to a steady
0.08V +/- 0.01V
Reconnect cable, take car for 5min spin then disconnect and repeat
both tests:
- Same sets of values result – i.e. firstly 0.70V at idle then after
a 10min rest 0.08V at idle
Questions:
1) Is this method of testing valid with the sensor’s signal cable completely disconnected – i.e. no valid signal (?) is being sent to the CIS-E control unit so it’s not trying to do any adjustments, or should I somehow be tapping into the signal without completely opening the connector?
2) Do the values I’m noting actually tell me anything about the integrity of the sensor?
3) What should I do next to either confirm/exclude the O2 sensor as the root-cause of my problems?
Hope those in the know have persevered through all this text and don’t mind sharing their wisdom…..
Kind regards,
Neil (London)