E300 hesitation resolved; runs better w/o ECU control
 

Big thanks to this forum especially to inputs of Stevebfl.

My 1992 E300 24 valve cisE suffers from severe warm hesitation with black smoke since I acquired it 2 months ago.
and last week goes into limp mode with max speed of 20 kph
and carbon build-up on plugs probably because of wiring harness
issue.

Coz of difficulty of sourcing and cost, i have it rewired for $100 by an electrician as temporary measure.
so far it has been doing well against the advise of an MB enthusiast coz it might affect performance as experienced by some.

But anyway despite rewiring i still have warm hesitation, black smoke, 0-100 in 20-30 sec, stalling even if i unplugged the coolant temp sensor, R16/2 resistor and using 95 octane gas.

After reading the forum last night, this morning i tried to adjust the AF (CO) because stalling and backfiring indicates lean mixture. after several rotation stalling resolved, i test drove it and runs fine. I planned to unplug Ox sensor but dont want to dirty
my hands

but after lunch with warm weather stalling returns.

FINALLY, I TRIED UNPLUGGING THE EHA (the final step in ECU control of fuel metering), the stalling resolved and runs better
and smoother.

NEXT DIAGNOSTIC PLAN is to unplug Oxs, replug the EHA
and unplug or replace sensors
that are telling the EHA to run lean
1. CTS
2. altitude sensor
3. air temp sensor (none)

Hope to find the culprit sensor and that the ECU is not the problem and return ECU control mode for better emissions.

Without EHA, car runs like a carburated engine without computer
control and probably can be suggested as an emergency
measure but make sure that the AF mixture is first adjusted with EHA removed because it mighty be on the lean side and the engine will stall.

follow up:

after removing the O2 sensor and replugging the EHA,
my car have not stalled the whole day.

also have my EHA current under observation by placing a multimeter in series and the current hovers around 0.02-0.03 ma
with 1-3 seconds increase in current when pedal is pressed.


when ignition is on and at start, the current peaks at 19 ma.

I follow these kinds of posts if for no other reason than to enhance my confusion on the CISE. If the EHA is getting what appears to be proper current and the O2 sensor is unplugged, then where is that signal coming from??? :confused:

input signals come from temp sensors, altitude sensors,
air temp sensors, air flow potentiometer, Oxs etc.

when any of sensor fail, the computer defaults to a lean mixture.

example: a failing coolant temp sensor tells the ECU that the engine is overheating although its not and the current give to EHA is to lean the fuel delivery.


The EHA is the one that electronically controls fuel metering in the fuel distributor for optimal fuel delivery and if unplug you go the mechanical metering through your accelerator pedal.

however with EHA, , it is more responsive probably because of throttle switch signal at high/full load, a higher current or addtional fuel is delivered on top of basic measurement.

My understanding is that if any sensor input is interrupted then the system goes to open loop or 'limp mode'. At least I think it does with the KEIII. At that point you are all mechanical with no electronic tweaking of mixture control.