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ke6dcj 08-06-2001 01:12 AM

Fuel Trim? OBDII cycle and CHECK ENGINE ?
I have changed out the O2-sensors with FACTORY MB O2-sensors for the
1995 C36. These are a 4-wire heated oxygen sensor, and yes, they do
have a small-opening at the tip vs. the 1995 E320 4-wire heated sensor
with no tip opening.

Unfortunately, I'm still getting a CHECK ENGINE light. However, armed
with the OBDII procedure cycle (below), I have confirmed that it comes
on during STEP 5-DECELERATION, which tests EGR, Purge and Fuel Trim

An OBDII scan showed the following:

MIL status=129, ECU Monitor status=7
Misfire monitoring COMPLETE
Fuel System Monitoring COMPLETE
Component Monitoring COMPLETE
** Catalyst Monitoring INCOMPLETE **
Evaporative System Monitoring COMPLETE
** Sec. Air System Monitor INCOMPLETE ** (what is it?)
02 Sensor Monitring COMPLETE
02 Sensor Heater Monitoring COMPLETE
Fuel Sys1-Open Loop Mode

Short Term Fuel Trim (FT): 271%
Long Term FT, B1: 4%
Short Term FT, B2: 271%
Long Term FT, B2: 271%
Cmd. Sec. Air Status = 4
# of O2 sensors= 8 (why eight?)

A scan of the OBDII fault codes revealed one manufacture specific fault
code, and one generic OBDII fault code.
Factory fault code: P1132- Manufacture controlled Fuel & Air Metering
Generic OBDII fault code: P0135 (O2 Sensor Heater Circuit Malfunction
(Bank 1 Sensor 1).

1) what is fuel-trim, and is 271% too high? What is the range and under
what circumstances?
2) I've noticed that the 1993 E320 M104 wagon does not have a fuel
accumulator, nor does the 1995 C36, can this contribute to too high a
fuel-trim and thus the CHECK ENGINE light? If so, should I remove the
3) could having a higher rear-end (3.07 vs. 2.87) cause a mismatch of
fuel-trim to RPM/speed, thus activating the CHECK ENGINE light?
4) should I clean out the PCV steel-lines to EGR valve with a used
speedo-cable? (a known problem, see the bottom of:
5) is the CIS dual-pumps too much pressure vs the single screw-pump of
the C36? I wouldn't think so, because the 1995 E320 M104 wagon has dual
CIS pumps but again, not accummulator.
6) why am I still getting generic OBDII fault code P0135 (O2 Sensor
Heater Circuit Malfunction) even with a new sensor?
7) Fuel-Pressure: 1428 KPa (is this too much?)

Some history:

1988 300TE with a 1995 C36/M104 AMG engine, tranny, and engine
computer. Because the W202 C-class has a different ABS teeth count vs.
the W124 E-Class we have gone to 1995 E320 ABS and Electronic
We also are running a 3.07 diff vs. the stock 2.87 diff.

We are still running the dual CIS fuel-pumps with the fuel accumulator.
I was experiencing intermittant CHECK ENGINE lights and the codes were
pointing to a LEAN CONDITION. Since we had retained the original two
fuel pumps from the CIS M103, I decided to change the fuel-filter,
thinking it may have an obstruction.

After changing the fuel-filter I have gone approx. 3-months without any
CHECK ENGINE lights. Just two weeks ago, I decided to change to a 3.07
limited-slip rear differential. And the next day, I got a CHECK ENGINE
light while going through STOP-N-GO traffic.

OBDII Cycle Instructions:
A complete driving cycle should perform diagnostics on all systems. A
complete driving cycle can be done in under fifteen minutes. To perform
an OBDII Driving cycle do the following:

1.Cold Start. In order to be classified as a cold start the engine
coolant temperature must be below 50C (122F) and within 6C (11F) of
the ambient air temperature at startup. Do not leave the key on prior to
the cold start or the oxygen sensor diagnostic may not run.

2.Idle. The engine must be run for two and a half minutes with the
air conditioner on and rear defroster on. The more electrical load you
can apply the better. This will test the O2 heater, Passive Air, Purge
"No Flow", Misfire and if closed loop is achieved, Fuel Trim.

3.Accelerate. Turn off the air conditioner and all the other loads
and apply half throttle until 88km/hr (55mph) is reached. During this
time the Misfire, Fuel Trim, and Purge Flow diagnostics will be

4.Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for 3
minutes. During this time the O2 response, air Intrusive, EGR, Purge,
Misfire, and Fuel Trim diagnostics will be performed.

5.Decelerate. Let off the accelerator pedal. Do not shift, touch the
brake or clutch. It is important to let the vehicle coast along
gradually slowing down to 32km/hr (20 mph). During this time the EGR,
Purge and Fuel Trim diagnostics will be performed.

6.Accelerate. Accelerate at 3/4 throttle until 88-96 km/hr (55-
60mph). This will perform the same diagnostics as in step 3.

7.Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for five
minutes. During this time, in addition to the diagnostics performed in
step 4, the catalyst monitor diagnostics will be performed. If the
catalyst is marginal or the battery has been disconnected, it may take 5
complete driving cycles to determine the state of the catalyst.

8.Decelerate. This will perform the same diagnostics as in step 5.
Again, don't press the clutch or brakes or shift gears.

stevebfl 08-06-2001 08:54 AM

Where to start???

"** Sec. Air System Monitor INCOMPLETE ** (what is it?) "

Yes, I think I can answer that question. The whole list you have given are called monitor tests. They represent specific testing required to maintain OBDII standards. The testing can be done in various ways by various manufacturers and is sometimes not required or inactive. These tests can be intrusive at times and are generally done (quickly) when the conditions are correct.

In the case of secondary air. This is the AIR pump. Air thats is directed to the exhaust during cold running to continue the oxidation process and heat the cat. The monitor test which only is conducted under certain conditions, excites the pump and looks for an immediate reduction of O2 sensor voltage below 100mv (I think). The test as run by the scanner powers the pump after closed loop and asks for the above voltage after some real long time like 20 seconds. In reality the voltage drop is almost instantaneous if the pump is working. BTW any Air leaked through this system after closed loop would probably cause a P1132 code among others.

The sec air system has an off/on control valve after the pump, before the exhaust. It must be closed after cold running to prevent air from being aspirated through the pump into the exhaust.

Got to go but for the rest I must say that many manufacturers and OBDII regs state adaptation values different; block learn, integrator values, percentages, etc. I do not recognize the numbers your scanner gives. The 271 doesn't ring any bells.

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