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Arench, Master Technician
Category: Mercedes
Satisfied Customers: 1490
Experience:  30-years experience auto and truck repair.
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S600: I own a 1996 Mercedes S600 Se***** ***** Wheel Base and

Customer Question

I own a 1996 Mercedes S600 Se***** ***** Wheel Base and currently have a problem where the O2 Sensor, O2 Heater, Catalytic Converter, Secondary Air Injection, and the Evaporative Inspection and Maintenance (I/M) status are not being set. Have changed all four O2 sensors (upstream and downstream), check spark on all 12 coils, replaced the fuel filter due to low pressure at the distribution manifold, and changed the air filters. The Bank 1 (Master) shows a Long Term Fuel Trim (LTFT) of -1.6%, and bank2 LTFT of 4.7%. I can see the Motronics 1.0 systems go into Open Loop mode when the car starts from a cold condition and later transitions into a Closed Loop mode, but with a CL-Fault indication on both banks. Car runs smooth and with lots of power. Any good suggestions? I need at least 3 I/Ms to get tho ready in order to pass the NJ inspection.
Submitted: 2 years ago.
Category: Mercedes
Customer: replied 2 years ago.
Also, there are NO Fault codes or pending code that I can see using my OBD2 scanner.
Expert:  Arench replied 2 years ago.
Good morning my name is ***** ***** I will try to help you sort this out.I have an emissions test and repair shop in Hartford, CT.First you should know your car is notoriously hard to become ready.As much as 1200-1500 miles of driving may be needed to get ready or set malfunction codes.One drive cycle is almost two hours.As a point of general knowledge: the three main stumbling blocks for readiness on this car areengine coolant temperature too low (below 80c), fuel trim above or below 1% and fuel level gauge inoperable. Knowing this we can already see a possible issue with your fuel trim. You need to investigate possiblevacuum leaks or clean or replace the mass air flow. Monitor the fuel trim with hot engine and slight throttle 1500-1700 RPM in park and expect to see fuel trim much closer to zero. Work toward this goal. Replace thermostat is engine runs too cold. Air conditioner control is to remain off during drive times. Fuel tank level should be around 1/2 to 3/4. Your drive cycle: (print this) Start and idle until coolant temp reaches 90cDrive with steady throttle in city ~40mph for 7 minutes.Idle in park for 4 minutes.Drive on highway with steady throttle ~55-65mph for 20 minutes.Idle in park for 2 minutes.Drive in city ~40minutes for 7 minutes.Idle in park for 2 minutes.Turn engine off for 1 minute.Start and idle for 20 minutes.Turn engine off for 1 minute.Drive any speed with steady throttle for 20 minutes.End of test.
Customer: replied 2 years ago.
Thank you for your quick reply. The car warms up fairly quickly to 80C and the fuel gauge works, therefore will focus on the LONG term fuel trims. As you can see from my question, the LTFT are out f the band that you indicated; inside + - 1%. I assume you meant plus or minus 1%. Is that correct? Given that the other two "stumbling block" are out of the way, how can I bring the fuel trims into the "good range?
Expert:  Arench replied 2 years ago.
Fuel trim out of whack is generally vacuum leaks, mass air flow, anything that interferes with fuel delivery.
Quote from above:
"Knowing this we can already see a possible issue with your fuel trim. You need to investigate possible vacuum leaks or clean or replace the mass air flow."
Customer: replied 2 years ago.
is it possible to work on the I/M Status one at a time. For example O2 Sensor first. Should it be the first to get to READY. From the Live Data on my scanner I can see that the car goes from Open Loop to Closed Loop and at that time it should be using the O2 sensors to control the fuel/air mixture. Why is it showing CL-Fault?
Expert:  Arench replied 2 years ago.
The car tries to set all the monitors at once. The logic is not like that of more modern cars.
I don't know why it's showing fault. Most likely it's because the fuel trim is too far out of range.
Customer: replied 2 years ago.
I cleaned the AMFs a few weeks ago and the scanner show a reasonable airflow while in idle. Can you suggest a good air flow at other RPMs. I placed an order for a USB HD camera that I can connect to my laptop a look between the fire wall and the engine. It should arrive today. My local independent MBZ mechanic did a water bottle test - pour water around the intake manifold, but did not detected/found any.
Customer: replied 2 years ago.
Lets say that bank 2 is trying to compensate for a leak and that is why the LTFT is at 4.7. But bank 1 is lean (-1.6), what can cause this condition? A leaking injector/s?I rather type and save the thread for future reference.
Expert:  Arench replied 2 years ago.
a smoke machine is better for finding air leaks than anything else.
Sometimes cleaning is not enough on a mass air flow. Sometimes they need to be replaced.
Air flow is relative to your car and air pressure where you live, you can not use comparative numbers. Think in terms of more or less air flow, not whether it's "right".
Some times I put a piece of foil tape blocking part of the inlet side edge of the mass air flow to raise MAF readings.
Weak engine, weak compression, poor cylinder balance can all cause MAF reading to go off, not just the fuel system.
Exhaust leaks in the manifolds will also cause false fuel trim readings.
Your biggest concern that I see is that bank1 and bank2 do not match and there is a large differential between the two. This indicates an undiagnosed problem.
Expert:  Arench replied 2 years ago.
Fuel injectors have to be tested as a group for spray pattern and volume delivery. You can not condemn them based on circumstantial information. If one is leaking or some are clogged that will throw all the rest if the diagnosis in the wastebasket but we have no information about the injectors at this point.
Customer: replied 2 years ago.
I am going offline for a while, will be back this afternoon.
Expert:  Arench replied 2 years ago.
Okay, lots and lots to think about here. I will be around this weekend between pool visits. Good luck!