Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help

Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Tech Help

LinkBack Thread Tools Display Modes
Old 12-27-2006, 10:45 PM
Registered User
Join Date: Dec 2006
Posts: 11
M103 W126 part throttle mixture adjustment


I'm working on a US non California 1990 300SEL with 185k on the clock. It was just taken in for Massachusetts state emissions testing, and failed the NOx portion of the test. Max allowable emission is 3.00 grams per mile, measured was 4.14 grams per mile. CO and HC emissions are well within the allowable limits.

This happened 2 years ago, last time this car was tested. After running through the usual suspects such as fresh oil/filter, new spark plugs, verifying no vacuum leaks, verifying acceptable spark timing, and verifying EGR operation, I ran the car on injector cleaner (the kind with a small bottle of lovely bioaccumulative chlorinated solvents hooked directly to the fuel distributor and powered by compressed air, not stuff poured in the tank), then setting the lambda duty cycle as monitored with a dwell meter across pins 2 and 3 on the x11 connector to near 50%, and within 10% of each other at idle and 2,500 rpm. I tested at no load with a 4 gas EGA and found that NOx numbers were still high just revving the engine up in neutral. As I richened the mixture with the allen screw on the airflow meter past the specified setting, CO and HC came up a bit and NOx fell a lot. I recorded the lambda duty cycle readings at this setting and test drove the car. It ran poorly, with a massive hesitation. When I reset the duty cycle to spec, the car ran great, but NOx emissions were high again at no load high rpm.

So, when the car went for its retest, I adjusted the airflow meter while watching the lambda duty cycle at the x11 connector to get back to the setting I knew lowered the NOx value. It worked, the car passed emissions, and I adjusted the FI back to where the car ran well.

Fast forward to today. Those same settings no longer worked, and the NOx was too high.

Within the last few months, the car has been using quite a bit of oil and blowing blue smoke. It managed to burn enough oil to apparently foul its plugs about a month ago, when the car was parked by its owner (Hi, Dad!) until I was available to work on it.

I replaced the valve seals, which were good and worn, along with a few gaskets and seals here and there. I also replaced the fuel filter, verified compression and leakdown numbers were good, verified good system fuel pressure, forgot to record the differential pressure between the upper and lower chambers in the fuel distributor, removed all the injectors to clean them, reinstalled them such that their spray patterns were all pointing the same way (none were perfectly conical, but all were "acceptable" as shown in the service literature and Bosch FI book, so I had them all point the majority of the fuel down across the incoming airflow), verified the ignition advance was within specs, replaced the spark plugs, and adjusted the allen screw on the airflow sensor to bring lambda duty cycle to spec. I am able to adjust the screw and see the adapted duty cycle change from near zero to 100 percent, but I can not seem to get the idle and 2,500 rpm numbers closer than within about 20% of each other, with idle being the higher value. As I was able to monitor the lambda duty cycle, you can conclude that there were no codes being shown at the x11 connector. I unplugged the coolant temp sensor, then the lambda sensor, to verify codes would be shown if a sensor were faulty. The car no longer blows blue smoke, and it seems to run great, except for the NOx emissions. After cleaning and "indexing" the injectors, the car idled more smoothly than I can recall it ever doing so before.

I am not ruling out the possiblity that past oil consumption has killed the catalytic converter, but before replacing it, I would like to rule out other possibilities.

I hooked up a multimeter to monitor closed loop lambda sensor voltage while driving the car. At hot idle, it fluctuates rapidly between about 0.3 and 0.7 volt. Under steady load while driving, it does the same. Under full throttle acceleration, I see about 0.8 volt. But on part throttle acceleration, it drops to almost zero voltage for several seconds. When I ease up enough on the accelerator to level off at a constant speed, the voltage climbs up to about 0.7 volt then begins fluctuating across stoich. This lean period during part throttle acceleration coincides with the portion of the emissions test where the car is accelerating somewhat briskly to about 30mph on the chassis dynamometer, and where the test recording shows NOx emissions are the highest.

Adjusting the allen screw on the airflow meter so far rich that idle lambda duty cycle is pegged at 100% and the idle closed loop lambda voltage is parked near 0.95 volt still does not bring part throttle acceleration lambda sensor voltage up from rock bottom.

I have not yet made a setup to monitor EHA current, and of course as such I have not yet actually monitored EHA current.

I am tempted to adjust the EHA valve to increase the pressure differential between the upper and lower chambers, in hopes that it will create a richer part throttle acceleration mixture, I will be able to use the airflow meter adjustment screw to bring the idle mixture back into line, and the lambda sensor and computer can sort out the rest. I own a CIS compatible fuel pressure gauge, an OTC 6552, so I will be able to record before and after numbers on upper/lower chamber pressure differential.

I've searched this site and others, and have read much about EHA adjustment, EGR system cleaning (I think I'll be scrounging up a speedo cable soon...), and R16 resistor and ignition timing retard. I also have a fair bit of experience using exhaust gas analyzers to tune carbureted and multiport programmable fuel injected engines on brake dynos. I have fully digested the Bosch FI book, own and read the Mercedes service CD set, and worked as a Audi/VW dealer tech for a few years. But my experience with CIS systems is, aside from this car, nonexistent.

I would appreciate any constructive comments or advice.


Last edited by patrick_mb; 12-28-2006 at 09:34 AM.
Reply With Quote
Old 12-28-2006, 08:18 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
IF you monitor the current flow to the EHA & adjust to to positive/negative around 0 Ma & the car still won't pass due to NOx then the catalyst is the most likely problem.

Adjusting the EHA to allow for more differential pressure will cause the car to fail CO.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator
Reply With Quote
Old 12-28-2006, 09:37 PM
Registered User
Join Date: Dec 2006
Posts: 11
Thanks for the kick in the pants to monitor the EHA current. I made a harness to sit in between the car's harness and the EHA valve that I could plug my digital multimeter into. And I hooked up my fuel pressure gauges. I also recorded some numbers before and after. Mostly I was concerned with the before numbers with the EHA current, lambda duty cycle, and upper and lower chamber pressures in the fuel distributor so I could put everything back after I botched it up. All numbers are after the lambda values adapted and began fluctuating.


EHA current: -0.3 mA to +0.5 mA
Lambda duty cycle: 66% to 71%
Upper chamber: 80 psi
Lower chamber: 76 psi

2,500 rpm no load
EHA current: -4.5 mA to -5.8 mA
Lambda duty cycle: 46% to 53%
Upper chamber: 80 psi
Lower chamber: 75 psi

At this point, I pulled the EHA valve off, removed the plug, and adjusted the 2mm allen screw. Then I reassembled everything, adjusted the allen screw on the airflow meter to get zero current at the EHA valve at idle, and tested again. I did this four times, turning the EHA screw clockwise 1/6 turn, 1/4 turn, 1/2 turn, then back counterclockwise a bit, so that with the EHA current at idle set to jump back and forth across zero, the EHA current values at 2,500 rpm no load were closer to zero, and the lambda duty cycle numbers at idle and 2,500 rpm were closer to each other.

With the EHA allen screw turned clockwise about 150 degrees from stock and the airflow meter adjusted accordingly to compensate:

EHA current: -0.4 mA to +0.2 mA
Lambda duty cycle: 62% to 68%
Upper chamber: 80 psi
Lower chamber: 74 psi

2,500 rpm no load
EHA current: 1.3 mA to 2.5 mA
Lambda duty cycle: 66% to 72%
Upper chamber: 80 psi
Lower chamber 74.5 psi

Then I put everything back together, pulled up the carpet on the passenger side floor, hooked up my meter to read closed loop voltage from the lambda sensor, and took the car for a spin. Even without looking at a meter, I could feel right away the car was much more responsive on part throttle accleration. And the lambda voltage was almost always jumping around between about 0.3 volt and 0.7 volt, even during moderate acceleration. The only time it went lean was when I lifted off the throttle to decelerate, engaging the fuel cutoff. This is exactly what I was hoping for.

Thanks for all the help. This site is a great resource.


Last edited by patrick_mb; 12-28-2006 at 10:15 PM.
Reply With Quote
Old 05-04-2011, 12:15 PM
Registered User
Join Date: Apr 2011
Location: south africa
Posts: 250
very interesting info. Thank you for documenting your findings

Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On

All times are GMT -4. The time now is 01:21 AM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page