theoretical engine tune control o2 sensors - Mercedes-Benz Forum
LinkBack Thread Tools Display Modes
post #1 of 6 (permalink) Old 06-23-2019, 08:37 AM Thread Starter
BenzWorld Member
Date registered: May 2019
Vehicle: 1994 s600 coupe
Posts: 121
Mentioned: 0 Post(s)
Quoted: 18 Post(s)
(Thread Starter)
theoretical engine tune control o2 sensors

been scanning and working on computer control cars since CCC and OBDI first came out in the early 1980's
and later OBDII came out 1990's
never thought much of it, it was easier for me to get more power with a carburetor.
OBD systems were just a necessary evil, because they became embedded in the vehicle by the 1990's,
and could not be completely disconnected and disabled. in the old days we could just bolt on a carburetor.
and disable the entire OBD system, not anymore. not that easy.

so I got to thinking, in open loop on the V12 the computer puts in its own 450mv signal,
and keeps the o2 sensors output signal shut off,until the coolant temp sensor reaches operating temp
then it turns over mixture control to the o2 sensor inputs, which vary based on what they sense in exhaust,
from 50 to 900 mv. low value being a lean mixture sensed more oxygen, high value being a rich mixture sensed less oxygen. fuel being primarily hydrocarbons HC, there's less o2 in the exhaust stream when HC levels are high. one displaces the other.

well it occurred to me, if the computer can put in its own 450 mv signal in open loop,
why can't I put in my own signal too ?
what if I injected a signal of let's say, 600 mv into the harness connector,
and pulled the wire off the o2 sensor.
would the computer be tricked and vary fuel injection to that new value.
open loop using 450 mv is too rich for daily driving.
so a higher value of 500-700 mv would lean it out,
and make more power,
this would sort of be like changing jets in a carburetor, they're fixed values.
not varying. (although carbs had power valves and metering rods that did enrich at part throttle/full throttle, vacuum controlled)
typically a setup like this may be on the rich side at idle, in order to be dead nuts on at wide open throttle
if it's dead nuts on at idle, it will typically be lean at WOT

having said that...take it a step further. introduce a varying control using a potentiometer or resistance value,
whereby you can vary, from the drivers seat, the voltage the ECM "sees" and assumes is coming from the o2 sensor
now you can tune from the drivers seat, on the fly, and for track or street conditions, or for weather conditions.

the big question now is, what will the ECM do, when it sees a steady o2 value, rather than a varying one it is expecting ?? my previous 02 sensors did not vary, they were defective. the computer still did not put on the check engine light, or store any codes. yet when the car warmed up, it did go into closed loop anyway, and one oxygen sensor struggled to fluctuate from 40-400 mv. the other stayed steady 50 mv.

that's what made me go "...hmmmmm"...maybe I can manipulate this system for more HP, and wrest control of the mixture system away from the ECM programming.

to take it another step further, I believe most coolant temp sensors turn over control to o2 sensors and go into closed loop, by grounding the circuit at a certain temperature.

could you not just ground the circuit with a toggle switch, thereby putting it into open or closed loop at will ? sort of like a hand choke on a 1940 chevy on a carburetor, complete manual control. I have a mechanic friend who routinely grounds the coolant temp circuit on cars, to force it into closed loop, to check the system sensors. usually he did this to test and replace a defective coolant temp sensor, or o2 sensors, and wanted the system in closed loop like right now, not later.

there would be a market for such a device, imagine saying goodbye to all these computer control problems, by simply over-riding it with a dial and a switch, and putting in your own values with a knob. a digital output meter that could read and supply any chosen value to the o2 sensor circuit, or simple big knob with rich and lean on both sides of knob, like a volume knob on a stereo.

are the light bulbs going off out there ?
any input or ideas welcome. has anyone tried it ?? there are no right or wrong answers here, just ideas.

if someone sold such a device today, and it was EASY TO INSTALL, that's the crucial point, I'd buy it and put it on my car immediately. on any of my cars. I could always shut it off and return control to the OBD system if I wanted.

imagine the flexibility of such a system. I see guys tuning the ecm's to get a tighter fan on the o2 values, to vary from 650-950 mv, rather than 50-800 mv. but what I'd want to do, is be able to just not use the varying fan at all. just put one constant value in, and stay there. the only reason it varies is for emissions, nothing else. because it spends half its time too lean, which pollutes less. keep that in mind, OBDII is primarily an emissions system that has control of the car. think of it as a greedy stingy king or queen, that is saying "there's no bread, let 'em eat cake"...

it's saying "there's no fuel, let 'em eat air"
1994v12 is offline  
Sponsored Links
post #2 of 6 (permalink) Old 06-24-2019, 05:38 AM Thread Starter
BenzWorld Member
Date registered: May 2019
Vehicle: 1994 s600 coupe
Posts: 121
Mentioned: 0 Post(s)
Quoted: 18 Post(s)
(Thread Starter)
adjustable o2 simulator

these devices already exists, but the adjustment screw is down on the sensor under the car, in the exhaust.
they need to make one with a box you can mount under the dash, and adjust from drivers seat.

and this one doesn't put in a static steady rate signal, it varies from high to low like a real o2 sensor
but it's better than nothing.
Attached Images
1994v12 is offline  
post #3 of 6 (permalink) Old 06-24-2019, 08:38 AM
BenzWorld Member
Paul600600's Avatar
Date registered: Jul 2015
Vehicle: 2005 CLK320
Location: Germany
Posts: 361
Mentioned: 30 Post(s)
Quoted: 113 Post(s)
my $0.02
The one in the link you posted will be able to make you mixture leaner or richer by adjusting the O2 sensor output.
Just a pot that would "fake" a fixed voltage while in closed loop would not.
Most ECU's take these measurements ++
-Atmospheric pressure
-Coolant temp.
-Air intake temp.
and use this information to calculate spark adv. and Injector pulse duration and timing and stuff like 2nd air inj.
on more modern engine you will find even more sensors.

Now for example if you drive with constant load and for and you O2 reading moves to not match the calculate value the ECU will start correcting this by changing mixture until the desired value is reached.
If you use a pot to "fake" a fixed value the ECU will never be able to reach it and will keep adding or subtracting fuel. Therefore you will never reach a stable condition.

If you use the special box on the O2 sensor you will tell the ecu that the mixture is for example 13 even if it is 12 then the ECU will add and subtract acording to the fuel table + or - the setting on your box.
Paul600600 is online now  
Sponsored Links
post #4 of 6 (permalink) Old 06-24-2019, 01:23 PM Thread Starter
BenzWorld Member
Date registered: May 2019
Vehicle: 1994 s600 coupe
Posts: 121
Mentioned: 0 Post(s)
Quoted: 18 Post(s)
(Thread Starter)
ok I get it,
are you saying that in closed loop, the ecm is looking for a varying o2 signal, and not a fixed signal ?
I noticed, a defective o2 sensor will read a fixed value, and the ecm appears to use it,
and screw up the mixture accordingly, making it too rich.
the problem being, when an o2 sensor gets lazy or dies, it sends a fixed low value,
a false lean reading, while in closed loop.
causing the system go too rich trying to compensate for the false low value,
the ecm goes full rich
there is a limit to how much richer or leaner it can go,
i.e. there are stops built into the ecm in closed loop
short trim fuel trim is +/- 10%
long term fuel trim is +/- 15%
in the reviews for that o2 simulator, buyers were saying they picked up more mileage and driveability/power,
I suspect due to an overall leaner mixture. these engines do seem to run on the rich side.
1994v12 is offline  
post #5 of 6 (permalink) Old 06-24-2019, 03:52 PM
BenzWorld Member
Paul600600's Avatar
Date registered: Jul 2015
Vehicle: 2005 CLK320
Location: Germany
Posts: 361
Mentioned: 30 Post(s)
Quoted: 113 Post(s)
That's correct a failing o2 sensor will drive the correction to the limiter ans stay there. And your mixture will be way off.
It is possible that on these older ones this happens without the ecu taking any action.
Newer ecu's will detect faults or implausible inputs and work around them by going into open loop or switch to use only one sensor.
If it comes to drivability and fuel efficiency I kind of trust the MB engineer's to tune the engine well. Therfore I don't really think changing the o2 sensors to make it richer in all situations is a big benefit.
If you looking for more power you need to change fuel and spark tables in the ecu.
There are tuners out there that can do this for you.
If you want all the control you could switch to a aftermarket ecu like a megasquirt ms3. This will allow you to change any setting you can imagen.

Sent from my SAMSUNG-SM-G870A using Tapatalk
Paul600600 is online now  
post #6 of 6 (permalink) Old 06-25-2019, 03:22 AM Thread Starter
BenzWorld Member
Date registered: May 2019
Vehicle: 1994 s600 coupe
Posts: 121
Mentioned: 0 Post(s)
Quoted: 18 Post(s)
(Thread Starter)
ok, that's exactly what I noticed, and was getting at.
the failing o2 that reads around 20-50mv signal steady,
is being taken by the ECM and being used as a plausible input,
causing system to go full rich on one bank.

made me think 'hmmmmm..."
what if I just put in my own input.
I unplugged that one bad 02 and the computer puts in 450 mv value
450 mv is better than 50 mv, but still too rich.
wondering what it would do if I fed in a 550-650 mv signal steady.

sort of slowly creep up on it progressively leaner, like jetting an old carburetor.

what I'm trying to accomplish and crack, is this varying o2 signal thing, the engineers put in.
engineers do a lot of nutty things based on demands for fuel and emissions standards,
that they otherwise would not do without those restaints. no engineer in his right mind, would put an o2 system in, looking for all out horsepower and driveability. these are emission control systems, first and foremost.

its going to be trial and error. when I ask these questions of the aftermarket parts suppliers, their knowledge base reference is later models. the early 1990's W140 had a very unique oddball ECM system on it, analog computers, and a mixture of LH, EZ, and SIM4, and other programming. it wasn't straight OBDI or OBDII. some sort of intermediate hybrid system. there's only one way to tweak it, cut and try, trial and error.
1994v12 is offline  
Sponsored Links

  Mercedes-Benz Forum > Mercedes-Benz Sedan Forums > W140 S-Class

Quick Reply

Register Now

In order to be able to post messages on the Mercedes-Benz Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode

Posting Rules  
You may post new threads
You may 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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Title goes here

video goes here
description goes here. Read Full Story
For the best viewing experience please update your browser to Google Chrome