Can't get Lambda leaner, 1983 380SL - PeachParts Mercedes-Benz Forum

erubin · #1 02-12-2006, 06:06 PM

1983 380SL.

Background: Car failed smog. I tracked down that the O2 sensor was no good. Replaced it and now the car can go into closed loop. Lambda is very low so...

I am adjusting the lambda by turning the 3mm adjuster CCW = leaner. lambda increases and stays steady up to 36%. If i continue leaning it out the car starts hesitating and finally stalls. The leanest i can get it at idle is 36%. Does this suggest an air leak?

I sprayed carb cleaner at all the vacuum connections i could see as well as the intake manifold rubber rings that connect the upper and lower manifold but could not detect a leak. Would you still be convinced of an air leak or could something else cause this. Car runs well (smooth idle, plenty of power) but i don't want to bring it back to smog untill i understand why I can't go leaner than 36%. At idle the duty cycle is very stable.

O2 sensor is oscillating as expected.

BTW, when I measure Lambda at diagnostic terminal #3 with key on engine off the duty cycle shows up as 0% (0 VDC was also measured). I'll just assume that this year and model does not put out a fixed 70% or 85% with KOEO.

I've spent many hours reading over all the great info on this site but still am stumped.

Duke2.6 · #2 02-12-2006, 06:48 PM

If you read any of my posts you should understand (at least somewhat) how to use the gas concentration data on your test report to analyze your emission test results.

It would help if you post the numbers and your analysis.

Why are you leaning it? You want the idle duty cycle to average near 50 percent. (It should fluctuate, and the "average" is about halfway between the high and low meter reading fluctuations.) I like to set the idle average mixture slightly rich (meter reading range fluctuation between 50 and 55 percent, for example) since cold starting is better, and this is okay if the 2000 rev duty cycle average is within ten percent. Fifty percent means that the base mechanical mixture is right at stoichiometric, so the control system spends as much time richening as leaning to maintain a stoichiometric average.

If you get the duty cycle too far from 50 percent the system runs out of control authority to maintain an average stoichiometric ratio, and there won't be enough free O2 in the exhaust for proper catalyst reactions.

Too lean a base mixture can fail emissions just as surely as too rich a mixture.

Duke

erubin · #3 02-12-2006, 09:43 PM

Let me clarify:

The highest lambda value I can adjust to at idle is 36% duty cycle (positive lead at term 3 and neg lead at term 2). I can lower the duty cycle (screw CW) but if i try to raise it (screw CCW) to get the duty cycle near 50% the car hesitates and stalls when i go above 36%. I will stop calling this enriching or leaning the mixture since that is the wrong way to describe it.

Duke, you said to target arround 50% but I can't get there. Can you tell me the likely cause(s) for this scenario?

I checked for air leaks and found none. Should I keep looking for air leaks or are there other likely suspects if the engine can't be raised to 50% lambda?

I assume my smog values are not very useful since it was smogged before i replaced the defective O2 sensor.

FWIW, here are the smog test results at 15 mph (25mph values are not much different) with a defective O2 sensor.
CO2 5.56%
O2 3.89%
HC 538ppm (gross polluter)
CO% 11.69% (gross polluter)
NO 92ppm (pass easily)

stevebfl · #4 02-12-2006, 10:03 PM

Not sure exactly what you are looking at but the worst problem dealing with duty cycle is point of view. When looking at this properly a lean compensation for a rich mixture has the duty cycle oscillating lower than 50% say 35 -40%.

Another absolute is that the mixture in closed loop is always the same whether the correction necessary to achieve it is 30-35% or 70-75%. SO! If you are in closed loop you really are never changing mixture as the control system takes what you are giving and stays the same. That is what is meant by closed loop.

I'm afraid we my have some comments here that are looking from a different points of view. We all know that 30% duty cycle is also 70% duty cycle. It depends what point of view one is taking.

erubin · #5 02-12-2006, 10:18 PM

Thank you all,

<<"When looking at this properly a lean compensation for a rich mixture has the duty cycle oscillating lower than 50% say 35 -40%">>

So my control system is compensating for a rich mixture. Does this mean something is not working properly. It stays at 36%, hardly any fluctuating at idle.

When I turn the lambda adjusting device screw CW the duty cycle on my digital meter goes down. When I turn the screw CCW the displayed duty cycle goes up. When i raise the duty cycle above 36% the engine stalls. This part does not confuse me. What does is that I'm told to target around 50% but the engine stalls if i try to adjust higher than 36% (by turning screw a little more CCW). Any suggestions?

With KOEO, duty cycle is 0% and the voltage on term 3 is battery +.

Exhaust smells fine.

Does the fact that I can't adjust the base setting higher than 36% at idle indicate a problem that should be explored? If I can't adjust higher than 36% what should i adjust it too?

Raymr, what does 30/70 mean? Are those target duty cycles or something?

stevebfl · #6 02-12-2006, 10:27 PM

I would suggest calibrating your tool.

There is a very simple way. Disconnect the O2 sensor. Start the engine and you will be reading 50%. If you aren't fix that first. I'm not sure what you are reading but I'm pretty sure it isn't closed loop control.

You actually don't even need to start the car but the fuel pump relay has to be jumped otherwise as lambda control is taken off one terminal of the FP relay.

raymr · #7 02-12-2006, 10:12 PM

Does it run better with the new sensor installed? I give my cars the subjective 'sniff' test. If the exhaust smells like clean hot air, they have always passed emissions.

Regarding duty cycle, I thought I read that earlier models are something like 30/70. I haven't looked at mine in a while, but I'm pretty sure its not at 50% and it passes the state test very easily.