PeachParts Mercedes-Benz Forum - View Single Post

ctaylor738 · #8 01-07-2003, 11:05 PM

I kinda sorta think I have this figured out.

Keep this in mind. WW failed the emissions test right before the chain jumped. Not running very well at all then compared to now. I am fairly sure that it had a vacuum leak of some sort then which seems to have gotten fixed with the head replacement. And I think the new chain helped the valve timing.

So anyway, I disconnected the new oxygen sensor and put my DMM on it and started the car. As the car warmed at idle, the voltage came slowly up to .27 and stabilized. By holding the RPM at 1500, the voltage came up to about .78. When I dropped it to idle, the reading gradually dropped back to .26 volts.

Now you need to read Steve B's dialog with Cap'n Courageous on the Tech forum to understand this and I am not sure I have it totally right.

The oxygen sensor generates a current as the mixture richens. The computer measures the current and leans the mixture by telling the valve to open and divert fuel from the injectors. As the mixture leans, the voltage drops and and the computer responds by closing the valve. All of this is happening in milliseconds. But here is a quote from Steve B

"[The] system will correct mixtures less than .5v by adding fuel (within the 10ma correction range we have already tested) till the mixture becomes richer than .5v at which time the current will reverse and the system lean out till it switches the other way. Once the O2 sensor is hot and everything is adjusted then the .3v to .7v O2 sensor swings will be kept in check by about a 4ma total mixture correction."

The light comes on! At idle with .27 volts, I WAS NOT GETTING ENOUGH VOLTAGE FROM THE SENSOR TO PULSE THE VALVE. The mixture is a bit lean. So I am gonna run up tomorrow and get the idle re-tested with my slightly lean mixture and then richen it up a bit so the lambda controller can regulate it.

So my theory is that before the chain jumped I had some basic problem, possibly vacuum related, causing a rich condition that the system could not deal with because the O2 sensor was bad. With that now fixed, the mix at idle is lean enough that it does not need "help" from the mixture valve.

The other point that Steve makes which I did not fully grasp is that the lambda system only "trims" the mixture - it cannot cure basic problems with the injection system.

Hope this makes some sense.