Trying to adjust Lambda but this makes NO sense!

[Ron Brooks]

Car: 1988 300SE

I suppose I should preface it with, "it makes no sense to ME" which may not be saying all that much!

With the car running and my duty cycle meter connected pos. to #3 pin and neg to a chassis ground I get 34-40 at idle (rich) and 48-51 above idle (right where I want it).

Same set up but with neg connected to the battery negative post the meter stays at 20.9 to 20.1 no matter what the rpm is.

I thought the neg battery post was the best ground?

I should also mention that whether I adjust the screw 1/4 turn or 1 full turn rich or lean it seems to have no effect on the readings. It does certainly have an effect on the engine smoothness.

What started this was my mechanic saying the car was running rich. It seemed to load up at idle because froma a stop at a light it would studder but once you got to 20 mph or so it was fine no matter how fast you were going. I figured I'd hook up the old meter and adjust the Lambda a bit leaner. The above is what I found. It makes no sense that the duty cycle reads different at a chassis ground v. battery ground does it?

Plus, how can the car seemingly be running properly above idle but so rich at idle?

Thanks for any opinions,

Ron B.

[sbourg]

First thing I would do is measure from the negative battery post to chassis ground, engine running, using the voltmeter function. You may have a bad ground strap if you read greater than a few millivolts.

Steve

If the the car is running rich, then rather than to try to set the lamda, perhaps one should test the O2 sensor to see if it is malfunctioning. Since a large part of rich/lean is controlled by the O2.

[Ron Brooks]

I actually never thought to check the O2 sensor because it's only a about 5 months old. But, I suppose if the car has been running rich, the O2 sensor could be blown. I'll check it out.