Abe,

I have to say I had my doubts about this "sci-fi" Lambda ECU test procedure. I've been a home Volvo mechanic for about 10 years and this was a new one for me (never too old to learn though). I performed this test and got no reaction out of the B21F motor... no change in idle or in the frequency valve (it is running very constant and fairly loud at all times). I then did this test on my 245 Turbo to see how subtle the changes should be and it did register changes exactly as you pointed out.

As Roger mentioned the temp sensor (in my case all there is besides the gauge sensor is a thermal time switch under the #4 cylinder intake port) could cause this if is it disallowing ECU operation because it thinks the car is cold. Is there a way to test this sensor in the car (probably not). I suppose I could pull it and check resistance while heating it with a heat gun.

As a clarification, I believe the car is running rich because it smells that way... no smoke or rough running. Actually it's running very nice, but I do want it to pass a smog test so I can register it.