I have a W126/91 300SE. The economy/vacuum gauge is about 1/4 swing from max economy/vacuum at idle. It swings out a little more if the AC compressor is on. I can cruise on the freeway with a higher economy/vacuum reading. Does this mean anything?

Thanks,
Sixto
91 300SE

Mine is the same as yours. I think this is normal.

In my 1991 420SEL with 192K miles, the economy gauge rests comfortably all the way over to the left at idle. It moves slightly to the right when the A/C compressor is on. When I was having problems with the idle (sticky exhaust valve) the needle regularly moved one-quarter of an inch to the right during idle. Since the idle problem was fixed and the valve adjusted, the needle stays to the left. I don't know if this is true for all M-Bs or whether your model is a little different from the V8s. The fact that you can get a better reading at highway speed may be relevant, however.

I have a '88 190E and was having the same problem. My culprit was a leak in the vaccuum line. Hope this helps. Seems like an easier problem to fix than the other.