Newbie has 300se mixture question
 

Hello!
Recently purchased a 1989 300se (167K) and have noticed that that mpg has sank from 20 to around 14. We adjusted the mixture screw a little (not much) to run a little richer. Car starts normal, however from a dead stop the car chokes a little when I open the throttle. Also, the "economy" gauge needle sits about 1/3 of the way from the left when at idle. Idle rpm is normal, we checked.

Question: Is this normal? This is my first Benz and I don't really know how to judge performance and economy. Any help is appreciated!

If the vacuum gauge is low like that, you almost certainly have vacuum leaks, which you need to find and fix. Climate control and door locks are prime suspects as well as the engine fittings and hoses. These will totally screw the car's running.

Next, you can't just adjust the mixture because it is electronically controlled. You use a meter with a duty cycle or an old-style dwell meter. There are literally hundreds of posts on this. Getting the mixture right by setting the on-off ratio is the basic first step in getting these cars to run right.

Vacuum gauge? I'm assuming that's the "economy" needle that you're referring to. Where should it be located at idle?

The economy meter is in fact a vacuum guage. It should be close to "pegged" at idle.

OK, do you mean the needle should be buried to the left or right?

Left - it should go down when you accelerate.

OK, that's what I thought. The needle should be pegged in the black at idle and swing to the right(red) when accelerating. Makes sense. One question though:

If I can't adjust the mixture because it's electronically controlled, what in the heck is the "adjustment screw" for? The on-off ratio? Thanks!

Look at this from the DIY section:

http://www.peachparts.com/Wikka/M103VacuumLeak

Good question!
 

What I meant is that you cannot adjust the mixture because the computer is reading the output of the oxygen sensor, and changing the enrichment. So if you richened it with the adjustment, the O2 sensor increased its voltage,and the computer responded by leaning it through changing pressure in the fuel distributor. You can only change the mixture if you go far enough either way to completely override the electronic system, which is a bad idea.

So the way you adjust the mixture is to set it at hot idle so that the on/off ratio is about 45%. If you have any doubts about your O2 sensor, replace it with the appropriate Bosch generic.

Look toward the end of th first page of this thread for my version of how to adjust the mixture:

http://sl107.com/groupee/forums/a/tpc/f/2346035941/m/8251096042