1990 M103 economy gauge shows vacuum problem?

[francotirador]

Hello: I did a search hereon regarding the economy gauge in my 1990 300SE with the M103 engine. I perused 17 pages to find questions and answers regarding the same. My understanding is that when at idle with the a/c off the needle should be pegged to the left. And that's the way it was until a few days ago.

Now, with the car in park, the engine idling and the a/c off, the needle is about 1/8 from the left. When I put the car in D and hold the brake, the needle moves to the right a little more for a total of about a quarter of the distance. With the a/c on, the car in park and idling, the needle is about 1/3 from the left. When I put the car in D and hold the brake with the a/c on the needle moves to the 1/2 mark or so.

If I'm driving the car and let off the accelerator the gauge pegs to the left. Oh, and the car sputters a few times upon hard acceleration.

What I'd like to know is where to begin checking for the problem. I didn't find enough information in the archives to tell me it's definitely this or that. For example, in some posts regarding the same problems, the OP was advised to check the oxygen sensor & others said the fuel filter. And, of course, many suggested a vacuum leak.

So, given my description of the symptoms I would appreciate advice on the best place to start. My car has 65k on the clock.

Thank you in advance.