Rough idle @ economy guage not working 420 SEL
 

I lent my 1988 420 SEL to my father inlaw and when I got the car back the idle was rough. He had filled the car with gas so my first thought was bad gas. I have already run Three quarters of a tank of new gas through the car and it is still idling rough. I also noticed that the economy gauge had stopped working. Could a vacuum leak cause a rough idle?

Thanks

Dave Schwartz

Most certainly.......

Absolutely! and you can start your search from the line for the Economy gauge. A hand-held vacuum pump and digital multimeter are the most valuable tools I have for my MBs. Look in the manuals on the W126 CD-ROMs--there are good vacuum diagrams there. Cheers, Andrew Seidel.

Ok here are some things to check. You will find a small vacuum line running from the back of the engine to a small rubber "Y" connection just in front of the fuse box. It is usually strapped to the large black vacuum line going to the brake booster. One of the connections goes to a yellow vacuum check valve that has 3 connections on it. One of these connections (on the Y connector) goes to the line leading to the economy guage. Check these vacuum circuits carefully. I have seen the Y connector get hard, start leaking and generally fail. This would cause a vacuum leak and affect the mixture of the engine. Also check the vacuum connection on the back side of the engine for leaks or broken parts.