Jack D. Heres a little trick. On the early 300's In side the fusebox
You acutally have to remove the fusebox cover,and the fuse box assembly. Where all the wires. inside the box its self. There a 12 pole harness connector. Its rectangular. pul it apart. Its a solid purple wire. I think its the female end. Pull the plug apart what you want to do is ground that wire i thinks its either pin#10 or 11 wire.Its the purple one. You ground it to the chasis it steps up the idel to addl 50 rpms more you'll feel it smooth out. This is a early upgrade due to alot of the cams
the timing was retarded to get them to pass emmisons.

So try this. I did this on my old 300E when I had it It help
Also the airslide valve could be worn out also. Which is the valve the guys mention to you on the post.

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500E Sold
2006 Mini Cooper S
Modified,pulley,injectors,suspension
2009 E350 Sport AMG package