This is a repost. I'm afraid the original is too far down the list to get any notice. My car is a 1990 240 w/165k, tried a few different types of spark plugs (Bosch copper & platinum) but the only ones that work (relatively smooth engine) are the Bosch +4's.

...Some more history that I don't think I shared (maybe I did, I'm too lazy to read my previous posts on this thread), the combustion chamber is really carboned up, all cylinders. Can't say how thick it is of course but I ran some BG44K through the car about three weeks ago...was expecting the pistons to look at least close to the BG44K advertisement. The shop that I take the car to also did the fuel rail injection cleaning, shouldn't the pistons be cleaner!?

With a "thick" layer of carbon on the piston, what would that do to the mechanics/physics of the combustion chamber? I run 91-93 octane in the car. What would make carbon build up inside like that?

Thanks for your help, I want this engine smooth! (and pass emissions...)
Bean