I have a 1999 Mercedes-Benz C230 that had ~144,000 miles on it.  The idle was very rough and noisy, and the engine wasn't as smooth as it should have been in general, especially at lower engine speeds.  The problem was so bad that the engine would almost stall at idle after first being started.I tried Lucas "Upper Cyclinder Lubricant and Injector Cleaner", and it didn't seem to help at all.  Then I put a full 20-oz bottle of Chevron Techron through the fuel tank, and it also seemed to do nothing.  In each case, the product was used as directed, and the car was driven until the fuel level was very low.  That is, I didn't refill the fuel tank and dilute the product in either case.  In the past, I've also used other brands of fuel system and fuel injector cleaners, including Gumout and STP.  None of these products seemed to have the slightest effect.Finally, and extremely skeptical, I tried Sea Foam Motor Treatment.  I put a full 16-oz bottle in the fuel tank after filling up.  The improvement was dramatic and immediate.  The engine ran much more smoothly and quietly than before.  The engine no longer idled like a diesel farm tractor.  I was extremely surprised, but still skeptical, because I wasn't sure if it had actually cleaned anything, or if it was simply helping because it was being burned with the fuel.  So, I withheld judgment until the fuel tank had been driven to near-empty and refilled, meaning that the amount of Sea Foam left in the fuel would be negligible.This was no placebo effect.  Even after refueling, the car ran much better than it did before.  It now idles more nicely than my Hyundai, which still seems bizarre to me.  I am extremely pleased, which doesn't happen often, trust me.  I'll be putting a can of Sea Foam Motor Treatment through my fuel tanks before every oil change from now on.My other car, a Hyundai Sonata, had an annoying ticking sound when it would idle, especially when cold.  I would have described the sound as a "lifter tick", but I'm not sure what it was--a sticking valve?  I don't know; I'm no mechanic.  I added the recommended amount of Sea Foam to the crankcase, and the sound went away immediately, or nearly so.  I've since changed the oil in that car, and the sound has not returned.Some people criticize Sea Foam based on its age.  I find these criticisms ridiculous--as if "older" equates with "worse".  Do these people ever address the actual chemistry of the product, or provide actual reasons for their disapproval?  No, they just complain about how long it's been around.Sea Foam made me a believer.  Very highly recommended!