i have a 2006 300c that idles rough so i took it to dealer and they said the sparkplugs needed to be replaced. so i replaced them , used champion double platnium and now the rough idle got worse, any ideas why ? somewhere i was told that i should of used regular spark plugs instead of platinum anyone know if this is true?

 

put the orginal plugs in the U dont need plat if it did not come with it

 

Always try to use the original brand and model of spark plug. The computers now days aren't smart enough to handle changes!

Also, rough idle is caused by much more than spark. The idle air control valve, the fuel injector system, the throttle position sensor, the O2 sensorsand the EGR valve come into play. The real sneaker of thebunch is the EGR. If it bleeds at idle there is no code, and no smooth operation. The only test for this assumes that you can hold about 18" vacuum on the intake for about 10 seconds (if I remember correctly) without the engine running. And that is only if you can be sure that all other sources of leakage are guaranteed sealed. It's a tough diagnosis believe it or not. Sometimes it's just plain easier (not to mention cheaper than paying $80/hr for diagnostics) to replace the darn thing.

 

This is a relatively common complaint on these cars (including mine). Most have found a "reflash" of the computer does the trick, there is a TSB on it.