Cabrio 2002 Bucks

My 2002 Cabrio bucks when idling at a stop after the engine is warm. It is quite prevalent in stop & go traffic.

I just had it tuned & mechanic can’t seem to diagnose problem.

Manual or automatic?

My first guess would be a stuck EGR valve. But that’s just a guess.
Perhaps another shop will be interested.

automatic. Forgive my ignorance, what’s an EGR Valve?

Exhaust Gas Recirculating valve.

You have a system in your car that allows a wee bit of inert exhaust gas to displace some of the oxygen in the intake when the engine is under load in order to keep the cylinders from getting too hot. This is done to prevent preignition and to prevent excess production of oxides of nitrogen, NOx, both of which can result from high cylinder temps.

Since the EGR valve passes axhaust gasses it can get gummed up with carbon residue. If that happens it can stick open and allow exhaust gas to enter the instream when it should not be, like when idling. That can cause erratic idling.

But I repeat, this is just a wild-A long-distance guess. Only by looking at the car can it actually be diagnosed.

Thank you.