I have a 1996 pontiac bonneville with a high idle problem. I tried the mass air flow sensor, idle positioning sensor, intake gaskets. I bought this car and have been trying to find the correct reason why it idles 2 1/2 to 3000 rpms when I start it and drops when the engine warms up. The car is omitting an smell like rotten eggs and smells like the engine is burning something.
The catylitic converters are dirty or the EGR system is dumping a lot of unburnt hydrocarbons back into the system which is too much for the catylitic converter.
Both components either need to be replaced or cleaned. You can find catylitic converter cleaner that you add to gasoline at mosr auto parts stores, but if it is too dirty it will not work very well.
The EGR may be cleaned but with the mileage yyou have the vacuuim actuated diaphragm is probably old and has failed. It is supposed to only dump exhaust at higher RPMs and when it is always open it gives that sulfur smell.