Cadillac Repair: 95 Deville, vacuum line, fuel mileage

Question
QUESTION: I have a 95 Deville that is pinging going down the road. It's also not running perfectly smooth, kinda surging as you come to a stop at lights and such. I did a tune up a while back before all this started and it ran fine for a few months. (didn't really need the tune up just doing maintenance) I just replaced the ignition control module hoping to fix the idling problem, which helped but not completely. Also the fuel mileage has steadily dropped in the last week or so from about 20 mpg to about 16.5 mpg.

ANSWER: IS or HAS the CHECK ENGINE light ever illuminated while driving the vehicle and or have you seen any message display on the information panel ?

---------- FOLLOW-UP ----------

QUESTION: Sorry should have givi=en you that, no messages and no check engine light.

Answer
TRy a higher octane of fuel for 2 tank fulls and see if the knock goes away. If it does, You can use a product or similar type product known as TOP ENGINE CLEANER which will remove the carbon from the cylinder walls and on top of the pistons. Just follow the directives.

Also check the EGR vacuum line and valve.