1991 Cadillac Seville What could it be?

Engine Mechanical problem
1991 Cadillac Seville V8 Wheel Drive Type unknown Automatic 122700 miles

My Caddy began running funny. I noticed that the fuel economy had been continously dropping and then it began running really rough and the idle began surging.

So I went and bought the parts for a full tune-up and found that was I over my mechanical aptitude head and instead took it to a mechanic a friend recommended. Before I even got to him the "Service Engine Soon" light came on. He did the tune-up and also replaced the Oxygen Sensors which he requested be done. The "Service Engine Soon" light no longer comes on but now the fuel economy is all the way down to 10.9 miles per gallon (still dropping) and the engine is running rougher than ever. It has almost stalled out on me while coming to a stop and then accelarating (before and after the tune-up) but has never actually stalled out.

While in Park the idle surges.

It runs louder now (almost sounds like a lawn mower) and stutters while accelerating. It's giving me little power to acceleration going from a stop until 4th gear when I can also no longer can hear it sputtering or stuttering. I can feel it's putting a heavier load on the transmisson. At around 3000 rpm it it backfires in both the Park and Drive position.

It won't pass smog and the muffler shop said it wasn't the catalytic convertor and said it sounded as if it were missing and running on only 7 cyclinders possibly six.

The engine is giving off a burning smell that smells to me like both oil and wires and this smell also creeps through my air conditioning vents into the car. I also smell gas from time to time.

I was told by another friend to take it back and have the mechanic do a compression test. Also to have him check the EGR valve. I'm told that the new spark plug wires or the Oxygen Sensors could be faulty. Or even that the mechanic didn't really change all the plugs, but the guy seems honest to me and I doubt that that's what really happened. He said it was the mechanics responsiblity to find out why it's running like it is but the mechanic has said he did all that he was supposed to do and at this point said he doesn't know why it's running like it is. He has said that he would run some more tests but that it's time consuming and that leads me to believe he will ask for more money. The tune-up already cost me $380.

I have very limited knowledge and patience with vehicle repairs and problems. This car ran very well until it began surging. Since then it has been one issue after the next right into the condition it is running in now. I am afraid that driving it is only causing much more damage. As of now it's my only means of transportation so I am desperately seeking to find the problem and get it fixed it, if it's even possible. Could the head gasket or something worse be going? Could you tell me what to have checked out or what I should do at this point? Thank you for any help or advice that you could provide me with.