Rough idle, service engine light, and a funky exhaust smell....

I have a 1990 oldsmobile regency 98 and recently i have been getting some problems with it. first of all, i have a service engine light that turns on not instantly but after i have been driving for a while, its like after the engine warms up good it turns on. also i get a exhaust or gas fume smell not inside the car but after im done driving and walking away from the car, it also has semi rough idling, i recently changed the plugs to bosch platinum 2's and changed out the spark plug wires and pcv valve, it seemed to help my response. oil has been changed recently but i am getting a sort of gas smell in the oil, i added some marvel mystery oil into both gas and oil i also changed out air filter and am planning to change the fuel filter because i don't it has ever been done. im at 100,700 miles on the car and the engine runs fine power wise. its just that smell, engine light, and the occasional rough idling that worries me. just yesterday it died out on me like 6 times but after cooling down for the day i haven't had any problems like it. my head gasket looks like it has a very little bit of leaking. my gas mileage is around 200 miles to the tank if im lucky is that pretty good for this car?

sorry if my post is so long im just trying to include all the details i possibly can. if you read this long thumbs up to you for sticking to it :smile:

First order of business is to check for diagnostic trouble codes.It is not difficult on this vehicle and all you need is a jumper wire. Here is a link that shows you how:
http://www.troublecodes.net/GM/

Many auto parts stores will scan for codes free of charge if you wish to go that route.

Post again with the results. I have an 89 98. Great car with a few electrical gremlins. I would buy another one in a heartbeat.

i just tried the service engine thing and i got the code 12 and 44 i believe. i think code 12 is just saying its in diagnostic mode. the 8th vin digit is "C" so i looked and it said that 44 means o2 sensor (lean exhaust).. what can i do to fix it? how do i make my a/f ratio more rich. could a hole in the exhaust cause this?

im gonna go consult my haynes but feel free to help me out with this. oh yeah i opened up the hood and it seems alot louder then usual sounds kinda like a krrrrrrrrrrr constant sound coming from where the belt runs. i've replaced the water pump once before and alternator. it sounds somewhat like its coming from the powersteering/ water pump area. could this be the culprit? any help will be great thanks guys.

*edit
i just rechecked the code and it says that code 44 means Oxygen sensor voltage was under 0.2 volts for up to 4 1/2 minutes of closed loop operation. which sounds about right because the service engine light turns on around 4 - 5 minutes into driving... i'll try and check o2 sensor.

An exhaust leak can cause code 44 to set provided it is upstream of the O2 sensor. The O2 sensor is located in the exhaust manifold so look for a damaged manifold or leaking gasket.

Check for vacuum leaks at the throttle body base, intake manifold and all vacuum hoses.

The O2 sensor or the sensor circuit can cause 44 to set as well. Visually inspect the circuit wiring for damage. If you are familiar with checking electrical circuits the O2 SIGNAL is BLK/PPL at terminal YD2 and the O2 GRD REF is TAN at terminal YD3. Both are located at the YELLOW/ORANGE 32 PIN C-D CONNECTOR #3 at the ECM

Low fuel pressure is another possibility. Check fuel pressure if possible and compare to spec. Check for external fuel system leaks and if the fuel filter has never been changed (you said you were thinking of changing anyway) change it. Injectors could be dirty or blocked.

Edited to include:
Not a likely cause but a possibility which is easy and free to check. Disconnect the MAF sensor electrical connector. If the engine runs smoother you may have a faulty MAF sensor or circuit even though a corresponding code is not present. Consult your manual for location and illustration. Be sure to reconnect when done.

I replace parts as a last resort and only after I have checked everything I possibly can. Emission system components cost $$$!

i was thinking that the funky smell might mean my cat converter is bad but i just called a muffler shop and asked about a funky smell. he said usually that smell is from the cat working not broken. he said that the smell is usually caused by a fuel problem so i going to change out the fuel filter and o2 sensor and check the exhaust for leaks. thanks for the help. will keep updated... oh yeah is changing a fuel filter hard to do by yourself? i got jackstands and some wrenches but should i consider getting some one to do it professionaly? i really don't wanna spend too much to get things like that done if i can do it myself, i just don't wanna deal with a mess especially when it comes to messing with the fuel lines..

thanks again.

PROBLEM FIXED!!! thanks for the pointers and help. turns out all i needed to do was replace the oxygen sensor and reset the ecu. now there is no cel on and its running great!. thanks again for all the help

Outstanding! I bet your gas mileage improves. 200 per tank is pretty bad.

You should have no problem replacing that fuel filter. Just read the manual and be careful.

Well done!