2002 Buick LeSabre – Code P0172 – What Next?

Hi everybody. I started getting into ETCG and doing my own car maintenance about 9 months ago. Everything has been going well, except now I need a little more help than what I’m able to find via videos and already posted forum threads. So here I am.

I’m looking for advice on what I should try next to solve this engine code:
P0172 : Bank 1 System Too Rich
-I reset the light twice, but it comes back and stays on, so something needs to be fixed.

Basic Info:
Car Type: 2002 Buick Lesabre Custom
Engine: K type, V6, 3.8L, firing 1-6-5-4-3-2
Miles: 94,000
My grandpa was the 1st owner of this car. I bought it from him about 2 years ago now and have put on about 7k miles on it since.

—The car seems to run fine. It would previously idle low (400-500 rpm) and a little rough, but that was fixed with the MAF replacement.
— Occasionally the car starts poorly. There is no hesitation in the start (it doesn’t turn over many times before starting). However, sometimes it will just barely start (almost giving the impression that it isn’t going to take off), then the rpm kicks up to ~1,300 for maybe 1 second, and then it drops down to idle and runs perfectly fine as if nothing ever happened (idling smoothly at ~600-700 rpm).

Previous Work Done:
(mid January) – I’ve had this code (P0172) for about 4 months now.
(end of Oct. 2014) – About 3 months before I got this code I replaced the MAF sensor (with this one) (because I initially had a code specific for the MAF sensor and the shop I took it to cleaned it but said it probably needed to be replaced [that’s when I started learning and replaced it myself]).

(early March) – About a month after getting the code P0172, I replaced the engine air filter (with this K&N filter).

(early March) – I also replaced the fuel filter around the time I replaced the air filters. I don’t think it had ever been replaced before. (This is the new one)

(end of April) – I replaced the PCV valve ~ 2 weeks ago simply b/c the part was cheap and I read that it could cause some problems if it wasn’t working correctly. (Part used)

***I’m coming up on the 100,000 mile mark, the manual says to replace the spark plugs. Could changing these a little early fix the problem? Also, when doing it, can I use a normal type of socket wrench or will I need something special? (I was going to pick up these)

Other Concerns:
Occasionally (maybe once every week and a half), the “check gas cap” indicator comes up on the dash. I figured this could be a related problem. I have purchased a new cap, except I haven’t replaced it yet since I wanted to figure out how to fashion the old strap onto the new one. (I got this one)

***I had the oil changed back in early February (it was cold and I didn’t want to do it myself). However, about 1 month later I checked my oil and they had filled it a little full (maybe an extra 1/4 – 1/2 quart. So I emptied some of it out. The oil that I put in was light gold in color, however the stuff I emptied was almost completely black. My concern though is that I noticed it smelled like gasoline. — I’ve read that these engines have problems with the “Intake Manifold Gasket” leaking. Could this be the problem with the oil? And if so, any suggestions on how to fix it or which part to buy? I’ve looked into it and it seems like a fairly big project..

Thank you,
Aldon