Engine Mechanical problem
1997 Buick Skylark 4 cyl Front Wheel Drive Automatic 120,00 miles
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I have a 97 Buick Skylark with the 2.4 4 cyl. I bought it used with this problem.
First of all I used a code scanner from Advanced auto, and it pulled the 2 codes, P0171, and P0172 (mixture too lean, mixture to rich)
The symptoms are as follows...

-when engine is cold, and starting to drive it will hesitate until warmed up

-intermittently driving down the road at normal speed, usually 40 or higher, it will stumble, hesitate, or act like its not running on all 4! (its doesn't do it all the time! If you let off or push harder it will usually clear up, but this happens all the time!

-if you put it in park, rev the engine to around 2500 + RPM's ( it has to tach, so I'm guessing) the RPM's will surge, the only way to stop it is to back off the gas...pushing wide open throttle does not stop the engine surging.

-sometimes if you look at the exhaust, you can tell its running too rich

-it never stalls, and seems to run very good at idle with no load

-the check engine light will obviosly come on most of the time. But it will go off when its running good and the code is cleared

Now here is what I have tried so far!
-new plugs and boots
-new O2 sensor
-new fuel filter
-replaced some vacuume lines that looked questionable, (MAP and PVC tube) others look good

went to junk yard and one at a time tried the following used parts...that did not help!!!

-TPS
-MAP
-ignition module
-ignition coils
-MAF

none of these solved the problem!

The plugs look good, its doesn't burn oil, when it runs good, it has plenty of power. My next thought was tring a new( used) EGR, could that cause this problem?
Any thought or comments would be great...This is my beater car, so I don't want to spend much money, if I can fix it myself! It is in good shape for the year.
Also, the gas tank does have a small leak, but only when its completely full...I can't imagine that is the problem here, but I could be wrong!

Thanks