pc171 and pc174

[paszallar]

I have a 96 sts and the engine light came on. The codes i'm getting are pc171 and pc174 fuel trim lean bank one and two. at first i was only getting pc174 and the engine light would go off after a couple of times driving the car. then it would come back on. lately I'm getting both pc171 and pc174 but the engine light still comes and goes. The car seems to run fine, no cold start problems but it is harder to start when warm after driving. any ideas on what to look for? i've read a couple of posts on the subject with newer models, would the same info apply to the 96? also the fuel gage doesn't work right. It only shows its true level when you start the car. You can drive 150 miles and the gage doesn't move untill you restart the car. It appears to be accurate according to the dic info. when i refill the tank it takes pretty close to the amount the dic says it used. any ideas

[Ranger]

Those codes point to a vacuum leak that affects both banks. Probably on the intake manifold or TB maybe.

[Cadillac Jim]

I've always found the fuel gauge to seem a bit sticky. Accurate when you start up, as you say, but not prone to moving much during a drive. On long drives it will move, though.

The most likely thing to cause the PC171 and PC174 are vacuum leaks. As Ranger says, you can check the gaskets on the intake manifold and throttle body; I think that there is an O-ring on the throttle body assembly somewhere. Other sources of vacuum leaks are a PCV valve stuck open, a broken or cracked vacuum hose, the oil filler cap loose or off, or the dipstick not all the way down or missing. The FSM also says that a clogged air cleaner element can cause lean codes.