[wachuku]

'97 Northstar, started getting intermittent codes:

• P0171 fuel trim system lean bank 1
• P0174 fuel trim system lean bank 2

I did some searching and read about the "plenum" issue for the 2000's Northstars, but mine is a '97.

Apparently I should be searching for a vacuum line leak? Any words of advice specific to a '97 STS?

 

[STSS]

You can spray some starting fluid under the hood where you think there might be a leak in the intake. With the car at idle, you should notice a rise in RPM if there is a leak that is sucking the fluid in.

Turn off your HVAC before doing this, if you suck that starting fluid in the car it sucks.

 

[Ranger]

Check the intake manifold over pressurization valve behind the P/S pump.

 

[wachuku]

Update . . .

Oh boy. Well, no more codes for about a week now (P0171 & P0174 resolved).

The fix: don't forget to replace oil cap after adding oil.

 

[ewill3rd]

Yep that old oil cap will do it every time.
That crankcase is designed to be sealed. The PCV system is metered and if you leave the oil cap off it will ingest tons of extra air leaning out the fuel mixture.
If you don't put the dipstick back in the tube all the way the same thing will happen too.
Glad you found it.

As mentioned suspects on a pre 2k vehicle are the backfire valve behind the PS pump, the intake gaskets themselves, and vacuum line leaks going into the cabin or to the canister.