P0171 bank 1 too lean

Have you tried putting a fuel system cleaner in the gas tank, and then driving the car for awhile to see if code stops ?

This guy covered the usual items for 'lean bank'

PCM software needs to be updated Vacuum leaks (Intake Manifold Gaskets, vacuum hoses, PCV hoses, etc.) Mass Airflow Sensor (MAF) Plugged Fuel Filter or weak Fuel Pump Plugged or dirty Fuel Injectors

without success but then discovered an unusual failure:

If you have a definite P0171 DTC, then best you determine why Bank 1 is being deemed too lean. The DTC alone is providing insufficient data - it's a diagnostic starting point, nothing more. An intake system air leak can cause such an issue so look at the intake system carefully for cracked hoses and/or loose hoses or ducting. So can partially blocked injectors. Take a look at LTFT and STFT data. Look at live data, look at freeze frame PIDs. A 20+ year old car is likely to have perished hoses and ducting and can be a PITA to locate when they become the cause of air leaks.

If you aren't familiar with OBD2 data, consult someone who is. You need to know what data is *normal* and what data is *within spec*

Dicking with the idle speed *will not fix the issue*. The idle speed is merely a symptom of idle air enleanment, not the cause.

Why plugs? That's not likely to be related in any way, it is not like you are getting a spark error.

Your first job is to figure out if it really IS too lean or if the computer is deluded.

You need a real scan tool so you can see if the other bank is being forced too rich or not.

With an inline engine where everything is one bank you can smell the exhaust and tell if it's too rich or too lean, but you're going to need the proper tool for this one.

Why would you want to increase the idle RPM? If the engine really IS running too lean, that's a good way to burn some valves. On the other hand if the engine is running rich and the computer thinks it's too lean, that also does nothing to help the problem.

--scott

Interesting story and there are many like this. But the dealer scan tools should have been able to test for it. Look up OBD2 Mode $08 - control operations of on-board systems. Mode $08 allows the tech to control certain operations of the EMS allowing a scan tool to do bidirectional control of an onboard system or test. Most solenoids and emission equipment can be controlled and cycled independently of conditions and commands. Factory scan tools and the more expensive aftermarket scanners offer bi-directional functionality thus have Mode $08 capabilities. This is a very handy method of testing for equipment failure without the need to wait for the EMS to command the operation. Would have been very handy for you, no need to wait out the time or an idle situation.

HTH