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3.8L P0171 High STFT

09-23-2022, 12:24 PM
1997 Lesabre 3.8L starts great, idles a little rough but gives bat-out of hell acceleration. P0171 comes and goes.
Upon cold start live data shows closed loop for about 60 seconds and fuel trims normal. ( I'm assuming because no sensors in play). Then when open loop, STFT maxes to around 20%. STFT drops significantly with RPM raised to 2500rpm which indicates possible vac leak.
Here's what's been done.
1. Fuel pump replaced about 3 yrs ago. Airtex from RockAuto. GM manual wants 48-55psi. Pump gives right at 48psi.
2. Fuel filter replaced.
3. FP regulator replaced
4. Water test, starter fluid test, propane test and smoke test all given both with engine cold and hot looking for vac leaks. None found.
5. I did test and clean MAF and EGR. They seem to be ok.

*Can either of the O2 sensors mimic the vac leak, or is there more live data I can do that would help diagnose?

*With ECM dumping fuel do I run risk of damaging cat, or is the computer keeping the exhaust safe for the cat?

* Maybe just need to look harder for vac leak?

Stumped. Any ideas appreciated.

09-23-2022, 02:08 PM
After cold start you would be in closed loop within seconds if you have a heated sensor. Open loop would occur at full throttle. When do you see open loop? Is the o2 sensor switching up and down?

09-23-2022, 02:56 PM
Ouch! Sorry. Trying to do this on cell phone. Can't see #/%#*.

Engine starts cold fine and idles in open loop for ~ 30 seconds with fuel trims good. THEN closed loop and STFT shoots high. If memory serves LTFT is not outrageous but will check again. I do not recall OL at WOT but will check again. Am using Innova tool with limited live data but I believe I can check O2 sensors and post back.

NOTE: A little history. Last year, car would start hard then lope terribly. Never could find idle. Found vac leak in that stupid vac line rats nest near throttle body. Fixed and no longer does that but still the lean condition. This happened AFTER all the "fixes" listed above. Car is driveable but ECM still trying to correct a lean condition.


10-08-2022, 12:35 PM
Sorry guys. Slammed past couple weeks.

Currently, CEL is not on but P0171 shows on scanner.
Live data shows:
System in Closed loop
O2 S1 cycling back and forth from 0.100 to 0.875
O2 S2 meandering around 0.095 to 0.110 so not much movement there.

10-08-2022, 01:29 PM
Is the S2 o2 sensor behind the cat?, or in the other cylinder bank?

10-08-2022, 06:38 PM
S1 is upstream O2 sensor and S2 is downstream O2 sensor of cat. Converter.

BTW, in closed loop STFT is maxed at +24. LTFT is currently at +16

10-08-2022, 07:07 PM
Bad maf can cause this but usually you would have performance issues, the maf also calculates altitude. The s1 sensor should switch up and down frequently in closed loop, if not it is bad. Remember the upper intake manifold had frequent failures between the egr and coolant passages, causing mysterious coolant loss.

10-08-2022, 09:22 PM
MAF has been swapped with another another ( I have 2 more '97 LeSabres).

Pre-cat O2 is switching but post-cat O2 stuck around .085-.110.

All three family Buicks had upper manifold and upper and lower Intake manifold gaskets replaced several yrs ago. I practiced on a 4th wrecked Buick, so pretty confident no airleaks (proper fastener sequence in 3-4 passes as per GM shop manual).

Isn't Post-cat O2 sensor supposed to be cycling as well?

10-08-2022, 10:44 PM
Post cat o2 is just a monitor that the cat is doing its job. It should be steady

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