Lean Code P0171, but LTFT is -25

[zrjn89]

Hi folks,

I've been a lurker on these forums for the last few months and I've encountered a problem on my 850R that I can't wrap my head around.

I'm getting a P0171 code but my LTFT reads -25. I've pulled the spark plugs from the car to check and they were black and smelled like fuel. I'm just not sure how I get a lean code, but I'm running extremely rich.

My STFT is all over the place and I can't seem to pin any sort of pattern. I did notice recently that when I accelerate my STFT goes rich very quickly.

Boost gets up to 10psi but I do feel acceleration isn't quite as snappy as I'd expect it to be.

Overall, the car starts up fine and drives without much issue.

I've included a list of mods that I have currently and the steps I've taken to try and address the issue. I've also included a freeze-frame based on the results of my scanner.

I plan on doing a compression test, but not entirely sure how relevant it might be.

Any help would be appreciated, as I'd like to get this running right and fix this code once and for all.

Problem Solving Steps
Intake manifold gasket
Checked vacuum lines
Upgraded intake hoses, and rechecked connections
Upgraded vac lines, rechecked connections (vacuum gauge reads between 16 to 20)
New coolant hoses
New plugs and coils
New front and rear O2 Sensors

General Maintenance
PNP Switch
Water Pump
Timing Belt
Oil Dipstick Seal
Transmission Dipstick Seal
Oil Filler Cap Gasket
Thermostat
ETC
Serpentine Belt
Heater Hose Junction Replacement
Oil Cooler Gasket
PCV
Intake Manifold Gasket
New battery

Mods
19T Turbo
Do88 Intercooler
Do88 RIP Kit
Green Injectors
VAST Tuning Base Tune
IPD Downpipe and Exhaust
IPD Transmission Cooler
Walbro Fuel Pump

[RickHaleParker]

I'm getting a P0171 code but my LTFT reads -25. I've pulled the spark plugs from the car to check and they were black and smelled like fuel. I'm just not sure how I get a lean code, but I'm running extremely rich.

Leak in the exhaust system.

1. Air gets sucked into the exhaust.
2. O2 sensor detects the unburned oxygen.
3. ECM interprets it as too lean, dumps more fuel in the cylinder to compensate.
4. Extra fuel does not burn because there is not enough oxygen in the cylinder.

[zrjn89]

This makes sense, should I just replace the exhaust manifold gasket? The downpipe, and the exhaust look to be in great condition.

[bronco]

If you can see the gasket you could try waving a plumbers torch at it. See if the scanner reads the ecm adjust the fuel trim.

i mean it works for intake gaskets ........... Exhaust it might not.

[RickHaleParker]

This makes sense, should I just replace the exhaust manifold gasket? The downpipe, and the exhaust look to be in great condition.

Find the leak before you start changing parts. Try using your ears and eyes first. You might plug the tail pipe start the engine then use your eyes and ears. The back pressure will shoot out the leak.

Smoke test the exhaust system. Plug and inject smoke up the tail pipe or plug tail pipe then inject smoke into a O2 port.

Here a neat DIY idea if you have a air compressor.

[RickHaleParker]

I mean it works for intake gaskets ........... Exhaust it might not.

Should work on the Exhaust side if the torch is lit. When the lit torch is on a leak, the flame will consume the oxygen. No oxygen getting in to the Exhaust the reading should change.

[bronco]

I mean it works for intake gaskets ........... Exhaust it might not.

Should work on the Exhaust side if the torch is lit. When the lit torch is on a leak, the flame will consume the oxygen. No oxygen getting in to the Exhaust the reading should change.

i was thinking unlit it might draw in propane and richen the exhaust . Actually , propane directly into the intake with or without an exhaust leak should show up on STFT too I'd think

[RickHaleParker]

i was thinking unlit it might draw in propane and richen the exhaust .

Spray a flammable on a hot exhaust?

[bronco]

i was thinking unlit it might draw in propane and richen the exhaust .

Spray a flammable on a hot exhaust?

If it did anything it would only light the torch ? Anyway , don't say "spray" say "waft, gently"

[jimmy57]

Have you checked intercooler and the connector hoses? The negative numbers indicate taking fuel away. I would expect the rich P code but scan tools do make mistakes. The black plugs back up the negative numbers from the ECM dialing down injector open time. What is fuel pressure? Is fuel pressure regulator leaking fuel through its manifold sense hose (vaccum hose)?