P0171 - Can't Fix! *Solution Found*
 

2002 3.0L with 199k

Hey Everyone,

I recently made a 500 mile trip to visit family for the holidays. On my way there, gas tank got dangerously low as I kept missing exits. Fast forward a few days, go to drive again and 15 minutes in I get a constant Cylinder 1 misfire. Run some diagnostics for spark, fuel, compression. Test it, P0171 (Lean Bank 1) comes on along with Cylinder 1&2 misfire codes.

Before I knew it was a fuel problem, I checked spark and compression, both are great on cylinders 1, 2. I then changed the fuel filter, cleaned MAF/IAC, and replaced a fuel injector on Cylinder 1. Checked for vacuum leaks by unplugging IAC, truck idle worsened and almost stalled, indicating no vacuum leak. Also added some injector cleaner to an already full tank and drove for about 45 minutes.

None of these things helped. Can't figure out what's causing this problem on only one bank. Will test fuel pressure tomorrow but need to rent a tester from AutoZone.

Any help is much appreciated, I'm stranded in another state and need to get back home for work.

SOLUTION:
Replacing the ignition coil resolved the Lean Bank 1, Cylinder 1 misfire, and cylinder 2 misfire codes. Even though spark was present, it must have been weak or out of sync.

Symptoms included steady, regular misfiring, especially at idle and takeoff. Reduced fuel economy and CEL's were present. Symptoms were reduced at higher speeds and when cruising.

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The P0171 is most likely because of the misfire,

O2 sensor is measuring the unburned oxygen in cylinders 1 & 2

Fixing the misfire should also fix the P0171

Did you reset the CEL after you made repairs? If not, it may take a few drive cycles for it to go out if the problem is resolved. How did you check the spark? Is the misfire very noticeable? When you do a compression test, it's best to check all the cylinders and compare to each other.

Yes, I reset CEL after each repair and it continues to come back. Misfire is very noticable, especially at idle/slow speeds. Goes away with throttle but can tell I don't have full performance. I tested spark with an in-line tester, so it's just a visual test. My compression test came back at 150 psi for both of the problem cylinders. I do compression tests somewhat regularly and 150 is the normal compression for my engine.

Update:
​​​​​​Here's what I've done so far:
- Tested for spark on Cylinders 1&2 with in-line tester - positive
- Replaced Spark Plugs/Wires- Tested compression (150psi on all 6 cylinders)
- Replaced Bank 1 upstream O2 sensor- Replaced Cylinder 1 Fuel Injector- Cleaned MAF/IAC- Tested for vacuum leak by unplugging IAC (no vacuum leak)
- Replaced Fuel Filter- Added fuel injector cleaner and have ran 3/8 of a tank with it
I'm about to test fuel pressure. If fuel pressure is normal I have no idea what else I can do, am truly stumped.

Can you get a fuel injector node to see if the injector is getting a signal?

I tested fuel pressure at the rail. Perfectly normal at 65 psi idling.

This led me to the last ditch of replacing the ignition coil. Even though all spark plugs/wires were working per the in-line spark tester.

Problem was resolved after replacing the ignition coil. It must have been missing a spark or not producing one powerful enough. Either way I didn't suspect it because of the lean code. Thanks for the help!