In writing this, my intent is to give hope to those still trying to solve their P0171/P0174 DTC woes. I read several accounts of members on this forum and others, chasing this problem. I read these accounts hoping they would lead me to that one part, that when replaced, would fix my problems. My vehicle is a 2005 STR with 83K miles purchased this summer to replace a similar STR (RIP). Here’s what I found…

First, a brief background on what these codes really mean. These codes indicate that the ECU is adding more fuel than the airflow at the MAF sensor would normally prescribe. The ECU is adding this fuel because it is sensing too much oxygen in the exhaust at the upstream O2 sensors.

With these codes lit, on your SDD you will see LTFT values in double digits at idle and decreasing with increasing engine speed (for me 19.7 on both left and right banks at idle). You may also find the STFT numbers positive as the ECU adds more fuel on top of the maxed-out LTFT values in an effort to try to burn the excess oxygen.

On the oxygen supply side, this extra oxygen may be coming from vacuum leaks between the MAF and the cylinder head. It may also come from miss-firing cylinders. On the fuel side, this lean condition may be the result of low fuel pressure or plugged injectors.

If you are like me, a DIY tech/mech, and have limited experience with your SDD or just using a cheap scanner, your ability to pinpoint the exact source of the problem may be limited. If you are clever, you can probably find patterns in the O2 sensor readings, notice the ignition advance bouncing around, or find the MAF flow values are low and go right to the problem. This isn't me, and in practice this may not really matter.

You are dealing with an aging vehicle, and you probably have several marginal systems that when tolerances are stacked, result in the DTC issue. Sure, you might be lucky and eliminate one problem and get the LTFT back to where the DTC goes away, but your success will be short lived until the other systems degrade to the point where the DTC is back.

I spent a lot of time with my un-lit propane torch, watching for the STFT to drop as I found the one problem spot. For me, it was the oring on the brake booster hose. Further inspection would show that someone had already replaced it (leaving the old oring and plastic clip in the valley), but it had been partially pushed into the elbow, and remained a leak. I ripped the super charger off, (replaced the dreaded coolant hoses while there), installed the “permanent” booster hose fix, and was ready to celebrate. No luck. Yes, there was an improvement, but not sufficient. Further investigation with the propane revealed no other obvious leaks.

Re-starting my investigation from basics, I replaced every oring: CCV, part load and full load tubes, and evap canister feed. When they were off, I checked these hoses/tubes for leaks with compressed air. Then I zip-tied the vacuum hose connections for the EGR and fuel pressure regulator. Next, I replaced the air filter, fuel filter, spark plugs, and cleaned the MAF. Success! LTFT values are now in the single digits, STFT hovering around zero.

The lesson I learned was that when dealing with aging equipment: you likely have multiple contributors to the problem. In this case, small leaks on the many hose connections, occasional miss-fires from tired spark plugs, weak injector spray patterns, and marginally plugged filters inhibiting flow. When stacked together these conditions may exceed the P0171/P0174 DTC threshold.

In my case, the engine didn't seem to run bad with an obvious miss. My clue was that when checking the fuel trims at 2500 RPM then dropping back to idle, the STFT values would go negative (good), then slowly creep back into the positive range and become unstable. I think the plugs would cool off and begin to randomly misfire, but not enough to set the misfire DTC.

My advice is to start with a good baseline: fresh plugs and filters, clean the MAF, add a can of fuel injector cleaner, and change the orings (whether they look OK or not).

If you still have problems, move on to the harder stuff covered elsewhere in this forum: EGR, EGR pipe, O2 sensors, temp sensors, fuel pump, evap canister, etc.

Best of luck in your quest.


Jeff

 

Yes you found one of the hidden gotchas on the STR. That power brake vacuum connection on the super charger intake elbow is a strange design. I recommend fixing it permanently with a screw in brass elbow. About $6.
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clubairth1 -


I agree and followed your instructions except that the brass adapter I used had a 3/8" pipe thread on the induction elbow side. Thanks for the detailed write-up.




Jeff