2007 Chevy Trailblazer P0171 Plz help

[H.USA]

Hello, I have a Chevy Trailblazer 2007 LT 4wd high miles, I have a P0171 and I've been trying so hard to figure it out without going to a repair shop.

The Frame freeze data show a P0171 recorded at Coolant temp 206, Idle RPM , LT Fuel trim 26%, the Oxygen sensor and the MAF readings appear normal to me.

Live Data now is showing LT 29% on idle numbers improve to below 10% if increase RPM even slightly and get worst very quick on IDLE. I also noticed RPM isn't 100% stable on idle but nothing major. Sometimes the check engine light will suddenly go away to come back later, I also notice a gas cap light on from time to time if that matters

I already cleaned Throttle body, MAF , my Air filter was recently replaced. I visually double checked all the air hoses no obvious crack, Breathing cap was cracked so i replaced it but nothing changed. I tried a water test on the manifold gasket but the engine isn't shivering when I toss water, I don't have access to a smoke machine. I can hear a little whistling from the manifold but I'm not sure. couple days ago I had the Throttle body control error but it cleared when I cleaned it and it never came back. Another test of pressing the brakes on Live date show a huge increase in Short term fuel trim to 32.5%. 

Please help me because The car isn't worth spending thousands on it and it might not even work, is there anything else I can try, is it possible Fuel pump or fuel filter? 

 

Thank you so much

Edited January 26, 2019 by H.USA

[H.USA]

I also notice a gas cap light on from time to time if that matters

[Jsdirt]

I'm betting on intake gaskets. Do you have a rough idle on a cold start up that smooths out as it warms up? Usually with bad intake gaskets, you'll find them upon COLD startup using the water test (spray or pour along the gasket joint between the intake and head, and also in between the plastic upper intake and lower intake, if present). Sometimes there will be hard starting if they're real bad.

 

Sounds like the one or both gaskets are borderline bad, since your fuel trims aren't maxed out yet. To rule out a faulty o2, spray some brake clean, propane, or any other flammable gas or liquid into the intake, and see if the o2s are forced rich. Then, you can yank a vacuum hose and see if they're forced lean. To check the MAF and fuel delivery, a WOT run just in 1st gear should peg the o2s rich - if they're not going rich, there's a fuel delivery or MAF issue (so long as the o2's are confirmed good).

[riverbanks]

To add to above mentioned, sometimes the cause is the sensor itself,so the rich/lean test is needed,but you said"whistling"I would focus on that, unmetered air leaking somewhere,so,one by one, disconnect vacuum lines and plug off to isolate leak,and your gas cap is,po455?large leak,vent valve problem or cap,is cap oring cracked?

[Jsdirt]

If the purge valve is leaking, you've found your vacuum leak.

[H.USA]

Thank you very much for your help, After doing extensive researches I learned that on these particular models the 10 bolts on the Intake manifold could be loose so i checked today and they were all loose, I have to admit it was Hell to reach some of the bolts but I did it at the end! Numbers improved, LT is now 3  to 7% while driving it and up to 14% on Idle but I still have to drive it for the computer to learn for some reason I think erasing the Data from the scanner is not erasing the LT fuel memory.  If this doesn't fix it I'll go after the gasket and the purge valve.

Thank you again!

[Jsdirt]

No problem - glad to help if I can! I wish someone had helped me 11 years ago while I yanked my hair out over this lemon of a '00 Jimmy I owned. My first OBD-II vehicle, and boy did I regret that purchase! Trial by fire on that toilet ... every possible code that could be thrown, was thrown ... and always right before inspection time ... 

 

On higher end scan tools, there's a section for "Fuel Trim Reset" - this will set fuel trims back to zero, short and long term. Not to be confused with "Fuel Composition Reset", which is for flex vehicles, when that wonderful algorithm GM engineered into the newer ones goes crazy, and reads anywhere from 20-80% alcohol content when you've been using E10 (or straight gas, in every other state besides MA). Just FYI.

[silveradosid]

does this wonderful gm engineering trigger the service engine light

[Jsdirt]

If the fuel composition gets too far off, it will start throwing rich codes, since it'll be dumping almost double the fuel in, thinking it's E85.

[silveradosid]

thx might explain why my 2014 flex is getting such bad fuel mileage i will check to see if my scanner has that option of reset

[TBlaz]

With my 2006 Trailblazer I6 I had had the P0171 code on and off for awhile, but it eventually became a permanent thing. Both the method of spraying carb cleaner around and the use of a smoke machine indicated that the intake manifold was leaking. I decided to just tighten the bolts and see if that would solve the problem instead of replacing the manifold gasket. There are ten bolts and the ones closest to the front of the engine were not loose, but as I moved further back the bolts became progressively looser. Towards the middle of the manifold the bolts were definitely loose requiring perhaps a quarter turn to tighten them, the ones toward the rear took perhaps a full turn, and the very last one probably two full turns.

 

I used a variety of 1/4" drive tools to get the job done., including a variety of extensions including wobbler extensions, universal joint, and regular and 10mm deep socket.

 

I did unbolt the ECU to improve access, as well as removing the large vacuum hose going to the brake booster. The front ones would be much easier if the alternator was removed, but I left it on. I also disconnected the battery because toward the rear of the engine the hot terminal of the starter is fairly close to where you will be working with your sockets.

 

Each bolt posed its own particular challenges. I suggest you do not do this when you are pressed for time.

 

The problem is solved!