Hi guys i have a quick question. When you check the fuel pressure on a 04 trailblazer is the pressure shown on the gauge before or after the fuel filter? Im asking because i have a p0171 code ive been struggling with thank you.
Fuel pressure question
- Thread starter Trailblazen04
- Start date
Thank you then i guess that rules out my fuel pump and fuel filter as its in spec.. i also had my intake manifold changed today to no change in the p0171 i have also checked with carb cleaner for air leaks and also changed the air injection valve, does anybody have any more suggestions? Anything would be very appreciated
Mooseman
Moderator
Is there an exhaust leak, possibly from a cracked manifold? Could also be a bad O2 sensor.
Bad or gummed up injectors? Do you have access to a scan tool that can do an injector balance test?
Bad or gummed up injectors? Do you have access to a scan tool that can do an injector balance test?
I have the torque app im not sure if it can do that. If i remove the shield off the manifold would there be can i check for cracks?
Mooseman
Moderator
Torque can't do that but it can show live data like fuel trims which can be useful.
For the cracks, you would most likely hear a leak, kinda like a ticking sound. You could take the cover off or look on the backside with a mirror or boroscope if you have one.
For the cracks, you would most likely hear a leak, kinda like a ticking sound. You could take the cover off or look on the backside with a mirror or boroscope if you have one.
Mooseman
Moderator
That carries some risk of breaking the manifold to head bolts, and it's always at the hardest spot at the rear. Could also be already broken manifold bolts. However, if you don't hear a leak, it's likely you don't have one. Manifold cracks are usually very evident.
The Mysterious P0171 Code may have another culprit involved. If your Trailblazer is equipped with the SAIS (Secondary Air Injection System) which will be evident by looking on the Passenger side of the Engine head as a centrally attached Aluminum Solenoid with Hosing and Electrical Connector in plain view... consider the possibility that if the Solenoid has gone sideways... it will serve as an unwanted source of pumped or 'inhaled' Air down into the Exhaust Stream.
If you can test whether or not the Solenoid and SAIS Pump are working and the Valve body stays locked OPEN ... even of the SAIS pump under the vehicle feeding it is either NOT working or fails to activate ...generally during initial Start-Ups ... it will still provide additional, unwanted air into the Exhaust Stream and throw off the performance of the Up-Stream O2 Sensor.
The SAIS System is present in later model Trailblazer and Envoys; ostensibly to improve the efficiency of the CAT and get it working Very HOT quite soon after Start Up... But it is NOT an essential component ...and as a homologous structure ...like a Human Appendix... it can be removed and blocked off at the Engine Head with a Small Cover Plate and Gasket SAIS Delete Kit.
This video shows the methodical Scanner Diagnostics the VOP (Video Original Poster) followed to determine that a Stuck SAIS Solenoid was #1 on his Repair Runway. If you wind up getting a "GYMKO" Tech 2 Kit... later on, you can disable the SAIS feature in the PCM:
If you can test whether or not the Solenoid and SAIS Pump are working and the Valve body stays locked OPEN ... even of the SAIS pump under the vehicle feeding it is either NOT working or fails to activate ...generally during initial Start-Ups ... it will still provide additional, unwanted air into the Exhaust Stream and throw off the performance of the Up-Stream O2 Sensor.
The SAIS System is present in later model Trailblazer and Envoys; ostensibly to improve the efficiency of the CAT and get it working Very HOT quite soon after Start Up... But it is NOT an essential component ...and as a homologous structure ...like a Human Appendix... it can be removed and blocked off at the Engine Head with a Small Cover Plate and Gasket SAIS Delete Kit.
This video shows the methodical Scanner Diagnostics the VOP (Video Original Poster) followed to determine that a Stuck SAIS Solenoid was #1 on his Repair Runway. If you wind up getting a "GYMKO" Tech 2 Kit... later on, you can disable the SAIS feature in the PCM:
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Mooseman
Moderator
Mooseman
Moderator
Although you said you checked for vacuum leaks, did you check the resonator to valve cover hose?
Mooseman
Moderator
gmcman
Member
- Dec 12, 2011
- 4,656
From what I gather.....P0171 is basically saying the PCM has to add fuel. This is generally from unmetered air among what's been discussed with injectors and sensors.
My questions are:
1: Are you using AC Delco O2 sensors?
2: Does the engine idle smoothly at 600 RPM?
3: Have you used a vacuum gauge on the throttle body where vacuum hose enters near the butterfly? If so, please let me know what the reading is and is the needle steady...not fluttering but can move slowly back and forth.
4: There's a vacuum port in the front of the intake, facing the passenger side. On some models, this is not pulling vacuum, on some it is. Does it have a cap and/ or is it leaking vacuum?
5: Check the molded vacuum hose between cyl 4 and 5 on the intake for cracks or leaks.
6: Check the brake booster for leaks. With the engine off, slowly remove the check valve from the booster...it should sound like its pulling vacuum when the valve is removed.
7: Only do this if comfortable....but you can try using a propane torch with no flame obviously...but let it pour propane near the intake runners at the head....does the RPM rise?
Please let me know.
Thanks.
My questions are:
1: Are you using AC Delco O2 sensors?
2: Does the engine idle smoothly at 600 RPM?
3: Have you used a vacuum gauge on the throttle body where vacuum hose enters near the butterfly? If so, please let me know what the reading is and is the needle steady...not fluttering but can move slowly back and forth.
4: There's a vacuum port in the front of the intake, facing the passenger side. On some models, this is not pulling vacuum, on some it is. Does it have a cap and/ or is it leaking vacuum?
5: Check the molded vacuum hose between cyl 4 and 5 on the intake for cracks or leaks.
6: Check the brake booster for leaks. With the engine off, slowly remove the check valve from the booster...it should sound like its pulling vacuum when the valve is removed.
7: Only do this if comfortable....but you can try using a propane torch with no flame obviously...but let it pour propane near the intake runners at the head....does the RPM rise?
Please let me know.
Thanks.
gmcman
Member
- Dec 12, 2011
- 4,656
If you have the torque app, the O2 sensors and fuel trims will help a lot.
Here's two screenshots of one page I have to monitor fuel trims and O2 sensors. Granted, my converter is original and is starting to show its age, but works ok.
This is at idle, the left graph is the upstream O2 sensor and it's not needing to respond as fast.
This is at 60 MPH, give or take 1-2 MPH, but the upstream O2 sensor is much more active.
What happens is the sensor gets lazy over time and if you have a slow responding sensor like the 1st pic when at cruise you could possibly get some codes.
Here's two screenshots of one page I have to monitor fuel trims and O2 sensors. Granted, my converter is original and is starting to show its age, but works ok.
This is at idle, the left graph is the upstream O2 sensor and it's not needing to respond as fast.
This is at 60 MPH, give or take 1-2 MPH, but the upstream O2 sensor is much more active.
What happens is the sensor gets lazy over time and if you have a slow responding sensor like the 1st pic when at cruise you could possibly get some codes.
Ok thank you when i get home ill set it up. Do i just post screenshots like yours or is it something else?
Mooseman
Moderator
Hey guys thanks for everbodys suggestions i had been with my wife at the hospital for several days so i left the trailblazer at a local mechanics. It is finally running great again he said the oxygen sensor was dead. Im concerned that his scanner pulled up extra codes that were not showing on my torque app. He pulled p0134,p0135 aswell as showing the p0171.
gmcman
Member
- Dec 12, 2011
- 4,656
Hope the wifey is doing well.
Your mechanic could have pulled up codes stored long ago, hard to say but very possible.
Great that it's running well again, cannot stress enough that when it comes to sensors, don't chance aftermarket and go with OE AC Delco. Not sure what was in there or if the OE sensor finally went south...but just some tips moving forward.
Your mechanic could have pulled up codes stored long ago, hard to say but very possible.
Great that it's running well again, cannot stress enough that when it comes to sensors, don't chance aftermarket and go with OE AC Delco. Not sure what was in there or if the OE sensor finally went south...but just some tips moving forward.