03 Envoy XL -- Bank 1 & 2 System Too Lean

Model XL SLT 4WD, Engine "P" with 103,000 miles

I have been getting a Service Engine Soon light P0171 and P0174 setting repeatedly (have reset twice just to make sure it wasn't a one time thing). It doesn't come on right away, or every time it is driven -- and the only noticeable symptom is rough idling (but that is only noticable sometimes).

Recent maintenance items were air filter change 3 mos ago, tune up (new plugs and wires) 1.5 months ago and the light started about 3-4 weeks ago. I changed the fuel filter a week ago and added fuel injector cleaner. I just added fuel intake/fuel system cleaner on my next fill up (today). Also sprayed the Mass Air Flow sensor with MAF cleaner today. I tried to get it to run through a drive cycle with a code reader attached and it wouldn't "complete" after I followed the cycle directions. Will try that again tomorrow (idle 4 min, drive 18 min 45-55 mph, idle 2 min).

I am waiting to see if all the above makes the issue disappear this week -- but am not convinced I am working in the right spot and don't want to be missing a better place to start troubleshooting. I realize that the most "recent" fix was the tune up -- but I am not getting any spark plug codes or misfires.

The code information also mentions fuel pump, O2 sensors and vacuum leaks as possible causes -- but I am getting no other codes hitting that point in those directions, and no stalling or hesitations when accelerator is pressed.

Any other ideas would be appreciated so I don't keep chasing rabbit trails...

Thanks.

Welcome to AF. I assume this is the 5.3 8? Anyway, low fuel pressure, vacuums leaks, dirty injectors are the most common areas for these codes. See what happens after your recent work.

Because of Age + Mileage, suspect bad rubber, plastic, gaskets, or seals.
Call around for a Smoke test, and focus on the EGR, PCV, and Inake system.

Yes, sorry -- it is the 5.3 V-8 engine if the engine code I gave was the wrong "letter/number" to designate that.

I got no SES lamp this morning on a 20 mile drive to work, but it didn't complete the drive cycle again and was definitely running rough at idle when I started up this morning (cold morning though). I'm going to try to get it to dump values to a Code Reader triggered by the SES lamp coming on.

Can you be more specific on calling around for a smoke test? I assume that is something a shop can complete to locate vacuum leaks? Anything I can do in the home garage to try that (or is that over a rookie's head)?

Whoops, spoke too soon. Just started the car (cold after sitting for most of the day at work) and the SES light came on immediately at cold start (P0171 and P0174 pending). The SES light/MIL was OFF when I parked it at work this morning (I know because I had the code reader attached trying to get a drive cycle to complete).

After driving it for a short errand, I stopped at a stop sign, turned off the key and gave the reset code. Started back up, no SES, but this was a warm start instead of a cold start. Another short drive and the code did not set again.

I suppose it could be associated with a cold start, but not certain. Will test out this condition over the next couple of days to see if there is a link between cold start, rough idle and the code setting. I think I remember seeing the light come on during a longer drive not too long ago -- but not certain.

I found a video, see http://www.youtube.com/watch?v=FfiQ7qWiG-M

It's probably $80-$120 for test, but are the technicians going to tell the truth?
The smoke machine cost around $1200.

For $20 including S&h, I can send you a smoke test, see [email protected]

I have been looking for some reference to an EGR for this vehicle on RepairPath.com and cannot find it. Also stopped by AutoZone and asked for an EGR valve for the car and they don't pull anything up.

Is it possible that this part/system was renamed on this vehicle -- or does this engine not have an EGR system?

It might not have one, I got a 2000 Jag 4.0, that doesn't have a EGR.

See http://www.alldata.com

No egr on this engine.

Rented a fuel pressure gauge from Autozone. After a bit of searching, I found the test port on the passenger side located on the crossover pipe that connects the fuel rails.

On the initial ON cycle (engine off) -- it developed 51 PSI. A few cycles OFF/ON (engine off) it developed a maximum of 54 PSI. At idle, it settled to 47 PSI.

In the fuel gauge tester booklet -- it lists the 5.3 V8 "P" engine type as rated pressure of 55 - 62 PSI. Not sure if that means with engine off or running at idle.

Not sure if that is close enough or part of the problem. Do this sound like a fuel pump going bad (weak pressure)?

I didn't think about this before, but should i pressurize the system, and then turn off the fuel pump (no engine running) to see how quickly the pressure bleeds off? Would this indicate a tank leak?

Did another fuel system pressure test:

First accessory on: 51 PSI
Accessory on/off 3 times: 54 PSI
Five minutes after key off: 50 PSI
Ten minutes after key off: 44 PSI

Start it up see what you have, you seem to be on the low side, 62 is at full throttle, you need 55 running at no load.

I get 47 running at no load/idle speed (no accessories turned on, but engine running).

Since you say I need 55 at this situation and 62 at full throttle, sounds like I am down 15% or so on pressure. I just put a new fuel filter in -- so does that take me down the troubleshooting path to:

1) A new fuel tank filter (ugg - gotta drop the tank)
2) A new fuel pump (ugg - gotta drop the tank)
3) Looking for a hole in the tank/fuel line somewhere? (ugg -- gotta get a smoke test)
4) Bad gas cap? (wouldn't that be nice)

Does losing pressure at about 1 PSI per minute after energizing the pump (no engine running) lead me to a leak in the tank/fuel line/gas cap or is that normal?

Thanks for all of your help so far!

Test's are so much cheaper!
Fuel pressure Test
Compression Test
Smoke Test
Catalytic converter back pressure test.
Ignition Test.

Backyard Smoke Test -

1) Remove Air filter.
2) Wrap Air filter with glad wrap, or plastic bag.
The idea is to block all are going into the Intake.
3) Put Air filter back in box.
4) Remove the Brake booster vacuum hose.
5) Light a Black & Mild cigar.
6) Blow puffs of cigar smoke through the brake booster vacuum hose going into the Intake, and cover vacuum hose with thumb inbetween puffs.
7) Focus on EGR, PCV, and Air Intake system.
If you see any smoke, you found the problem. :smokin:

Good luck - danielsatur

Backyard smoke test performed. Have one lead I can follow up on with that, although its probably low odds -- but here's the story...

After I finally figured out how to keep the smoke in the intake hose (I actually had to take the air filter off the intake to the filter compartment and wrap the hose with a ziplock bag with about 6 doubled up rubber bands to keep it on when I blew smoke. That was all trial and error after wrapping the filter failed to keep the smoke back and just a single rubber band wasn't enough to keep the bag on the intake hose when I blew in the brake booster. (And yes, I did use the Black & Mild brand -- quite a treat).

The only thing that leaked (and it was just a little bit of smoke, but it was there) was just past the MAF sensor housing. That housing didn't seem to be "quite" seated with the intake hose just past it. Upon further inspection, both clamps (before and after) the MAF were a little loose and it seems that the last person to change the air filter "forgot" to screw down 2 of the 4 bolts securely (that would be the oil change place my wife took the car to). My guess is that being loose for a few months, the filter box got to moving a bit and might have worked the intake hose slightly out of 100% connection.

Anyhow -- its all put back, 4 bolts screwed down, the hoses all making positive connections and the clamps screwed down extra tight. I ran the car hard and didn't pull any codes, but we'll see. It doesn't seem to me like that little (very small) potential leak could screw things up that bad...

Another week of driving should prove it out. I'm going to rent the fuel pressure tester again to see if anything is different (and check the pressure at higher RPM, which I didn't do before).

Did you blow the smoke through the Intake brake booster hose, and cover with thumb inbetween puffs of smoke?

The plastic wraped around the air filter, should of been good enough to block all air going into the Intake.

I think you hit the nail on the head with the loose clamps, any unmetered air would of caused your P0171.

You Luckey, I just did a smoke test on a Mazda Tribute, the Intake gasket is bad.

I glad you injoyed the smoke!

LOL - If this fixed the problem, can I have $20

danielsatur

Did you blow the smoke through the Intake brake booster hose, and cover with thumb in between puffs of smoke?

Yes. And I noticed that with the bag and heavy rubber bands on the end of the filter box inlet housing -- it actually built a little bit of back pressure even with just lung power, where before, when I just had the air filter wrapped, the smoke was able to escape into the air filter box and exit the system that way without being able to build any pressure (lung pressure that is). Maybe I wrapped the wrong end of the air filter, but I couldn't really "seal" the round air filter in the wrapping? (In other words, I should have covered the portion where it attaches to the inlet housing rather than everything else?

The plastic wraped around the air filter, should of been good enough to block all air going into the Intake.


It wasn't -- at least the way I sealed it. And I am assuming you mean "backwards" into the intake, right? When I blow on the brake booster hose, the air is traveling the opposite way through the air filter box...

I think you hit the nail on the head with the loose clamps, any unmetered air would of caused your P0171.


OK, so even though the engine NEEDS air, it isn't just that the extra air isn't getting filtered, its that it is bypassing the MAF and so there is more air than the engine thinks it is getting -- and when the ignition takes place, it has a higher air/fuel ratio than it is expecting -- setting the code?

LOL - If this fixed the problem, can I have $20

Hey if it fixes the problem -- I don't mind paying you for the advice one bit (satisfaction guaranteed I suppose?). I think the jury is out still, but if it works, it would be a big save in my book. I would have dropped the gas tank next (big time and hassle/expense -- new fuel pump and filter probably is next in line).

I must admit, I didn't realize that paid advice was the "bent" of this forum -- so if that is the case, it would be helpful to make that clearer on the front end to a new poster. Regardless, that doesn't change my mind on paying if the advice proves to be the solution...

Check back in a week. And thanks again!

No one charges for advice here, he was kidding with you.

Whoops -- now I feel almost as stupid as I did when my 16 year old son walked into the garage and found me on top of my engine blowing on the brake booster line with a cigar in my hand...

...that was a tough one to explain.

So far, so good. The wife thinks the car is driving better and no lights have popped up yet. And I meant it about the value of the advice. I can see taking it to the shop and easily believing the problem was an $800 fix.

How about a Cubin cigar!
Your son would probably enjoy helping you, he can watch for smoke.

Thought we had it licked -- but on a 2-3 hr trip this week, the same codes set P0171 and P0174. Talked it out a little bit with a clerk from AdvanceAuto and he wasn't really buying the fuel pump/tank screen clogging as a possibility. Said that as long as the fuel filter was new and it had no problems keeping up when accelerating or stalling at idle -- then the pump was probably OK. Said they rarely go just a "little" bad. I remember being just slightly below the normal operating range at idle when I tested the pressure at the fuel rail -- and although I haven't checked it with the gauge at higher RPMs yet, but I don't feel any problems when giving it gas.

Suggested keeping on the vacuum leak trail again or maybe a leak in the exhaust system? Between the two O2 Sensors? Beyond that he suggested dirty injectors as a possibility or low end injector programming gone wrong (hard reset on the ECM?). I don't think I can run any more injector cleaner/fuel system cleaner through the gas tank. So I'm going to give the Seafoam through the brake booster hose a try.

I confirmed again that there is no PCV valve on this engine just to make sure I had that right.

I'd almost fly to Cuba to get those cigars if I could figure this out...

How was the fuel economy on the Trip?
1) Repeat Smoke Test, and have your son watch.
2) Consider replacing your upstream H02 sensors, these sensors are used as a feed back signal for the Air/fuel mix.

Just fixed a 2005 Ford Escape/ 130K miles DTC P0171
see http://www.automotiveforums.com/vbulletin/showthread.php?t=1040765

Fuel economy has been bad, bad. But I'll have to check it again after the latest attempt at a fix.

Just got done with the Seafoam treatment. Sucked a half a can through the brake booster hose with the car running. Let it drink it in with small sips, then dunked the hose and let it stall the engine (actually, it took the whole can and didn't die, so we turned off the engine).

Let it sit for about 30 minutes and then blew out the smoke (and smoke, and smoke). Ran it hard, full throttle a couple times 0-60 and drove it at highway speed for a while. Got another can of Seafoam and 1/2 went in the oil, half went in the gas tank.

Drove it for about 70 miles after that and it feels better at idle and the codes didn't come back on the 70 mile highway trip. I'm not a believer until it runs for a week like that though and makes it through a couple of cold starts without the codes coming back.

Went ahead and changed the oil without shutting the car off so whatever sludge came out with the Seafoam in the oil left with the filter and old oil.

I will gladly redo the smoke test and also replace the upstream O2 sensors if the codes come back. Will report back soon.

On the trail, and not giving up!

@ $3 a gallon, do a GMC drive cycle ASAP!
Do a Google ''GMC drive cycle''

There's certain rules for a drive cycle, so your OBD II system is readiness.
Example - Certain MPH of city driving, and so many MPH of Hwy driving.

If there's a problem in the drive cycle, a CEL is thrown, and a DTC can be pulled for diagnostics.

The trick is to go by the rules for a drive cycle, so you can see the problem resolved.

Most Inspection stations will just tell you to put miles on, it could take days instead of 15min.

Still working on trying to get a drive cycle to complete. And I am following the GMC Drive Cycle instructions you can get from Google searches.

On another note -- the car is running MUCH smoother at idle and drives MUCH better at all other speeds after the Seafoam treatment! My wife said it was a new car (that's the best feedback I've gotten yet through this whole ordeal).

Unfortunately, the codes came back after a short drive this morning. They didn't set right away on startup -- but we went about 2 miles at low speeds and both P0171 and P0174 came back.

Hooked up the fuel pressure gauge at the rail again and I still get about 52 psi with the engine off and the fuel pump turned off/on a few times. At idle, I am getting about 46 psi and after revving the engine up to 2000 RPM and then as high as 5000 RPM -- I am still only getting about 46 psi (small fluctuations as you change the RPM, but it settles back there).

I'm hesitant to change the O2 sensors, because I don't figure they have anything to do with the pressure at the fuel rail -- although I could be educated on that if the O2 sensors could "change" the pressure the fuel pump would be delivering at the rail.

I'll give it another backyard smoke test, but don't see the relationship between fuel pressure and a vacuum leak in the air delivery system. Again, I could be better educated here.

Should I be back to thinking a fuel tank screen or fuel pump problem OR a pinhole leak in the fuel tank itself? I'm not smelling or leaking gas, the cap seems OK. Not sure about the filler neck as another possible source, but I've heard they've had some problems with those from time to time on the Envoy too.

However, it seems that a small leak in the tank won't cause a problem at the fuel rail (past the fuel pump and fuel filter, etc -- those should be keeping a different pressure in the fuel lines than what the tank would see.

1) Black & Mild cigars are cheap!
2) Test's are a very good practice, and can rule out any problems.
3) I usually don't like to just throw money, parts, and service at a Auto, but
it's time for new upstream H02 sensors anyways, these signals are used by the ECU/PCU for the air/fuel MIX 14.7 to 1 ratio. We can rule they out too!

If smoke test passes, then focus will be on the fuel pressure.
Make sure the fuel pressure guage is good, try doing a fuel pressure test on another motor vechicle.
Fuel filter, electric fuel pump, and remove tank! ''ouch''

My understanding is the deliverabilty of engine is good, but poor fuel economy, and DTC's P0171,P0141

Notes
1) I have seen a bad injector seal causing lean codes.

2) Connect your OBD II scanner, and check I/M readiness (KOEO)
Monitors ''ok'', and ''n/a'' with no DTC's is complete.
Only do GM drive cycle monitor that's ''inc''

3) Any leaks in the fuel system would be a DTC for EVAP problem.

Great. I have the o2 sensors to replace -- but for the life of me I can't figure out how to reach my hand up around all the pipes down there to release the connector (let alone get any kind of grip to be able to plug the new one in). Am I supposed to be removing something in order to replace these bad boys?

Very frustrated. $110 in parts for the upstream sensors and can't figure how to plug them in. I can remove the sensor end no problem -- that is easy to reach, but the plugs are so far up inside the body of the car that I can't weave my hand in there to reach them.

Any tricks?

Try getting to the sensor connectors from under the hood instead of under truck.

$110 for upstream H02 sensors '''Ouch''

Don't do the downstream, save 50% & hold onto the old H2O sensors for spare.

I have used spare H02 sensors (4-wire) from my Jag to my Tribute ''OK''

They were $55 each and I just bought the upstreams. Having a hard time seeing them under the hood -- they seem just far back enough to be hidden by the back wall of the engine compartment.

Found new upstream H02 (4-wire) sensors for $37 free shipping,
see http://www.autopartswarehouse.com

Thanks for the link -- now if only I can figure out how to get to the connectors (from the top or bottom), I'll actually replace them...

I captured some data from the scan tool. The upstream O2 sensors are jumping all over the place -- but the downstream sensors are constant. This is all at idle speed with time in seconds at approximately 0.9 s intervals.

TIME 0 0.9 1.8 2.7 3.5 4.4 5.3 6.1 7 7.9

ABSLT TPS % 9 9 9 9 9 9 9 9 9 9
ENG SPEED RPM 547 552 547 556 557 557 548 551 558 563
CALC LOAD % 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9
MAF FLOW GR/SE 5.24 5.34 5.32 5.33 5.34 5.21 5.32 5.2 5.32 5.17
MAP KPA 34 34 35 35 35 34 35 34 35 34
COOLANT �F 181 183 183 183 183 183 183 183 183 183
IAT �F 66 66 66 66 66 66 66 66 66 66
IGN ADVANCE DE 18.5 18.5 16.5 20.5 19.5 18 16.5 17.5 17.5 18
ST FTRM1 % 0.8 0 0 0.7 0.7 0 0 0.8 0.7 0.7
LT FTRM1 % 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3
ST FTRM2 % 1.5 0.7 4.6 2.3 4.6 3.1 3.9 0.7 1.5 3.1
LT FTRM2 % 0.8 0.8 0.8 0.8 0.8 0 0 0 0 0
VEH SPEED MPH 0 0 0 0 0 0 0 0 0 0
O2S11 V 0.105 0.795 0.065 0.2 0.5 0.685 0.705 0.335 0.59 0.775
ST FTRM11 % 0.7 0.8 2.3 0 1.5 0.8 0.8 0 0 0.8
O2S12 V 0.805 0.805 0.805 0.805 0.81 0.82 0.815 0.815 0.825 0.82
O2S21 V 0.055 0.05 0.695 0.075 0.18 0.835 0.83 0.065 0.17 0.075
ST FTRM21 % 3.9 2.3 2.3 3.9 4.6 0.7 0.7 2.3 1.5 3.1
O2S22 V 0.85 0.85 0.845 0.84 0.845 0.84 0.835 0.835 0.835 0.83


TIME 8.7 9.6 10.5 11.3 12.2 13 13.9 14.8 15.7 16.5

ABSLT TPS % 9 9 9 9 9 9 9 9 9 9
ENG SPEED RPM 531 539 547 557 542 541 541 543 547 541
CALC LOAD % 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9
MAF FLOW GR/SE 5.3 5.3 5.28 5.28 5.29 5.28 5.29 5.27 5.28 5.26
MAP KPA 35 35 35 34 35 34 35 35 35 35
COOLANT �F 183 183 183 183 183 183 183 183 183 183
IAT �F 66 66 66 66 66 66 66 66 66 66
IGN ADVANCE DE 14.5 19.5 19 17.5 17.5 17 17.5 20 16.5 17.5
ST FTRM1 % 1.5 1.5 0 -1 2.3 2.3 1.5 2.3 2.3 2.3
LT FTRM1 % 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3
ST FTRM2 % 3.1 1.5 3.9 0 3.1 0.7 3.9 3.9 2.3 3.9
LT FTRM2 % 0 0 0 0 0 0 0 0 0 0
VEH SPEED MPH 0 0 0 0 0 0 0 0 0 0
O2S11 V 0.565 0.075 0.075 0.075 0.82 0.835 0.085 0.065 0.065 0.095
ST FTRM11 % 0.7 1.5 2.3 0.7 0.8 0 3.1 2.3 2.3 3.1
O2S12 V 0.82 0.825 0.82 0.82 0.82 0.815 0.815 0.81 0.81 0.81
O2S21 V 0.13 0.235 0.85 0.08 0.815 0.065 0.815 0.8 0.835 0.505
ST FTRM21 % 3.9 3.9 2.3 2.3 0.7 3.1 1.5 1.5 0.7 2.3
O2S22 V 0.83 0.83 0.825 0.82 0.815 0.82 0.82 0.815 0.815 0.81


TIME 17.4 18.3 19.1 20 20.9 21.7 22.6 23.5 24.3 25.2

ABSLT TPS % 9 9 9.4 9.4 9 9.4 9 9 9 9
ENG SPEED RPM 547 548 558 547 543 549 537 537 558 542
CALC LOAD % 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9 1.9
MAF FLOW GR/SE 5.27 5.35 5.23 5.38 5.26 5.36 5.25 5.24 5.25 5.15
MAP KPA 35 35 34 35 35 35 34 34 34 34
COOLANT �F 183 183 183 183 183 183 183 183 183 183
IAT �F 66 66 66 66 66 64 64 64 64 64
IGN ADVANCE DE 20 17 19.5 17 17.5 16 17.5 20.5 14.5 19
ST FTRM1 % 0.8 0 0 2.3 0.7 0.8 1.5 0.7 -3.4 -7.5
LT FTRM1 % 2.3 2.3 2.3 2.3 2.3 2.3 2.3 2.3 9.3 7.8
ST FTRM2 % 3.1 2.3 2.3 1.5 0.7 3.1 3.1 0.7 -2.2 -6.3
LT FTRM2 % 0 0 0 0 0 0 0 9.3 9.3 7.8
VEH SPEED MPH 0 0 0 0 0 0 0 0 0 0
O2S11 V 0.07 0.065 0.27 0.865 0.2 0.085 0.095 0.81 0.91 0.735
ST FTRM11 % 0.7 0.7 1.5 0 0.8 0.7 2.3 0 -4.6 -7.7
O2S12 V 0.805 0.805 0.805 0.805 0.805 0.81 0.805 0.805 0.835 0.855
O2S21 V 0.835 0.44 0.1 0.145 0.115 0.515 0.385 0.91 0.91 0.77
ST FTRM21 % 1.5 1.5 3.9 3.1 3.1 1.5 2.3 0 -4.6 -7.7
O2S22 V 0.815 0.81 0.81 0.81 0.805 0.805 0.805 0.81 0.865 0.895

Here is the full story so far tracking what I have done against suggested problems with DTC P0171 and P0174:


P0171 - System Too Lean (Bank 1)
P0174 - System Too Lean (Bank 2)

The Adaptive Fuel Strategy continuously monitors fuel delivery hardware. The code is set when the adaptive fuel tables reach a rich calibrated limit.

Fuel System:
• Contaminated fuel injectors
1. Ran fuel injector cleaner in one tank of gas
2. Ran fuel system cleaner (injector, intake, system) in one tank of gas
3. Ran ½ can Seafoam in one tank of gas
• Low fuel pressure or running out of fuel (fuel pump, filter, fuel supply line restrictions)
1. Changed fuel filter
2. Measured fuel rail pressure at idle (46 psi) and at high RPM (46 psi) and max pressure without engine running (3 on/off fuel pump prime cycles – 52 psi)
3. DID NOT CHECK YET: Change fuel pump or check fuel tank screen
4. DON’T KNOW HOW TO CHECK: Check fuel supply lines for restriction
• Vapor recovery system (VMV)
1. DON’T KNOW HOW TO CHECK

Induction System:
• MAF contamination
1. Used MAF cleaner/spray on MAF sensor
2. Reseated and tightened clamps at entry and exit to MAF and air filter box
• Air leaks between the MAF and throttle body
1. Did backyard smoke test with cigar through brake booster hose with air filter box cut off, did not detect any leaks
• Vacuum leaks
1. Did backyard smoke test with cigar through brake booster hose with air filter box cut off, did not detect any leaks
• PCV system concern
1. No PCV system on the Envoy XL 2003
• Improperly seated engine oil dipstick
1. Unlocked and reseated oil dipstick when adding Seafoam. Performed oil change 75 miles afterwards.
 
EGR System:
• Leaking gasket
1. DID NOT CHECK YET
• Stuck EGR valve
1. DID NOT CHECK YET
• Leaking diaphragm or EVR
1. DID NOT CHECK YET

Base Engine:
• Exhaust leaks before or near the HO2S
1. DID NOT CHECK YET
• Secondary air concern
1. DON’T KNOW HOW TO CHECK (Maybe Manifold Pressure?)

Powertrain Control System:
• PCM concern
1. DID NOT DO YET: Perform Hard Reset after replacing O2 sensors

OTHER ITEMS PERFORMED:
1. Did Seafoam engine treatment in vacuum line – ran hard to burn off contaminants
2. Added Seafoam to oil and then changed oil and filter after driving 75 miles
3. DID NOT DO YET: Change upstream oxygen sensors
4. DID NOT DO YET: Change downstream oxygen sensors
5. DID NOT DO YET: Check fuel pressure at tank if there is another Schrader valve

SYMPTOMS:
1. SES light and DTC P0171 and P0174
2. Hard time completing a drive cycle
3. Rough idle
4. Low fuel rail pressure
5. Poor gas mileage
6. Downstream H2oS always high “Rich” value (both above .8 continuously)
7. Long Term Fuel Trim Bank 1 staying at positive 2.3% value
8. Mostly positive Fuel Trim values on both ST and LT - rarely go negative

Is your son learning any of this stuff?
What's in common with both banks?
Air/fuel

Air- both banks running lean.
Unmeter Air leaks - Smoke Test Passed
New unstream H02 sensors for Air/fuel mix.
Cleaned MAF/IACT sensor, but are they good?
Do a Google ''Symptoms of bad MAF sensor''

Fuel pressure?

Well, I know when I'm beat. Three fresh attempts to get to those upstream o2 sensor connectors from both above and below and I'm convinced there is no way to do it without disconnecting some major component like a drive shaft or the entire exhaust system (well beyond me in the excitement category).

Gonna make my way to the stealership and let them put the o2 sensors in and maybe even get the fuel pump done (in that order).

Thanks for all your help and encouragement -- learned a lot along the way (especially with the smoke test and about fuel systems/sensors).

Stealership - You said it! $150hr

How about a local muffler shop?
They can do a catalytic converter back pressure test, install the upstream H02 sensors, look for leaks, and replace any bad rubber exhaust mounts.

Good point. I was thinking taking it to a GM dealership because I had read a technical service bulletin dating back to 2006 that said P0171 and P0174 were being caused by dirty injectors. They had a "proprietary" injector cleaning solution identified (GM cleaner and special tools to run the cleaner through the system) as a "fix".

Thought maybe that might work better than my injector cleaner, fuel system cleaner and Seafoam treatment (but maybe not). I might do that AFTER the steps you suggest above instead. Here is why...

I have been reading up a bit on the o2 sensors. It says that the downstream sensors should be settling in to an average voltage of about .45 if they are working correctly and your catalytic converter is working correctly, even though the upstream sensors should be jumping around from high to low (crossing the .45 several times a second between like .2 and .8) as the engine keeps things running properly for a fuel injected system.

So, with the scanner tool and the frame capture (or even just looking at the data live) -- I can see that the upstream sensors are doing just that -- jumping from low to high constantly. But I can also see that the downstream sensors are ALWAYS high (.68 at one test run highway speeds and .8 at another test run at idle). High sensor readings indicate a RICH condition.

If this were in the UPSTREAM sensors -- it would instruct the computer to lean up the fuel delivery to bring the condition back towards .45. I suppose if it was constantly doing this outside of norm values, then it would eventually set the P0171 and P0174. But I'm not sure what this means in the DOWNSTREAM sensors (as far as what the computer is telling the car to do to modify the air/fuel mix). I am getting a Long Term Fuel Trim reading on both banks at highway speeds at about positive 7%. The positive means LEAN condition - so engine, add more fuel (again matching the P0171 and P0174 banks to lean). However this seems opposite of what the downstream sensors are telling me (your exhaust gas is too RICH).

The downstream sensors are used to monitor if the catalytic converter is doing its job. So if the downstream is matching what the upstream is doing -- then the converter isn't working and the exhaust isn't being modified to remove pollutants.

BUT, the downstream sensor in a well functioning car should be settling in at about .45 volts (the ideal fuel/air ratio). So, the way I see it -- the downstream reading constantly high might mean that the converter is doing its job "somewhat" (because it is not matching the same voltages as the upstream -- so it IS changing the exhaust fuel/air ratio when passing through the converter). But maybe it is either not doing it well enough (bad cat), or it is constantly being fed an "average" stream before the converter that is TOO RICH and can only flatten it out somewhat to an average RICH condition.

So -- this leads me to the same place you suggested (in a long winded way I have to work out, because I want to know WHY, not just WHAT).

I should replace the o2 sensors in case they are either biased or not responding FAST enough(although I know the upstream ones ARE reacting because they are going low to high within good ranges) . Or I can check to see if there is an exhaust restriction (back pressure test) or problem with the catalytic converter (not sure bout that one).

If there were a leak in the exhaust system between O2 sensors -- the downstream would tend to read LEAN rather than RICH (right?).

And why are the banks LEAN but the downstream sensors RICH (unless both sensors are bad to the exact same extent, which seems a little far fetched).

Still on the path, but I will be engaging some shop help at this point...

I would cut H02 wires, splice, soldering, and use electrical tape for fix!
2-wires are for the heater, the other 2 for the sensor.

Didn't think about the splicing...

...if the part wasn't so damn expensive I'd be game. What if the wires aren't the same colors across manufacturers and I hook the wrong ones - blam, you fry the good sensor.

Sounds like a fun game to play though. You could always recover from it if it didn't work by just unplugging at the normal connector spot I suppose, just need to buy a new sensor.

Taking it in for an $80 smoke test this weekend. I called to ask about the labor cost to install the sensors and they won't install my parts (looking for an independent mechanic to do this now). Their part cost was 2.5 times what I paid for premium Bosch sensors.

Shop labor PER sensor for the upstreams was quoted at 3.7 hours (I confirmed this on Repairpal).

What they heck do you have to disassemble to get to those connectors that will take 3.7 hours? Does anyone have access to an Envoy shop repair manual that gives a sequence for the deinstall/reinstall of the o2 sensors on a 2003 Envoy XL 4WD 5.3 engine?

Check with http://www.alldata.com , Ask a local muffler shop how long doe's it take?

That was the local muffler shop's time quote!

They said 3.7 hours -- the online Repairpal (I am assuming they show the same information you get from alldata.com -- I used to subscribe there too) quoted labor was 3.8 hours on the low end.

I'm going another route taking it to a local mechanic recommended to me that has a full time shop job during the day and takes in cars at night. I can buy the parts and he thinks he can get all four changed in an evening so it should be a bit better cost wise than full shop rates. We'll see...

I would cut H02 wires, splice, soldering, and use electrical tape for fix!

2-wires are for the heater, the other 2 for the sensor.

Look for color coded wires!

All H02 sensors are the same except for the wire length, and connectors.

The heater signals are usually the same color, and should measure a resistance like a heating element.

Well - I didn't trust my smoke test, so just spent $90 and had one professionally done.

It passed the smoke test with no vacuum leaks or injector seals found to have problems...

On to the O2 sensors!

I glad it passed the smoke test, that tell's me you did a good job doing your own test.
LOL - I think a good cubin cigar $80.
It's always good to have a second opinion.
Did the Technician feel like the HO2 sensors were bad too?
H20 sensors, but how much is it going to cost to put them in?
MAF ?
Fuel pressure?

i have the same truck v8 and all i chased this code since i did the tune up in june till dec put 4 o2 in it fuel pump intake manafold gasket finaly i got some where i asked friend of mine who is a tech to look at it he pulled one plug and screwed it back in and said he never had a gm with out a egr valve run a bosch plug u running bosh? i changed the plugs ac delco platnums 50 bucks for 8 and poof codes gone 2500 miles no codes he told me the electroid is so small on the bosh it does not complety burn all the fuel then u got unburnt fuel in the cats confusing the o2 sensors , oh and there is a pcv valve it is on the drivers side in the back by the fire wall intop of the valve cover one other thing pass side upstream o2 if u reach through the fender well on top of the frame u can get ur hand back in there and get to that o2 plug it took me a freaking hour to change it put u can get to it with out droping any parts