[Cyynder]

2008 Acadia SLT-2 150k miles

I've been having random misfires while gradually accelerating and just maintaining speed. If i give it more gas or let off the gas while it's misfiring it stops and runs correctly and it runs perfect at idle. When i scanned for codes there weren't any so i was at a loss of what to do so i started with the easy and obvious...

So far i have replaced:
Spark plugs
Ignition coils
Fuel injectors
Thermostat
MAP sensor
Mass air flow sensor
Air filter
PCV valves
Variable valve timing solenoids
Checked for vacuum leak using propane

Throttle body and Throttle position sensor were replace 2 years ago when sensor went bad.

I have even taken it back to the dealer who was dumb enough to give me a lifetime powertrain warranty and got the timing chains replaced, for $12, the price of the oil filter.

Even with all of that i still have the misfire while gradually accelerating. Codes:
P0172 system too rich bank 1 and
P0175 system too rich bank 2 show up now too

While all of that has been going on i also have an ABS code, C0561 vacuum sensor "A/B" correlation. While driving there are no problems, during deceleration i get service stabilitrak and traction control off, then during acceleration they both turn off. I have checked the vacuum hose from the intake manifold to the master cylinder and it looks good.

I have a feeling that all of these issues are connected, somehow. I'm thinking the next item to replace is the master cylinder or maybe the ABS control module.

I am using the bluedriver OBD II bluetoth code reader that reads both engine and ABS trouble codes.

This has been extremely frustrating and any help or advise would be greatly appreciated

 

[JayTee2019]

Surely is frustrating to have that number of codes and glitchy codes and symptoms.

As you say, they all may be related somehow or not. I would venture that the P0172 and P0175 could be caused by "lazy" O2 sensors if they have never been changed.
If you monitor the electrical signals from them they should dance around a bit rather than stay at relatively steady voltage output. Even new sensors can be less sensitive than they should and not respond as fast as they should, but not even trigger fault codes.

 

[Cyynder]

As you say, they all may be related somehow or not. I would venture that the P0172 and P0175 could be caused by "lazy" O2 sensors if they have never been changed.
If you monitor the electrical signals from them they should dance around a bit rather than stay at relatively steady voltage output. Even new sensors can be less sensitive than they should and not respond as fast as they should, but not even trigger fault codes.

Which o2 sensors should I do first? Upstream or downstream, or just do all 4? The problem is i have codes for both banks so just doing 1 won't or shouldn't fix it...

 

[Marshall..]

2008 Acadia SLT-2 150k miles

I've been having random misfires while gradually accelerating and just maintaining speed. If i give it more gas or let off the gas while it's misfiring it stops and runs correctly and it runs perfect at idle. When i scanned for codes there weren't any so i was at a loss of what to do so i started with the easy and obvious...

So far i have replaced:
Spark plugs
Ignition coils
Fuel injectors
Thermostat
MAP sensor
Mass air flow sensor
Air filter
PCV valves
Variable valve timing solenoids
Checked for vacuum leak using propane

Throttle body and Throttle position sensor were replace 2 years ago when sensor went bad.

I have even taken it back to the dealer who was dumb enough to give me a lifetime powertrain warranty and got the timing chains replaced, for $12, the price of the oil filter.

Even with all of that i still have the misfire while gradually accelerating. Codes:
P0172 system too rich bank 1 and
P0175 system too rich bank 2 show up now too

While all of that has been going on i also have an ABS code, C0561 vacuum sensor "A/B" correlation. While driving there are no problems, during deceleration i get service stabilitrak and traction control off, then during acceleration they both turn off. I have checked the vacuum hose from the intake manifold to the master cylinder and it looks good.

I have a feeling that all of these issues are connected, somehow. I'm thinking the next item to replace is the master cylinder or maybe the ABS control module.

I am using the bluedriver OBD II bluetoth code reader that reads both engine and ABS trouble codes.

This has been extremely frustrating and any help or advise would be greatly appreciated

There are too many trouble codes in your car, right? Go to the repair shop and check it.

 

[JayTee2019]

Which o2 sensors should I do first? Upstream or downstream, or just do all 4? The problem is i have codes for both banks so just doing 1 won't or shouldn't fix it...

If you replace the O2 sensors, do all four especially at that mileage.

But first, given the nature of so many issues, I would suspect electrical grounds to the engine block, transmission and several on the chassis.
You might also check the engine mounts. They are known problems and if one or more are bad, they may allow too much movement as when accelerating/decelerating (stress cables.
Any time an engine is worked on like for the timing chains. . .. sometimes wire harnesses and cable connections can be messed with during that and later cause wonky connections.


Remove any you can find, clean paint and any corrosion off the mating surfaces, nuts and lugs. Apply an thin coat of grease to them and refasten them.
Electrical issues are often the culprit as a vehicle ages.
You may be surprised at what you find under ground points that may look good until you remove and clean them.

You might also check the fuse boxes under the hood and passenger side under the dash and right footwell panel.

 

[Cyynder]

There are too many trouble codes in your car, right? Go to the repair shop and check it.

It was in the shop for 3 weeks for this problem, they decided it needed timing chains. Got those done and it's still doing the same thing the day i took it in

 

[Cyynder]

If you replace the O2 sensors, do all four especially at that mileage.

But first, given the nature of so many issues, I would suspect electrical grounds to the engine block, transmission and several on the chassis.
You might also check the engine mounts. They are known problems and if one or more are bad, they may allow too much movement as when accelerating/decelerating (stress cables.
Any time an engine is worked on like for the timing chains. . .. sometimes wire harnesses and cable connections can be messed with during that and later cause wonky connections.


Remove any you can find, clean paint and any corrosion off the mating surfaces, nuts and lugs. Apply an thin coat of grease to them and refasten them.
Electrical issues are often the culprit as a vehicle ages.
You may be surprised at what you find under ground points that may look good until you remove and clean them.

You might also check the fuse boxes under the hood and passenger side under the dash and right footwell panel.

I thought about engine grounds too but it runs perfect at idle, as far as the engine mounts go, when it was in the shop the warranty told the dealer to remove the motor completely from the car so they could see everything. I have to believe that if there were issues with motor mounts it would have been taken care of at that time. Sorry, forgot to mention all of that in original post.

Guess i will go get the o2 sensors and give that a try.

 

[JayTee2019]

I thought about engine grounds too but it runs perfect at idle, as far as the engine mounts go, when it was in the shop the warranty told the dealer to remove the motor completely from the car so they could see everything. I have to believe that if there were issues with motor mounts it would have been taken care of at that time. Sorry, forgot to mention all of that in original post.

Guess i will go get the o2 sensors and give that a try.

Motor mounts are easy to check.

And since the motor was removed . . . all the more reason to suspect and check what things a well meaning tech may have left loose or undone.

 

[Cyynder]

Motor mounts are easy to check.

And since the motor was removed . . . all the more reason to suspect and check what things a well meaning tech may have left loose or undone.

Good point, I'll be working on it this weekend and will double check things like you said, got o2 sensors ordered so I'll have it up on jack stands and can get under and really check it throughly

I stopped and talked to another dealer today, they suggested o2 sensors as well. Also mentioned that there may be a problem with my power brake booster, might be a vacuum leak there which would explain and fix everything.

The more I think about this the more i keep thinking it has to be a vacuum leak, just have to find it. I'm going to do a leak check with propane again this weekend just to be sure

They also gave me the best explanation so far about why I'm experiencing the misfires with the codes for bank 1 and 2 being too rich. The ECU is trying to adjust the fuel mixture because of these codes, which is causing the fuel mixture to become too lean causing the engine to misfire occasionally. Not sure if thats whats really going on though, no one else I've talked to around here has told me anything else that could explain it.

 

[JayTee2019]

Good point,
They also gave me the best explanation so far about why I'm experiencing the misfires with the codes for bank 1 and 2 being too rich. The ECU is trying to adjust the fuel mixture because of these codes, which is causing the fuel mixture to become too lean causing the engine to misfire occasionally. Not sure if thats whats really going on though, no one else I've talked to around here has told me anything else that could explain it.

Yes . . that is exactly what the ECU will do if there is a vacuum or exhaust leak. The 02 sensors register more oxygen in the air/fuel flow than expected . . . . . so the ECU will then add fuel to "correct" and adjust to a proper air fuel mixture.
But it assumes all is fine fine with no vacuum or exhaust air leaks,
The ECU is calibrated to expect ONLY the amount of air it "knows" should be coming into the throttle body and intake that the MAF . . . Mass Air Flow. . . sensor sees that is connected into the air filter box between it and the throttle body.

If air leaks in downstream from that, the ECU does not "know" about that and tries to match with more fuel.

 

[SteelerFan]

Before anything, change all 4 engine mounts. The front one, beside the radiator is the easyest. I prefer Anchor brand mounts. DEA has decent mounts.
Rock Auto has them for about $30. BTW, I had the same codes in my 2010 Acadia SLE a few years back, changed all the mounts, and like magic, all the codes disappeared.!

2008 GMC ACADIA 3.6L V6 Motor Mount | RockAuto

RockAuto ships auto parts and body parts from over 300 manufacturers to customers' doors worldwide, all at warehouse prices. Easy to use parts catalog.

www.rockauto.com

 

[Cyynder]

Before anything, change all 4 engine mounts. The front one, beside the radiator is the easyest. I prefer Anchor brand mounts. DEA has decent mounts.
Rock Auto has them for about $30. BTW, I had the same codes in my 2010 Acadia SLE a few years back, changed all the mounts, and like magic, all the codes disappeared.!

2008 GMC ACADIA 3.6L V6 Motor Mount | RockAuto

RockAuto ships auto parts and body parts from over 300 manufacturers to customers' doors worldwide, all at warehouse prices. Easy to use parts catalog.

www.rockauto.com

I'm going to replace the power brake booster and o2 sensors this weekend since I've already bought them. I I'll do the motor mounts next if this doesn't fix it. Thanks

 

[Cyynder]

So I'm pretty sure i know what the problem is now. I went to swap the power brake booster tonight and when i pulled the brake booster sensor, it was full of brake fluid. Pretty sure that means my master cylinder is failing and leaking into the brake booster.

I stopped what i was doing when i found this and am waiting to do anymore work until i can get a new master cylinder and brake booster sensor.

I will then change the master cylinder, booster and booster sensor at the same time to avoid damaging anything else with brake fluid.

Feels good to finally know(hopefully) what's wrong and that i can fix it.