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Discussion Starter · #1 ·
Hi, I’m new here but running out of ideas for my 2011 Acadia SLT. When driving at a steady speed the engine revs rise and fall as if I’m gently pumping the accelerator, it does it in cruise control as well. Say from 1900 rpm up to 2100 and back again, constantly, while on the highway. Acceleration is fine and normal. If you try and target an specific rpm while parked it’s almost impossible, the rev counter waves up and down so I ruled out the torque converter for that reason.
The engine was recently replaced with a used one, (the car belonged to my friend before and he drove it with no oil after a leak and seized it) but this problem was present before the engine change.
Coils, plugs, injectors and fuel rail pressure sensor have all been replaced. Throttle body has been cleaned, I’ve also put some premium cleaner in the fuel as if that would make any difference. Car shows no OBD codes. I have a video of the revs but I don’t think this thing will allow me to upload it (not from an iPhone at least). The only thing I can think of now is that it could be the ECU. Any ideas please help.
 

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Discussion Starter · #2 ·
I should add, local auto workshops here in Dubai simply don’t have the knowledge to diagnose this, I’ve tried a few. The dealership won’t accept the car for analysis because of the engine change, flat refusal to even take a look so I’m really hoping for some ideas here on this forum. Thanks for your time. L
 

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Because the problem has been present on 2 different I would think it is not an engine problem. Rather is is an instrument problem. Also, because you do not specify that you can feel the vehicle changing speeds I also do not think it is an engine problem.

George
 

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Discussion Starter · #4 ·
Hi, thanks for your reply. I can feel the vehicle surging, it’s quite uncomfortable particularly when the engine is under low load. During acceleration it’s completely normal. i think it must be either an electronic control problem or a fuel system issue. The engine was replaced but it was only the block that was changed, all sensors, pumps and accessories are original.
 

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@lewisk : have you read through this thread? Has some good info ... especially @nanookvd regarding the APPS at the very end.

 

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Discussion Starter · #7 · (Edited by Moderator)
@lewisk : have you read through this thread? Has some good info ... especially @nanookvd regarding the APPS at the very end.
Colt hero thanks for your message. I did see that thread some time ago. I ruled out the APPS because the car also presents the same symptoms while in cruise control, which would suggest the position sensor is not the problem I thought. Does that make sense or could I be missing something there?
 

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@lewisk : the pedal still moves with Cruise Control (CC) engaged.

I’d seriously doubt it’s the ECM. That’s incredibly rare. More likely an input to the ECM is fooling it. If it was only doing it with CC engaged, then you might suspect the Speed Sensor giving erratic input, causing the PID algorithm to constantly make corrections about the Setpoint.
 

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Discussion Starter · #9 ·
Alright that’s good enough for me, I’m glad to have something to try. I’ll replace the APPS and see what happens. 👍🏼 Thanks.
 

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Discussion Starter · #10 ·
@lewisk : the pedal still moves with Cruise Control (CC) engaged.

I’d seriously doubt it’s the ECM. That’s incredibly rare. More likely an input to the ECM is fooling it. If it was only doing it with CC engaged, then you might suspect the Speed Sensor giving erratic input, causing the PID algorithm to constantly make corrections about the Setpoint.
Can I ask for a little more information? What did you mean when you said the pedal still moves with the CC engaged? I see it can be replaced for as little as $30 in the US, in Dubai is more like $300.

I’m only getting about 16mpg at the moment, pretty bad. It’s mostly highway driving, not sure if that gives any more insight.
 

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Im currently at 23 mpg on the current tank
on my 2010 with 181,000.
Im a sensible driver on the highway.
Conditions allowing, ill stick to the slow lane.

What about your oxygen sensors?
Ever been replaced? Probably lazy
 

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Can I ask for a little more information? What did you mean when you said the pedal still moves with the CC engaged? I see it can be replaced for as little as $30 in the US, in Dubai is more like $300.

I’m only getting about 16mpg at the moment, pretty bad. It’s mostly highway driving, not sure if that gives any more insight.
Yeah ... I certainly wouldn't pay $300 to replace a part that probably has a low likelihood of correcting your problem.

What I meant (about the pedal) was ... even with the Cruise Control engaged, the pedal still moves like your foot is pushing on it. You can test this out to be sure, and if it is moving in your Acadia, then the question is: Is the ECM still reading the pedal position (even when it's being "faked" by the CC module)? I don't know.

But up above, you said "particularly when the engine is under low load". That's when vacuum is greatest. Maybe you do have a vacuum leak?

The other thing is ... like others have mentioned ... what was the 2011 Acadia rated at for Fuel Economy when brand new? My 2011 Equinox, which I bought brand new (came with the 3.0L V6 engine) was only rated at 17 city and 23 highway! Absolutely dreadful numbers - and I knew it when I bought it, but it was the best value and the right-sized vehicle for me to buy at the time. The 2011 Acadia is even heavier than my 2011 "Tank" Equinox (just under 4000 lbs), so it was probably 16 city and 22 highway?? You're probably looking at 19 mpgs all-around fuel economy with that thing! :confused: Terrible ... I know!
 

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What I meant (about the pedal) was ... even with the Cruise Control engaged, the pedal still moves like your foot is pushing on it. You can test this out to be sure, and if it is moving in your Acadia, then the question is: Is the ECM still reading the pedal position (even when it's being "faked" by the CC module)? I don't know.
How can the pedal move on itts own when there is neither a servo motor in the module nor hard (cable or linkage) connection from the throtle body to the module?
 

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Colt...
The gas pedal is only a variable resister. No physical connection to the throttle body.
Or did i read your post wrong?
 

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@rbarrios : yeah - that’s why I threw in the disclaimer “you can test it out”. Wasn’t sure - even about my own 2011. Guess you have to go back a few more years ... like with my 2002 Impala...
 

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.... Guess you have to go back a few more years ... like with my 2002 Impala...
It's unlikely the pedal will move along with cruise control on any true 'fly-by-wire' throttle control system.
 

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It's unlikely the pedal will move along with cruise control on any true 'fly-by-wire' throttle control system.
Pretty certain I’ve felt the pedal move with Cruise Control engaged on both my 1997 Taurus and 2002 Impala ... but those both use cables.

On the electronically-controlled Throttle ... yeah ... that’s a one-way street.

OP should focus on a vacuum leak.
 

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Below is a summary of what I've done since I first experienced the rpm fluctuation and shudder around 1900 rpm (~ 45-50 mph) under light load Problem started about 140k miles and only happens at that specific condition when the vehicle is at operating temperature:

1. Dealer had overfilled the transmission fluid after a tire rotation and oil change. I brought the fluid back down to proper level. Seemed to have minimized the problem but was still apparent on occasion.
2. Received a fault code re: the APPS. Replaced with new genuine AC Delco pedal. Acceleration was more smooth & responsive. Appeared to have solved the problem or at a minimum, reduced it.
3. At 100k was when I performed 2x drain & refill on the transmission fluid for the first time. I performed another drain & refill at 144k and have one more to do. Seemed to have nearly eliminated it.


Some comments based upon my experience and the posts I've read here:

People have had success using ShudderFix & for me, this problem does not present when the vehicle has not reached operating temperature or vehicle in in tow mode. This points me to the torque converter and/or TECM becoming more sensitive to viscosity due to age, wear, etc. Some people have had success with replacing the torque converter while others have not. This may be dependent on how much transmission fluid the shop flushed/replaced when that work was performed. Your ability to duplicate the problem with the vehicle in Park under no load was interesting.

My vehicle has almost 146k miles on it and coming on 10 years with a new driver in the house. If I experience the rpm fluctuation or shudder, I either accelerate a bit or take foot off the gas pedal to make it go away. I consider it minor and it hasn't gotten worse. It seems to have improved over time and with the things I've done. I won't drop the ~$2-3k to replace the torque converter or ~5k to replace the transmission at this time and will just continue to drive it.
 
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Discussion Starter · #20 ·
Thanks for the comments guys. The low load characteristics prompted me to check vacuum, it’s normal, and I also replaced the coils since I think they may have been underperforming under low load if they were worn or faulty.

I did check the APPS with my OBD module, it’s only a basic one, but seemed to show normal limits and smooth operation.

I’ll look into changing the transmission fluid. I have already put a bottle of BG 44 fuel system cleaner through, in case there was a build up in the fuel system; made no difference.

Frankly if I can find a cheap APPS I’ll replace it just to eliminate it from possible causes.
 
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