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What is the reason for my throttle lag/engine hesitation?

  • TCM software problems

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  • ECM software problems

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  • Physical transmission issues

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  • Physical engine issues

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Discussion Starter #1
I have a 2015 GLK350 with about 21k miles on it. After I took it on a long 500-mile trip, and had to give it a 5-second WOT on the freeway at one point, from a standstill, for very quick (and noisy! - engine really revved up) emergency acceleration, now I experience throttle lag/engine hesitation when starting to move from a standstill or when trying to quickly accelerate from a slower speed. Also, the fuel efficiency has decreased some: on the trips where I was previously able to get 29-30-31 mpg (YES!), now it only goes up to 25 mpg; and on those where 26-27 mpg was the norm, now I can only get to about 21-22 mpg.

Having already read some of the forums on here, I've tried to follow the procedure to reset my TCM a couple of times. Each time, the reset does help "wake up" the throttle quite a bit, yet only at first; during the very first trip, it seems to slowly "dumb down" again.

Also, during idle, the engine seems to sound a bit rougher than usual; and during the initial acceleration from standstill, it now sounds a bit like diesel (this is a gasoline engine taking 91 gas.)

The vehicle is fairly new and is still under warranty, so I really can't imagine something going wrong in the engine or the transmission simply from that 5-second emergency acceleration on the freeway. Could this simply be a TCM or ECM software issue? Thank you for sharing your opinions and advice on the above.
 

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Discussion Starter #3
They say there's nothing wrong with the car, but there obviously is...

Ok, hooked up Actron's OBD II PocketScan, found pending code P0451. Cleared the code.

Took it to the dealer in the morning 3 days later for warranty diagnosis/repair. Did a test drive with them to demonstrate issues. Told them about the pending P0451 code that I cleared. Then they looked at the car during the day, and called me late in the afternoon to let me know there's "nothing wrong with the car" - said they couldn't reproduce the issues, that both engine and transmission were ok.

Picked it up. Drove it around some, and the issues are still there, and are starting to become more noticeable. For instance, when just starting from a standstill (e.g. at a traffic light), I have to depress the pedal much more than before, the response is delayed (by 1-2 seconds) and the engine appears to be working really hard (this is in D2) to get the card going and accelerating. Then shifts to D3, and it's easier from there on. Also, when driving on a freeway, and I have to accelerate quickly (e.g. when changing lanes), I depress the accelerator pedal; the car drops from D6 to D4, then "thinks about it" for 2 seconds, and only then increases RPM and begins to accelerate. Kinda dangerous on the freeway!!!

Please let me know if any further advice. Thanks.
 

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P0451 would not be the cause of this, so I do not know what else to say. I have seen so many where it was not possible to replicate the customers complaint, but as soon as it leaves the workshop it does it again.

I would talk to your SA/Manager and explain that this needs fixing as it is dangerous.
 

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Discussion Starter #5 (Edited)
Hello,

Thanks to everyone who've replied so far.

So, I took it again to the dealership, and provided them with the following detailed problem description: "The issues with throttle hesitation / loss of power / soft gas pedal / delayed throttle response continue, and appear to vary in degree, on and off. I've noticed that, when starting from standstill, removing my foot from the brake pedal sometimes barely causes the car to move, if at all. Then, gently depressing gas produces no response - some more pressure, no response - then giving it some more gas jerks the car and produces an exagerrated response. Also, just earlier today, I was driving on the freeway to work, and at some point, the gas pedal became very "soft" and had to be depressed halfway if not more to produce any response at all; as if the power to the engine has diminished so much that it couldn't propel the vehicle along. I've also noticed that the fuel economy displayed per-trip is oftentimes 4-5 MPG less than before, yet overall fuel economy is only 2-3 MPG less than before (still less though). At the same time, actual fuel consumption doesn't appear to have changed. This seems to indicate a disparity between what the car computer "thinks" the consumption is, and what it really is - hinting at the engine not getting enough fuel from the gas tank for any given position of the gas pedal. Finally, while right now there are no OBDII codes displaying, considering the P0451 code (evap pressure out of range) that I've read and cleared on the car a couple of weeks ago, this all seems to hint at the fuel line / fuel pump / fuel filter issues, as if the engine isn't getting enough fuel at times to produce the proper horsepower. Please warranty-diagnose in more detail and help determine the cause of these issues. They're really taking all the pleasure out of driving this car; and they were not happening at all prior to the original event (5-10 second wide-open-throttle on a freeway, emergency-accelerating from 10 to 65) after which these issues immediately began (as in, right after that event, the car's driveability changed abruptly.)".

They've taken it in for 5 days, and the shop foreman and his colleague have driven it for 26 miles and compared it to other GLK's they have in their possession. Then they've stated they could not find any problem, and have given the car back to me. They've also updated ECM software and reset initial adaptations, "as a goodwill gesture", at my request.

Still, the issue remains. It's still slower than before to respond to the throttle - in my opinion, anyway - and is consuming more gas than before, as well. Not sure where to turn with this now. Should I try another Mercedes Benz dealer, to see if they find something that this dealer has not? I'm a bit apprehensive that they'll again give it back to me declaring that there's "nothing wrong with it." To someone who hasn't driven it before, it would seem that way, in fact; while to me, its daily driver, it's obvious that the car behavior on the road has changed, and also exactly when it changed.

Any other advice? Please let me know!
 

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R.I.P Always Remembered
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Hello,

Thanks to everyone who've replied so far.

So, I took it again to the dealership, and provided them with the following detailed problem description: "The issues with throttle hesitation / loss of power / soft gas pedal / delayed throttle response continue, and appear to vary in degree, on and off. I've noticed that, when starting from standstill, removing my foot from the brake pedal sometimes barely causes the car to move, if at all. Then, gently depressing gas produces no response - some more pressure, no response - then giving it some more gas jerks the car and produces an exagerrated response. Also, just earlier today, I was driving on the freeway to work, and at some point, the gas pedal became very "soft" and had to be depressed halfway if not more to produce any response at all; as if the power to the engine has diminished so much that it couldn't propel the vehicle along. I've also noticed that the fuel economy displayed per-trip is oftentimes 4-5 MPG less than before, yet overall fuel economy is only 2-3 MPG less than before (still less though). At the same time, actual fuel consumption doesn't appear to have changed. This seems to indicate a disparity between what the car computer "thinks" the consumption is, and what it really is - hinting at the engine not getting enough fuel from the gas tank for any given position of the gas pedal. Finally, while right now there are no OBDII codes displaying, considering the P0451 code (evap pressure out of range) that I've read and cleared on the car a couple of weeks ago, this all seems to hint at the fuel line / fuel pump / fuel filter issues, as if the engine isn't getting enough fuel at times to produce the proper horsepower. Please warranty-diagnose in more detail and help determine the cause of these issues. They're really taking all the pleasure out of driving this car; and they were not happening at all prior to the original event (5-10 second wide-open-throttle on a freeway, emergency-accelerating from 10 to 65) after which these issues immediately began (as in, right after that event, the car's driveability changed abruptly.)".

They've taken it in for 5 days, and the shop foreman and his colleague have driven it for 26 miles and compared it to other GLK's they have in their possession. Then they've stated they could not find any problem, and have given the car back to me. They've also updated ECM software and reset initial adaptations, "as a goodwill gesture", at my request.

Still, the issue remains. It's still slower than before to respond to the throttle - in my opinion, anyway - and is consuming more gas than before, as well. Not sure where to turn with this now. Should I try another Mercedes Benz dealer, to see if they find something that this dealer has not? I'm a bit apprehensive that they'll again give it back to me declaring that there's "nothing wrong with it." To someone who hasn't driven it before, it would seem that way, in fact; while to me, its daily driver, it's obvious that the car behavior on the road has changed, and also exactly when it changed.

Any other advice? Please let me know!
This is an interesting one, if I were the Tech I would take it out to a fast road and beast it, then hit the city traffic with start stop situations and see if the computer learned my driving style, which it is supposed to do.

Just saying but maybe your driving style has caused this, given the computer is supposed to imitate your driving style, as I say just saying.
 

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Discussion Starter #7 (Edited)
Cound this be somehow related to an ECO feature malfunction?

Hi,

Something interesting I've noticed just now. I usually drive in E + ECO, and before this whole ordeal, ECO was working just fine. Now - and I saw a real example just a few minutes ago - when I was stopped at a red traffic light, the engine didn't shut off as it normally would. Pressed the ECO button once - ECO did ***NOT*** turn off; the green light on the button remained. Pressed it the second time - it did turn off. Pressed it again to turn on - and it turned on, and the engine shut off. Then, while in motion right after this, the car seemed to respond better; and when I tried to turn ECO off and on while in motion, sometimes it would turn off/turn on on a single press each time, and sometimes it would require two button presses to turn off. So it wasn't me just not pressing the button hard enough.

From this, I tend to conclude that, maybe, this whole thing has to do with some software issues; or, more specifically, with the ECO feature malfunctioning and 1) not shutting off the engine every time when I'm at a standstill and it's supposed to, and 2) also somehow throwing off certain engine parameters, resulting in this throttle delay I've been reporting earlier on.

Then, again, the fact that ECO doesn't always shut off engine now (and it always did before this whole thing began), *might* be not the ECO malfunction per se, but the ECO feature "malfunction" actually pointing at another kind of malfunction - like low vacuum, etc. - one of those several few conditions being present that would cause ECO to not shut off the engine on standstill.

Thoughts? Please let me know.
 
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