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M271 loss of power/unresponsive pedal, P0121

31K views 42 replies 6 participants last post by  roadstar zimbabwe 
#1 · (Edited)
OK, here we go again.
In continuation to this thread, http://www.benzworld.org/forums/c209-a209-clk-class/2652410-bad-throttle-pedal.html, I have similar symptoms again, but they appear intermittently. Let me describe exactly what I get, so you have a more clear picture. I know it's gonna be long, but I want to present the full story.

One morning, like a month ago, that I first started the car I noticed that the RPM would not go to 1100-1200 as every morning, but rather dropped to 600-800 immediately. I did not pay much attention, but when I pulled over I noticed that the engine had some hesitation to accelerate, though the pedal seemed to be responsive, i.e. pressing more on the pedal seemed to give the command to accelerate, but the engine was somehow hesitant to do so, as if I was using both the brake and the throttle pedal at the same time. I immediately remembered that the day before I made a long trip with the car and on the return leg I had refuelled. I then suspected that the gas I put in was bad. After switching off the engine and back on and going for a short trip on the freeway while running the engine at 3000-4000 between times, the problem seemed to have gone away and everything was normal again. The issue never reappeared, until 12 days ago (Thursday, March 9). On that day, the car was fine from morning till night, until I went over a few speed bumps. Seconds later, the engine begun again not revving up fast enough according to the throttle pedal inputs, but it would still go up to high RPM if I selected a specific gear. I mean, if I selected, say the 3rd gear, the engine would rev up to high RPM, but it did slowly, as if the throttle pedal input was very minor, though I was almost pressing it fully. I found it strange that this happened as soon as I went over a speed bump. I thought that I was unlucky again with the gas, as I had refuelled 5-6 days before, and I thought that the speed bumps somehow caused the gas in the tank to move and mix, possibly allowing some bad gas leftovers to pass through. I pulled over, switched off and on, everything was fine. Next morning, the same thing happened with the low RPM in the first start, and after switching off and on the engine, all back to normal. :confused:

On Sunday (12 March), the car started fine, but guess what, as soon as I went over a speed bump, the same hesitation appeared again... Switch off, back on, everything OK.
Please note here that I had no other issues, i.e. no CEL, shifting was smooth, no excessive fuel consumption, no rough idle, and when I say no rough idle, I mean the engine is so smooth you can hardly tell if it's running or not, especially when at operating temperature, and regardless of P,R,N,D. I was really puzzled.

Another full week passed without any issues whatsoever, until - guess what - I went over another speed bump!!! :eek Note that in all this interval I hadn't refuelled. This time, I felt both the acceleration hesitation, and also unresponsiveness from the throttle pedal. I pulled over and put the car in neutral. Pressing the pedal was very unresponsive, it would rev up the engine very very slowly and 5-6 seconds after I had really put my foot on the pedal. These are exactly the same symptoms I had when the previous throttle pedal was going bad. I decided to hook up my $20 dollar OBD reader, which showed the code P0121 - Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance Problem. Hmmm, something is suspicious here... After restarting the engine, yes, everything was back to normal... I still had no CEL. This day was Friday, 17 March.
Friday evening when returning home from work, I noticed the same unresponsiveness from the throttle pedal.

On Saturday, the car started fine. I decided to put some gas in, though the tank was still half way full. On the return leg, the unresponsiveness came back in a rather severe way. The engine was hardly responding to the pedal's input commands, making the car almost impossible to drive. I drove home carefully, and when monitoring my fuel consumption indicator, fuel consumption still looked normal to me, despite pressing the pedal almost full way in order to get the engine to rev up.
On Sunday it was the same story again, with low responsiveness/acceleration, switch off and back on all back to normal.
Yesterday, the car behaved completely fine.

From all the above, I concluded that it was either the throttle pedal going bad again, or I got really unlucky with bad gas, or a combination of the two, which is actually a rather rare coincidence, I would say.
The new throttle pedal that was installed in the car was an aftermarket HELLA (if I remember correctly), though I insisted to my mechanic to install an OEM one.

I called my mechanic and argued that the throttle pedal he installed was faulty, and he told me to bring the car in. I took it today morning ( the car was running fine by the way), and he confirmed with his diagnostic the P0121 code. His diagnostic said "Accelerator pedal sensor", but there was also another message regarding the M16/6 module (throttle valve actuator) and some bad value on its potentiometer ( or something like that, I don't fully remember the message). These were the only codes that were stored. The mechanic tried to convince me that it's the throttle body at fault and not the throttle pedal.
I asked if there would be a different code if it was the throttle body itself and he said that all these are connected and one gets the same code for both components. I found it hard to believe, and I suspected that the message on the M16/6 module was due to the pedal not sending correct inputs. Anyway, I decided not to do anything until I get a second opinion. On the way to work after a short stop for groceries I got the unresponsiveness again... :banghead:

At some point I worried that there might be something wrong with the fact that I replaced my spark plugs around 3 weeks before the first symptoms appeared, but in that case wouldn't I get misfires and/or other related issues? I mean, the engine runs silk smooth even when the issue is present...

Finally, note that I experienced this acceleration hesitation twice, like a year ago, with around 2-3 months interval time and it was again right after refuelling, both times. The first time, I took the car in, there were no codes stored and the mechanic cleaned the throttle body as preventive maintenance and claimed that it was bad fuel. The second time I didn't pay much attention because I was sure it was yet another bad refill. These two times the issue happened only once, with no recurrence between them.
I hadn't experienced this hesitation again until now.

I apologise for the long post.
I'd be very glad for any input related to this issue.

Chris
 
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#2 ·
Getting the other specific codes for the throttle would be helpful. According to SDS, P0121 (DTC 2002-16) is definitely a problem with the accelerator pedal and nothing else. Since this seems to happen when you hit a speed bump or stop and start the car, I suspect it may be a wiring problem. SDS does have specific steps to diagnose problems with the throttle.
 
#3 ·
Hi Rudeney,

Thanks for the quick reply, as always, and thanks for paying attention to my huge post :)
A faulty wiring, a bad connection or an indeed bad throttle pedal would have also been my guesses if I fully eliminated the bad fuel scenario. I'm not putting my hand on fire, but the other code on the diagnostic I think was P0075. But this one usually refers to the cam shaft solenoids, or?

Tomorrow I'm taking the car to another shop I used to go in the past, this guy is more reputable and has the original SDS Diagnostic, I just didn't take it there so far because he usually charges for even hooking up the SDS...

I'll keep you posted on how it goes.
Chris
 
#4 ·
P0075 would be a problem with the recirculating air flap for the supercharger (M16/7). This can cause the engine to be very sluggish if you are getting no compression from the supercharger when expected. Regardless, let us know what the shop tells you!
 
#5 ·
Aaah, maybe it's M16/7 that was displayed on the mechanic's diagnostic tool and not M16/6 then... Unfortunately I don't fully remember.
But, if this is the case, then it might as well be the recirculating air flap's fault.. On the other hand, why did the P0121 code appear, then?
Is there a diagram in which I can see which part the M16/7 is?

My worry is that maybe the guy yesterday erased the fault codes, and unless they are stored again, there won't be much to see today...
Anyway, the car is already at the garage. Let's see how it goes.
 
#6 ·
Chris - you have engine type 271.955 in 209.341

You must recall a recent thread that you contributed to here: http://www.benzworld.org/forums/c209-a209-clk-class/2774666-m272-e30-clk280-hesitation-issue.html

The OP eventually cleaned his throttle valve actuator (M16/6) and his 'hesitation' problem was solved. It is easy to access, so why did you not try that idea?

Of course it may be that your recirculating air flap actuation motor M16/7 has a fault. It provides outputs from 4 potentiometers (M16/7 r1 to r4) which could be bad.

This WIS drawing is your M16/7. Unfortunately to remove this the compressor has to come off and that entails quite a lot of additional work.
 

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#7 ·
Hi keyhole, thanks for the input.
According to my information based on my VIN, my engine is 271.940 and the model is 209.342, though. Are there significant differences between these two?
M16/7 looks a lot like M16/6 to me, actually. The throttle valve actuator (M16/6) is what we usually call the "throttle body", right? I have cleaned this last year, in fact, when I first experienced these hesitation issues. Could it be that it's clogged again?
And if M16/7 is so hard to access, I can't imagine how much grime is collected in there if nobody ever cleaned it...
 
#8 ·
M16/6 is the throttle body, and P0121 points to that as a problem. As Keyhole stated, it may just need cleaning.

M16/7 is the recirculated air flap that is part of the supercharger system. P0075 would point to that as a problem.

Let's get actual codes and go from there.
 
#10 ·
Keyhole and Rudeney, thanks very much for your valuable input.

The shop called me yesterday and said that they found no fault codes stored and he also took the car for a test drive and he could not reproduce the problem.
He also suggested to clean the throttle body as a first step, and I told him to go ahead. When I went to get the car he mentioned that it was not particularly dirty but he went ahead and cleaned it. On the way home from the garage the car was perfectly fine.
Today on the way to work it was still just fine (fingers crossed).
Let's hope that cleaning the throttle body did the job again.

I just don't understand why the car had those symptoms in the way that they appeared..
 
#13 ·
Unfortunately the problem of performance loss came back on Saturday.. My fuel level was becoming low by the way, so I stopped and refuelled. After that everything seemed OK again since then.
Of course I don't know if the engine really performs at 100% of its capabilities, but at least it doesn't feel as sluggish as when the problem is present.
I tend to believe that it's not something electrical, nor mechanical that's going wrong.

Guys, is there any chance my fuel filter is going bad? How much is the lifetime of a fuel filter? I've never been replaced it AFAIR, and I own the car for about 90,000Km.
 
#14 ·
My Service Book states: Fuel filter replaced (112 & 271 engines) - every 78k miles/ 120k km, or every 4 years.

However, although I do not meet the mileage rules yet , my 2004 CLK is still using the factory-fitted filter.

Incidentally, your fuel filter could be either inside the tank or under the car - according to your chassis number. See this post, which covers your 271 engine also.

http://www.benzworld.org/forums/c209-a209-clk-class/1594941-fuel-filter-location-clk-350-a.html

I doubt if the fuel filter is giving you this problem.
 
#15 ·
My fuel filter is under the car, I can see it when looking underneath. Well, the car has now 200,000Km, I got it at ~110,000Km and I doubt if the previous owner had ever replaced it until then.
I really don't know what more to think. I've been in contact with another two-three MB mechanics and none of them could give a single clue as to what's going on.
The fact that this comes and goes makes me believe that it's not something strictly mechanical. It also seems to be not so serious, otherwise I would get more intense symptoms, i.e. rough idle, stalling, excessive consumption, misfires, unusual noises, things like that.
I also have in mind the other two-three times this happened and it was really temporary (and right after refuelling, at all times).
From the symptoms and the instances they appear I end up with the conclusion that the engine is just not "fed" well at those times, in the sense that when the issue is present, one can feel the engine struggling to accelerate but something is holding it back. The electrical issues with the pedal, etc, might just be the aftereffects of the main issue, who knows..

I've also tried the TCU reset procedure, didn't seem to do much in general (if any).
For now the car feels OK.
I'll keep you posted with any updates.
 
#16 ·
Just a guess,but since it seems related to speed bumps and refuelling,maybe your first thought is correct.Contaminated fuel,maybe there is some water or something in your tank?
When you refuel or go over a bump it gets mixed up and causes misfiring.
Re the filter,it wouldn't hurt to replace it,it regulates fuel pressure too,so they cost about $80 but not too hard to diy.You will need some jubilee clamps to replace the factory clamps.
Good luck
 
#17 ·
Exactly. I mean, since I refuelled on Saturday the engine seems to be performing as expected again... It's just so confusing!
The fuel filter from the dealer costs around 110 Euros, and the Bosch from my local parts store around 60. It's not so expensive and I guess it's really about time it gets done actually..
If I decide to go DIY I'll probably get the one from the dealer.
Are there any other concerns I need to have in mind when performing the job myself?
 
#22 ·
Guys, some other ideas came to my mind, I've searched a bit online but I guess too many opinions don't really mean something, and I thought asking in the forum is more reliable than believing every other website ;-)

So, one thing that came to my mind is engine timing. Could an issue like this cause this problem or would it be more severe issues? Btw, my timing chain has been replaced around 25,000 Km ago.
Second thing is engine compression. Again, could this cause this intermittent performance losses? Or, would I experience other symptoms as well?
Just to keep you updated, since last Saturday the car has been running fine, apart from a single instance that the loss appeared and then it went away after restarting the engine.
 
#23 ·
The variable valve control system can compensate for a stretched timing chain. Performance is usually OK until the chain breaks. But I doubt that has anything to do with the problem because timing issues would either not be intermittent, or would throw specific codes. Same goes for engine compression issues - they are usually not intermittent.

Now, one thing that can be intermittent is a clogged exhaust system. When the catalytic converter dies, its internal material disintegrates and ends up in the resonator and muffler clogging it. it can be intermittent as the material melts and passes through. In this case, you usually get either really rotten exhaust odors, noises (like BB's in the exhaust) or O2 sensor errors. Although the catalytic converters are designed for about a 100K mile lifespan, it is not uncommon for them to last twice that long on MBZs.
 
#25 ·
Thanks, that basically confirms my existing knowledge and what I've found by searching online. Indeed, bad engine timing and/or loss of engine compression could cause performance loss, but they are serious problems and one should expect more severe situations.
So, my latest conclusion is that it has to do with something related to how the engine breaths and is fed with. I doubt that there's something wrong with my catalyst or exhaust, as I don't have any abnormal sounds/smells. In that case I would also expect O2 sensor-related codes. I've also checked my air filter (which is just 6 months old and got it from the dealer, by the way) and still looks like new, and there is nothing in the air filter housing that prevents air from travelling through. So I'm only left basically with bad fuel leftovers, bad fuel pump or bad fuel filter.. As the latter is not an expensive part and looks an easy DIY (thanks to Keyhole for the WIS!!) I might as well go ahead and replace it and see how it goes.

As always, I will post again for any updates.
 
#26 ·
Small update.
Car was sitting for two days (I use my bicycle to move around on sunny days!) and the issue appeared again today a few minutes after leaving home and while going up a small hill, then it seemed that it just went away on its own, while driving.
I'm starting to go crazy with this, thinking all sorts of scenarios.

Latest thing that came to my mind is that maybe something went wrong when I replaced my spark plugs?? I doubt that really this could be the culprit, because as I mentioned many times, this issue appeared twice before, while my older spark plugs were in place... But, I can't take off my mind the fact that I replaced my spark plugs a month or so before the issue begun appearing. I'm pretty confident that I have not cross threaded the spark plugs and I used my torque wrench to torque them to spec. I'm also quite sure nothing fell inside the plug holes... And, I got the spark plugs from the dealer.
But then again I read so many horror stories online about cross threaded spark plugs and the like, that the only thing that cause to me is fear for the worst! On the other hand, the fact that the issue is intermittent also kind of reassures me that it's not the spark plugs.
I would be grateful to hear any opinion on this.
 
#28 ·
I don't think it is related to spark plugs.
Watch this youtube video,not sure if it is the same as your problem,but worth a look..https://www.youtube.com/watch?v=ekqgzp1V_kE&t=445s
Thanks, Benzyle, very interesting. What the guy in the video describes the issue was like fits perfectly to the feeling I get from my engine. But, that air recirculation flap I think is what we usually call the throttle body, right? :confused:
I think that's what my mechanic pulled out and cleaned and actually programmed it using SDS so that the flap is in the correct position... I've read online that it's not a good idea to move the flap by hand as it confuses the sensor and it needs programming again. But the worst that can happen I guess is for the engine not to perform well until the flap position gets reprogrammed..
The guy in the video looked very confident and he said it fixed his problem. Maybe I just try the procedure with the key in the ignition as a first step without moving the flap, it could just reset the flap position. If I become even more desperate, maybe I go ahead and try what the video suggests.
 
#31 ·
OK, so the recirculation flap is really what we see easily from the engine bay, and it's what the guy in the video shows. The throttle body is I guess harder to access, if it's after the supercharger. Again, I believed that what my mechanic cleaned and programmed was what I thought was the throttle body which is in fact the recirculation air flap. So, if he cleaned/programmed the actual throttle body, then I guess the recirculation air flap was not affected. Actually, can one really clean the throttle body in around 1hr? This is how much it took him to do the job.

If it's really the recirculation flap at fault, it might start to make sense, as it is something that involves electronics which when failing they do this in random times, it is related to the supercharger which when not operating fine it would cause the engine to be sluggish and it also must be related to the throttle pedal which also might act up when things do not work as intended.

When I find some time I will try the fix suggested in the video.
 
#33 ·
OK, I took the time to try the fix suggested in the video in post #27 from Benzyle. The air recirculation flap looked quite clean, which suggests that it's what indeed my mechanic has cleaned last time, and not the actual throttle body. I attach two pictures.
Actually there was a drop or two of oil in the cavity. Is this normal? When moving the flap with my finger it didn't feel stuck or something, but it did required some force to actually move it. It seemed to be spring loaded and if I moved, it would return to the closed position on its own.
Anyway, after trying the fix and taking the car for a test drive it felt like a rocket!! But only for a few minutes, and then it became sluggish again. This lasted for next 2-3 days, but the last 2-3 days the car drove normal again... It's almost obvious to me now that it's this flap at fault, most probably the electronics are just giving up. But I don't get a CEL.
I'm taking the car to another MB specialists shop next week to get their opinion as well.
 

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#37 ·
The air recirculation flap looked quite clean, which suggests that it's what indeed my mechanic has cleaned last time, and not the actual throttle body.
Your photo is indeed M16/7. Throttle Valve Actuator (M16/6) has a similar operation and is also accessible, as shown in the WIS drawing below for engine 271 air intake manifold.

Unfortunately, it's not the visible part of the throttle body that is possibly causing this intermittent problem - the items would have to be opened up and CRC cleaner used to remove potentiometer wear debris (carbon dust) from affecting the output signals.

My WIS drawing for your engine lacks the explanation shown on the A140 TVA drawing below from my old WIS. Otherwise the devices operation are identical.

The drawing shows that the flap actuator motor is driven from the ECU, and then feedback signals are derived from 2 internal potentiometers showing the actual position of the flap. As the sliders move over the potentiometers after time debris can degrade the output voltages. After cleaning, operation should be improved.

Also I noticed a 'swirl flap motor' (M59). I wonder how that can affect engine performance..:confused:
 

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#34 ·
Well done Chris.
Normal for the last few days is good,right?
After further research I am sure that what you have in your pictures is actually the throttle body.It is after the intercooler isn't it.
I don't know exactly why,but the throttle body should not be soaked in or sprayed with solvent,or carburetor cleaners or silicone lubricants.
You may need to replace your throttle body if it does not work reliably.
To do that,disconnect the vacuum line,the electrical cable,undo four bolts,remove,remove the gasket and replace it with a new one,refit new TB ,torque to 9 Nm.
Good luck
 
#35 ·
Hi Benzyle,

I don't know if it's the throttle body, really, I'm also quite confused about the exact name! So this part is exactly under the intake pipe, it's the same part with the orange elastic ring as the one that the guy in the video shows. It seems to me that all mechanics refer to this part as "the throttle body".
If I understand correctly the route that the air follows, first it gets inside the air filter cavity and passes through the MAF sensor. Then it follows a pipe deeper inside which seems to come out and pass from the the harmonic balancer and into the intercooler (in the very front of the car, under the radiator) and then it comes up where it finally gets in this air recirculation flap and (I suppose) it gets compressed by the supercharger. Or is the compression taking place right after the MAF sensor? I'm not sure.

In any case, the part that I'm referring to is marked in red in the pictures of the M271 engine that I attach. It looks like a doable DIY for my skills, but wouldn't a new part require programming using SDS?
 

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#36 ·
Sorry I don't know if it would need programming,my guess would be not.Maybe just reset like in the video.
I think the air gets compressed after the MAF and is pressurised all the way through the intercooler and then through the TB.
As per the post from Keyhole,the recirculation flap is located on the supercharger.
Where I live used Throttle bodies are available on ebay for about $120.
 
#39 ·
Based on the note at the bottom of my last post, I attach another picture which shows where the modules M16/6 and M16/7 are located. This confirms that the TVA is indeed M16/6 and it's the part that we have been taking about for so long.

Now, something that keeps me back from replacing immediately the throttle valve actuator, is post #4 from Rudeney, who pointed out that the code P0075 initially stored on the car refers to M16/7 (not M16/6), whose "official" name is the "air recirculation flap actuator". According to this picture, this looks somewhat more involved to access.
 

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#41 ·
Why not clean both of them.
Well, of course this would be optimal, but I don't know how far I can get with my current DIY skills and confidence. It looks to me that M16/7 is quite deep into the intake manifold. If I manage to locate the part physically, I might be able to see how to get there. For now, I think I'll just focus on the M16/6 and monitor any improvements, then go deeper if necessary. If this thing's never been opened for 16 years and is really susceptible to collecting dirt and grime into the electric contacts, I can't imagine what's happening behind the plastic cover... :eek
 
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