TDI AEL: Rough Idle - Fix & Troubleshooting

For almost 3 years now, I've been driving my A6 TDI Avant, with varying degrees of success. Currently, it's giving me trouble in two places:

- The engine is running unevenly at idle. It doesn't happen all the time, but it's often noticeable and annoying. Especially at traffic lights, it might seem like I'm playing with the accelerator pedal. The frequency of the 'stuttering' is usually the same, but not always; sometimes it happens very quickly. Usually, the change is enough to be visible on the tachometer. More often than not, the engine calms down after a maximum of two or three minutes, but it can be easily triggered again by lightly touching the accelerator pedal.

It's very important: if you press the accelerator pedal slightly so that the needle on the tachometer shows a little higher (otherwise it's just below the imagined 9), and it's exactly at 9, the problem goes away. Doing this at every traffic light is extremely annoying, but I think that constantly revving the engine will definitely damage some parts in the long run.

What can I, as a technically proficient hobbyist mechanic who has unfortunately experienced a lot of problems with my vehicle in the last six months, check or potentially fix myself?

The fuel injection pump is electronically controlled, which means it has an electronic throttle. So far, everyone I've talked to says there's no way to adjust the idle speed, and that the adjustment mechanism inside or on the fuel injection pump is inaccessible or not repairable.

What I need is a cost-effective repair. A new fuel injection pump won't help. Perhaps it's possible to adjust the electronic throttle to a higher RPM by adding an aftermarket trim potentiometer?

- I've been having a completely different problem since the weekend: who hasn't heard that noise? You park your car in the garage after driving on salted, wet winter roads in the evening, and the next morning, when you start it, you hear a loud noise from the rear axle, probably because the brake pads are coming off the coated brake disc.

'I'm now sometimes hearing the exact same noise while driving, sometimes it's gone, and other times it's very noticeable. When my girlfriend was driving and we were walking alongside a slow-moving car, I could definitely tell that the noise was coming from the rear wheels. However, during a visit to the workshop, no one could find anything wrong. It seems like my A6 is about to take another chunk out of my wallet – help!'

I would appreciate any good tips, of course, including workarounds that might not be ideal for a professional workshop. This car now has 315,000 km on it, so we don't need to make it like a brand new car .

Tuppie.

Hello Tuppie,

Regarding the idle speed: It can be adjusted using a diagnostic tool (e.g., VAG-COM). The throttle body or fuel injection pump are not involved in this process; the idle speed is controlled electronically.

It's relatively inexpensive to build a diagnostic adapter yourself, or to buy one (e.g., here in Rainer's shop). The appropriate software is also available (in my opinion, even the free shareware version is sufficient).

Alternatively, someone in this forum who lives near you might be able to help. Many users here have the necessary equipment.

Regards,
Private.

Hello Private Area,

Thank you for your reply. I initially thought the same thing, that an electronic system with a control unit would surely have something like an idle speed setting. However, unfortunately, every VAG workshop I've contacted has claimed that such a setting doesn't exist and that it's fixed. The question now is: is that true, or is it just a claim to try to sell me a new fuel pump?

Based on my idea using a potentiometer to simulate a slightly pressed gas pedal, effectively raising the idle speed. If there's another way to achieve this, I'm open to it. Are you sure that this possibility exists for the A6 TDI (C4) with the AEL engine?

You can actually forget about the second problem; it's a wheel bearing. That's bad enough .

Tuppie.

What engine code are we talking about?

As 'privatbereich' already mentioned, it's possible to change this through adaptations using VCDS (VAG-COM). The question is, why don't more VAG enthusiasts know about this? We've discussed and speculated about this many times. Often, they simply aren't aware of it!

To adjust the idle speed, please use the forum search function; you should find something there.

Unfortunately, this is just a temporary fix. The root cause lies elsewhere. However, to my knowledge, MichaelII has already provided tips on that as well. I also used the search function for that.

Important: Please include the MKB (German business registration number) when posting. It makes it easier to provide feedback!

Sure, here is the translation of the text from German to English:

'edit: oops, those who can read have an advantage! the answer was in the title!!!! sorry!'

Based on the suggestion, my idea is to use a potentiometer to essentially create a constantly slightly pressed gas pedal - which would be like a slightly elevated idle speed.
A constantly slightly depressed gas pedal is something completely different from a slightly elevated idle speed!

Bei deiner Lösung würde der Motor immer eine Menge X Diesel einspritzen. Fazit:
- The idle speed is dependent on temperature and load.
- the engine revs down very slowly.
- no more abrupt shutdowns.

Overall, it's not very useful and it even encourages consumption.

Are you sure that this possibility exists for the A6 TDI (C4) with the AEL engine?
It's not 100%, but it's a high percentage. But it's electronic. With VAG-COM, this is done through adaptation. A regular workshop probably doesn't even have the right equipment. If the VAG technician isn't technically proficient, they'll just look in their program (ElsaWin) under 'idle quantity' and see only the mechanical adjustment option for the older engines (which I have personally verified ). The adjustment options for the electronic engine control are not listed there (or I can't find them). However, the adjustment options are also not listed for vehicles where I am sure that an adjustment is possible. (If someone could explain to me how to find the adaptation channels in Elsa --> please send me a private message).

Therefore, I strongly believe that the same applies to you, although the workshops you mentioned might have been somewhat... lacking.

Regards,
Private.

Hi,

Here's a quick note about the wheel bearing:
Specifically, the C4 can exhibit very unusual tire wear due to its rear-wheel drive configuration, which manifests as noises that are strikingly similar to a faulty wheel bearing.
I once replaced a bearing because I thought it was noisy, but I then realized that the driving noise hadn't changed at all.
A test using the spare tire then provided clarity...

Regards,
Olli.

@joergs, private area.

Of course, I understand that this method is flawed and is more like a temporary fix. The failure of the automatic shut-off feature would particularly bother me. On the other hand, it's currently driving me crazy to have to put up with the noise of the saw.

Somewhat off-topic: here in Munich, it's considered stylish to make the city impassable by placing dozens of red traffic cones. Driving a car, which means stopping at a traffic light 15-20 times on the way into the city center, quickly tests my patience .

Now, let's consider this: so far, I've consulted with 4 VAG workshops and one independent mechanic, who I believe is trustworthy and wouldn't try to sell me unnecessary parts. Each of them says that it's not possible with that model and engine. Here's a reminder of what the issue is:

Audi A6 Avant 2.5 TDI 103 kW, engine code AEL.

You say it works with VAGcom, but I'm having a problem with that. My laptop only has USB, no serial port, which increases the cost of trying it myself. Also, most COM-to-USB converters are quite quirky when used for specialized applications. So, is the only option to buy the expensive USB model? Well, no offense, but 'I'm not sure, but I think so' is a bit shaky. I've already invested enough money, and every time something unexpected comes up.

Therefore, where in Munich and the surrounding area can I find someone who can perform this adjustment - of course, for a fee?

Besides a potentially faulty control valve in the fuel injection pump, does anyone have any other ideas, perhaps something a bit more radical? I've already temporarily disconnected the NHG (which I've read is prone to causing problems), but other than the engine idling a bit rough, nothing major happened.

The fuel injection pump itself has three electrical components: one at the bottom with the Bosch logo, one at the top with a single-pole screw terminal, and the large central connector. A silly question: isn't one of those two related to idle speed?

I've already checked the air flow or mass sensor for any play in the valve, but I haven't tested yet whether the potentiometer at the top is sticking. Should I perhaps try holding the valve in a certain position to see if the sawing sound stops? Or am I completely off track?

@olli

Yesterday, I had the tires rotated from front to back as part of a routine service, but the noise persisted. So, it's not related to the tire rotation, or am I understanding you correctly?

It was also checked to see if the exhaust pipe was hitting anything, but there was no evidence of that. When fiddling with the left rear wheel while the car was raised, a similar cracking sound could be heard. Of course, the possibility of a problem with the axle suspension cannot be completely ruled out with that kind of movement...

Tuppie.

I can only offer one more thing to help you solve your problem:
A USB to RS232 adapter with an FTDI chipset.

At €12.50, it's not exactly expensive... However, it's probably only suitable for Windows XP through roundabout means, but that shouldn't be a problem.

I would gladly lend you my adapter, but as someone from Schleswig-Holstein, Munich is simply too far away for favors .

Regards,
Private.

PS: Are you sure that no acquaintance has an old laptop with an RS232 port ?

Hi,

It seems like it's really the wheel bearing. (But it's not actually that difficult to replace.)
I haven't experienced the rear axle making continuous running noises from the suspension yet (but you never stop learning...).
What does the sound like when you manually rotate the wheel (while it's on a jack)?
Are the brakes functioning properly? It's also apparently a common issue with the C4 that the rear brake calipers seize.
What does the whole braking process sound like?
If the rear brake pads are already heavily worn, you'll continuously hear a noise that sounds like you drove over a salted highway the day before.

Regards,
Olli.

Mittlerweile habe ich unter Fehlersuche & Anleitungen die PDF mit dem zerlegten Mengenstellwerk der VP37 gesehen, und das Bild entspricht genau meiner EP. Das Innenleben habe ich teils so erwartet (insbesondere den Geber) manches hat mich auch überrascht, daß der ganze elektrische Kram im Diesel schwimmt. HHuh?

In another article, I read about a Liqui Moly diesel injector cleaner, but I set it aside because I thought it wouldn't be suitable for my problem, as I assumed it wouldn't clean the path of the sensor (typically, one imagines it being located in a dry part of the pump). However, with this new understanding, the cleaner would actually reach exactly where I need it to – the quantity control valve.

Perhaps this will already help, but if not, I can take it apart and clean it manually; I'm quite experienced with fine mechanics and electronics. I definitely prefer this approach to the trim pot idea.

Regarding the USB converter, thank you for the tips. I will try that approach at a later time if this doesn't work. Increasing the idle speed just to prevent the sensor from rubbing against the dirty/worn area is more of a temporary fix; I'm more interested in a more permanent solution.

How can I best drain the LiquiMoly from the container without making a mess (i.e., without diesel spraying out of another hose)? Right now, the tank is full, so it would probably distribute poorly. I heard that in extreme cases, it should be drained directly without mixing it with anything...

Once again, thank you for all your tips - and of course, to the authors of the other posts that I came across.

Tuppie.

How can I best have the LiquiMoly sucked out of the container without making a mess (i.e., without diesel squirting out of another hose)? Currently, the tank is full, so it would probably distribute poorly - I heard that in difficult cases, it should be sucked out directly...


Beide Schläuche der Einspritzpumpe am Dieselfilter abnehmen und in den LiquiMoly-Behälter stecken. Motor anwerfen. FSure!

Since there's no fuel pump inside the diesel tank, nothing comes out of the hoses.

By the way, I wouldn't let the container be completely emptied. I think air isn't exactly good for the fuel injection pump.

Regards,
Private.

@olli

Perhaps this will provide some insight. I haven't heard the cracking noise when reversing. I've tried applying the brakes manually, and it seemed like the cracking disappeared during braking, but I'm not sure. The cracking noise might not appear for 50-100 kilometers and then reappear.

@private area

Here's a brief update: I tried using a diesel cleaning agent, but so far, it doesn't seem to be working. The engine sometimes makes a sawing noise, sometimes lightly, but often quite noticeably. Interestingly, this noise disappears immediately if I increase the engine speed even slightly by pressing the accelerator. Also, the previous engine speed is retained when I press the accelerator pedal firmly and then turn off the engine; it's then restored upon restarting. So, my method of adjusting the throttle might work if there's actually a potentiometer in the electronic accelerator pedal.

Arrrgh... now it's cost me another day and two bottles of additive, and besides a worn-out starter motor due to excessive revving caused by unmetered air, I haven't really solved much. Sch...! So, if anyone has an idea about whether there's a list of workshops that have a VAGcom, that would be great. I really can't afford to put more money and time into it myself, so I'd rather use the workaround with the potentiometer; the main thing is that it runs, it's not a new car .

Perhaps another tip: Yesterday, I unscrewed the mass airflow sensor (MAF) from the air filter housing and could see inside while the engine was running, which really shocked me. I expected the valve to be open to a value of +/- a few millimeters at idle. But that's not what I saw! The valve was moving quite a bit. It seemed to be moving a relatively long distance, perhaps in a ratio of 3:2 or 4:1, and then it would open with a whistling sound, about a centimeter or more, relatively rhythmically, but not in sync with the idle engine speed, maybe double the cycle time or more. The thought that a potentiometer slider inside the MAF sensor is doing all this constantly makes me instinctively think that it can't be right; it won't last long like that. It's already 100 cycles during just one or two traffic light phases! Has anyone of you looked inside to see if this is normal, or should the valve be mostly still?

For a general assessment: I drive it quite moderately, mainly focusing on fuel consumption, and it usually averages between 5.2 and 6.7 liters per 100 km, depending on whether I had to drive into the city or not. Since the last timing belt change at a so-called workshop, the engine has been starting extremely poorly without preheating. Another workshop fixed this by adjusting the injection timing, and since then, it pulls like crazy. I had almost forgotten that 140 horsepower actually means power - but under full load or during load changes, there are sometimes black smoke plumes. The real 'afterburner' feeling starts around just above 2000 rpm. Normally, it takes about 1-2 seconds for the full power to be available, which I don't find unpleasant, but I mention it because in the forum's used car review, it's stated that the power should start below 2000 rpm and be fully available within half a second. Can we conclude from this that there is an acute problem, aside from the mileage?

Looking forward to your helpful suggestions, with anticipation for some crafting! *ggg*

Tuppie.

Hi,
I thought the noise might be a typical wheel bearing sound, something like a light grinding or humming.
If it's a cracking sound, I would be more inclined to suspect the wheel suspension or the shock absorbers.
Just try removing the rear wheel and turning it by hand.
If it's a wheel bearing issue, you should be able to hear it. While you're at it, check the bearing clearance. Too much clearance is bad, but too little is even worse.

Olli.

@olli

The noise is indeed a more or less rhythmic cracking sound. It's a very hard sound. Last week, a master mechanic came along and put the vehicle on a lift and fiddled with the wheels, and the cracking sound occurred in a similar way then as well. As I said, I wouldn't completely rule out the shock absorbers/suspension, but I'd rather not invest money again based on just assumptions.

@all

'Yesterday, I used the diagnostic tool (DZT) to test the general condition of the engine. It showed 130 horsepower instead of 140, and the measurements seemed quite accurate. I suspect this is still within the normal range given the mileage, but should I be concerned about it and mention it during the next service?'

I'm increasingly seeing the root cause of the idling problem in the way the last timing belt replacement was performed. During that replacement, the vibration damper was also changed because it was very brittle. After that, the engine started very poorly, but that has since been repaired elsewhere. Immediately after the repair, the tensioner for the serpentine belt failed, and there was a visible crack and signs of damage on the bracket, which further erodes my confidence in the repair. Therefore, my questions are:

- Can incorrect installation cause idling problems?
- Is the constant oscillation (1-2 cm) of the LMM valve during idle normal?

And because I'm now seriously considering buying the VAGcom package, here's an important question:

- Can anyone reliably tell me whether the idle speed on an AEL engine can be adjusted directly or indirectly?

'So far, every workshop I've spoken to has told me that it's pre-programmed and that no adjustments are possible. Is that simply wrong, or can the original VAG device do less than the VAGcom? What's definitely the issue?'

Looking forward to your suggestions.

Tuppie.

Tuppie wrote:

@olli Perhaps another tip: Yesterday, I unscrewed the mass airflow sensor (MAF) from the air filter housing and could see inside while the engine was running, which really shocked me. I expected the valve to be open to a value of +/- a few millimeters at idle. But that's not what I saw! The valve was moving quite a bit. It seemed to be moving a relatively long distance, perhaps in a ratio of 3:2 or 4:1, and then it would open with a whistling sound, about a centimeter or more, relatively rhythmically, but not in sync with the idle engine speed, maybe double the cycle time or more. The thought that a potentiometer slider inside the MAF sensor is doing all this constantly makes me instinctively think that it can't be right; it won't last long like that. It's already 100 cycles during just one or two traffic light phases! Has anyone of you looked inside to see if this is normal, or should the valve be mostly still?

Hi,
I checked it out. It's also changing on my end. Also, it's not guaranteed that there's a potentiometer inside. The output signal is definitely a voltage, not a resistance. The voltage is stable on my end, as far as I can tell with an analog multimeter.

Hello everyone,

Current status in a brief summary:

- The hard 'tack-tack' noise coming from the rear axle: I removed the wheel, and both bearings definitely have some play. I'll disassemble it tomorrow, clean it, and grease it so it lasts a little longer. But that wasn't the problem. A quick look inside the brake caliper, and I knew what it was: a brake pad had come loose from its carrier, and depending on its position, it would either rub against the upper stop when the brakes were released, or fall down to the lower stop. 'Tack-tack-tack.' Consequently, it never happened when reversing, at high speeds, or when braking.

If we had known that from the start... it's always easier to be smarter after the fact, and you would have known it right away. To be honest, I never would have guessed it.

- The sawing idle: After talking to x people who told me there were no adjustment options, I decided to save the 200 euros for the complete VAGcom package and instead use my own methods. I removed the accelerator pedal and devised two solutions: one involved an idle adjustment screw, and the other was the mentioned trimmer. It turned out that I chose the electronic method because I couldn't find a suitable screw. After numerous measurements, I discovered that the relatively coarse ADC (Analog-to-Digital Converter) in the control unit makes a potentiometer more or less redundant, as only one resistance value is relevant.

Between pin 1 and 2 of the electronic throttle sensor, an additional 39kΩ resistor was soldered in, and the problem is gone. The idle speed is slightly increased (800 instead of 750 rpm) due to this, but the torque cutoff still works perfectly, which means for me: mission accomplished.

I intentionally used this sentence from recent history to express sarcasm: while troubleshooting, a master technician pointed out my expensive, level-regulating shock absorbers, noting that they look very rusty and are partially cracked, but no hydraulic fluid is leaking out – not yet, anyway... which means the next maintenance cycle will be interesting.

- 2 leveling dampers (1000 euros, no alternative accessories).
- Rear brake pads (as discussed before, gluing them probably won't work).
- Bearing grease, shim, and bearing cover (negligible, as it's just a precaution).
- Front tie rod ends (completely forgot, the one-sided tire killer, but the workshop can no longer remove or adjust the cobbled-together parts).

Then the time spent in the hobby workshop and getting the wheel alignment adjusted at ATU... all together, it's almost 1500 euros.

It's not that I like to brag, but as soon as one problem is solved, another one pops up asking for money. Where is all this money supposed to come from??? I'm starting to get fed up. The prices for VAG-ET parts are outrageous. It just can't be right that...

- LMM: over 500 euros.
- E-scooter regulations (just for informational purposes), around 400 euros.
- Leveling dampers: 500 euros per side.

'Come on, as a technician, you have to work on something like this for over a month! You have to consider the cost in German Marks! Somewhere, the whole situation gives the impression that customers are only there to boost the struggling economy, essentially to act as sponsors. My problems, however, are not being resolved. Does anyone see the absurdity of buying a ticking time bomb with 315,000 km on the clock, for half its resale value?'

A daring idea has come to mind: measure the wall thickness of the pipe and insert it from below into an inner pipe of the same thickness, fix it in place with some adhesive, and paint it nicely so that it doesn't rust; it should hold perfectly. In other words, armor and seal the damper in case it ever leaks. Currently, there is no apparent problem, except that it looks disastrous. Therefore, I don't quite understand the expense; it's like going to the dentist without a toothache. The local scrap dealer also said that I could install normal dampers and simply close the oil line, but VAG, as always, says it's not possible, it's not allowed, and they don't want to.

What's the matter, or where might I be able to find two such shock absorbers cheaply, if I shouldn't trust the armor? I also don't know where to get the pipe from.

Going from bad to worse.

Tuppie.