Engine Problems? Diagnose and Repair.

Hello,

'I've been driving an A4 2.5 TDI (110 kW) for several years, and until recently, I was very happy with it. However, sometime last year, the performance started to be lacking, just like it had been before. An inspection revealed (again) a faulty mass airflow sensor (MAF). The part was quickly replaced, and after that, it ran really well, at least on the highway. However, I still couldn't find the acceleration behavior below 2500 RPM to be satisfactory.'
In short, the engine presents a very contradictory picture: It has excellent top speed (220-245 km/h), even slightly better than when it was new, but absolutely no agility. To put it dramatically, it's like a high-speed train: You don't feel any acceleration, but you're constantly getting faster. The inertia is particularly noticeable when starting from a standstill, like at a traffic light.
I became particularly aware of this when I recently drove a new 100 horsepower PD Golf IV. The way the engine responded to the accelerator at the lowest RPM reminded me what makes a good TDI engine.
'Experienced drivers and mechanics say everything is normal. However, they are probably all used to driving gasoline cars, and since the engine really starts to pick up speed again above 2500 RPM, their reactions are understandable.'
Using VAGCOM, I checked all the measuring blocks and could only determine that everything is correct.
Okay, here's my question to the community: Has anyone here ever driven a V6 TDI engine until it completely failed, and noticed a similar power delivery around the halfway point (or perhaps towards the end?!?)? As a possible clue, here's a transmission symptom: When coasting in 6th gear at around 100 km/h, there's a whining noise.

Hello Thomas!

I have an A6 2.5TDi (110KW). It has now run 180,000km. Before that, I had a 115PS PD TDi in a Golf 4. The Audi feels much more powerful at low speeds compared to the Golf, but it gains speed more quickly. I think this is mainly due to the weight and the characteristics of the engine. If you compare the torque curve of the Golf and your 2.5 TDi, you will see that the Golf's torque rises very steeply early on, but then drops quite quickly. This probably contributes to the feeling of agility. The 2.5 TDi has a much more balanced torque curve, which probably also causes its weakness in the lower RPM range. However, it maintains its power for longer .
When switching from the Golf 4 to the Audi, I had to adjust my driving style in terms of acceleration and pulling power from the lower RPM range in a high gear. Where the Golf could still accelerate at 60-70 km/h in 6th gear, the 2.5 TDi doesn't offer anything that can be called acceleration .
I've also been wondering if something is wrong. I've been driving it for two years now, and so far I haven't been able to find any faults anywhere. I think it's probably just my imagination. Especially if you've ever driven a new PD TDI. The V6 has been on the market for 6 years, and it's still using the 'old' TDI technology, except for the 24 valves .
I also wrote a post a while ago about the so-called 'switch effect' when accelerating. Maybe you've experienced it too. It's most noticeable in 2nd and 3rd gear. When you accelerate from a standstill, there's a noticeable jolt around 2100-2200 RPM, and it feels like the car then accelerates much more strongly.
Until the bitter end, I have never driven a V6-TDi, and I sincerely hope that I never will.
'The transmission doesn't whine in my car. I also have the 6-speed transmission. After 180,000 km, everything has a bit more play now, but thankfully, there's no whining. Reverse occasionally hesitates, but all other gears shift smoothly and without any issues.'

Hi Garfield,

I had already thought that the comparison with the PD-Golf was a bit of a stretch, but I think the difference is simply too significant.
What you describe as the 'switch effect,' I've experienced before, and it's actually what I miss. It simply doesn't produce any power anywhere, almost like a modern naturally aspirated engine with a completely flattened torque curve.
The effect should ideally occur when the turbocharger reaches its full pressure at a certain RPM, allowing the electronics to release the maximum fuel injection amount. This usually results in a noticeable surge of power. When it was working correctly, I could easily overtake on the highway starting from 1800 RPM. Now, the procedure is to let the speed drop slightly, downshift, accelerate into the oncoming lane, and then pass at 3000 RPM. It's roughly the same as driving a 1.8-liter gasoline engine.
So, be happy with your wah effect!

After many days of troubleshooting with my AFN, one of the effects I experienced was this.

Cause: Delayed fuel injection. The injection timing was at its limit, barely reaching the target values, with a maximum of 12 degrees, which corresponds to the target value at approximately 3500 RPM. While the power output and maximum speed weren't drastically affected, ...

As I could see on the VAG-Comm diagram, the actual value of the injection start was lagging significantly behind the target value.
'Anders: When I accelerate from 0 to 3500 RPM, it reaches the target value, but with a 1-1.5 second delay, and the injection timing is at its maximum limit (96%).'
It took approximately 2 seconds to go from 0 to 100 for me.

The pump is adjusted - perfect.

Okay, so: Using VCDS (Vagcom), the adjuster should/is/will be logged.

If the fuel pump isn't delivering enough fuel at full throttle, and the actual performance is lagging behind, then adjust the fuel pump.

It's best to bribe a child (with money) to remove the timing belt cover and loosen the tensioner pulley. With adult fingers, you can't even reach that area (at least on a VW Golf A4 1.9), let alone tighten or loosen anything there.

Would anyone actually support my demand for corporal punishment for Audi engineers?

'Gear noise? It's quite normal with high mileage and can often last for a very long time. You might consider changing the oil; this will also show you if there's anything causing metal shavings or debris.'

To be more clear: As long as the compression and fuel injection are correct, the full power should be achievable. The challenge is figuring out *how* to achieve it.

Hello Garth,

I had already looked at the relevant measurement blocks. The difference between the target value and the actual value for the injection start was actually quite small. I need to check again to see if the delay you described is present.
What immediately caught my attention was that the target value is quite low: At 3500 RPM, I was only getting 10 degrees. I think the repair manual mentions something about a range of 9-15 degrees.
I think you've put me on the right track, though. A mechanic mentioned that the timing of the pump is slightly late from the factory setting, but still within the acceptable range. He suggested adjusting it to be 'slightly early.' Let's see if that makes a difference.

Thank you.

Hi Thomas,

TKN wrote:

... I particularly became aware of this behavior recently when I drove a new 100 horsepower PD Golf IV. The way the engine responded to the accelerator at the lowest RPMs reminded me what makes a good TDI. ...

"That's the worst possible comparison you could make among all the TDI engines."
The 74kW PD engine has the highest torque of all 4-cylinder TDIs up to approximately 1300 RPM. Furthermore, it has a torque plateau of 240Nm between 1600 and 2400 RPM (I believe it's electronically "limited" for the sake of the transmission). This makes driving with this engine a rather unusual experience. The 81kW and 85kW TDIs that I have test-driven so far don't come close to the impression my AXR made.
I therefore don't think your engine is "sluggish."


@GarfieldMZ

GarfieldMZ wrote:

The V6 has been on the market for 6 years now, and apart from the 24 valves, it still uses the "old" TDI technology

.

We had an article about that a few weeks ago. It showed that the "older" V6 TDI technology (VP44) actually performed better than common rail (CR). Even the new V6 TDI still relies on the VP44 system. It even achieves EU4 emissions standards without a particulate filter, something that CR has only been able to achieve so far with OPEL's "lean diesel" engines.

@GarfieldMZ
The V6 has been on the market for 6 years now, and apart from the 24 valves, it still uses the 'old' TDI technology .

We had an article about that a few weeks ago. It showed that the 'older' V6 TDI technology (VP44) actually performed better than CR. Even the new V6 TDI still relies on the VP44. It even achieves EU4 standards without a particulate filter, which CR has only been able to achieve so far with OPEL in the 'lean diesel' configuration.


Hello Bertil!

I didn't mean to say that the V6 with the VP44 is bad! It's just 'different' and not like today's (racing) common-rail TDIs. I think that VAG deliberately tuned them for this enormous acceleration from low RPM. This was the best way to dispel the diesel's 'prejudice' (turbo lag and 'not responsive'). Three months ago, I drove an A3 with the 130PS TDI. It's really amazing how much power comes from low RPM. Accordingly, the tires on the front of the rental car looked like this .
I hope that the newer V6 isn't as prone to problems as the one in my car's model year. Starting with the constantly failing mass airflow sensor and the breaking-in camshaft, that engine is not exactly 'easy to maintain' .

Is there a kind of pressure accumulator in the VP44 fuel injection pump? If so, how can you test it?

Regards,
DENNIS

Hello Thomas,

'If the timing of the pump is slightly off, exactly what I described will happen. Mine was also right at the (late) limit. A little too early is no problem; it's better than being too late.'

Hello,

Quote:

I hope that the newer V6 isn't as prone to problems as the one in my car's year of manufacture. Starting with the constantly failing mass airflow sensor and the breaking-in camshaft, that thing is not exactly "easy to maintain"

.

"Among other things, the entire valve control system has been redesigned in the new V6 engine. It now features roller rocker arms, likely also due to the previous camshaft issues."

I have to say, Euro 4 for that heavy machine with a VP 44 - impressive.

Best regards, Albrecht.

Hello,

I dug out an old log file again.
The deviation between the target value and the actual value is actually quite small. At higher speeds, the actual value even tends to be slightly ahead of the target. I can't detect any delay here (unfortunately). So, the control system doesn't seem to be at its limit yet.

RPM Ign, Tim Ign, Tim Duty Cycle
TIME
MARKER STAMP /min ° ° %
9.71 2088 4.2 4.4 61.9
10.08 2343 4.6 4 55.2
10.44 2598 5.2 5.2 53.7
10.8 2877 6.2 5.8 48.9
11.16 3178 7 7 47.7
11.52 3457 7.8 7.8 43.4
11.88 3689 8.8 9.6 42.6
12.24 3944 9.8 10.8 41.8
12.6 4246 7 11.4 48.1
12.96 3666 7 0 46.6

What surprises me is that the control system only requires 9.8 degrees at an engine speed of nearly 4000 RPM. Normally, it's 9-15 degrees. It couldn't be due to an excessively high temperature, as I performed the measurement last winter.
Under what circumstances does the engine control unit (ECU) request 9° of advance, and when does it request 15°? Assuming, of course, the same conditions: full throttle and 4000 RPM.