A4 2.5 TDI BDG - Power Loss/Performance Issues

Hello,

'For some time now (approximately 15,000 kilometers), I've noticed something on my A4, which currently has a mileage of 49,000 kilometers.'
The following problems are present: The engine feels weak across the entire RPM range, and the maximum torque is reached later. The maximum speed (as stated in the vehicle registration) is difficult to achieve and is slightly lower than it was before the problems started. The emissions control warning light occasionally illuminates, which is always accompanied by a brief 'jerk.' A visit to the mechanic yielded no results, as the error memory was empty.
Now that I finally have a laptop again, I did a test run. Third gear, going uphill. The error memory was also empty.

'It's noticeable that the turbo pressure increases somewhat later than usual (according to the manufacturer's specifications, the maximum torque is already reached at 1600 rpm), but otherwise it behaves normally. Since the test drive was uphill, the slightly slower pressure build-up of the V6 engine in the lower gears could be the reason. The mass airflow sensor (LMM) seems to be working correctly. Perhaps someone here has further ideas. How can a stuck-open EGR valve be diagnosed using VAGCOM?' 'Accessing the terminals for expansion or disconnection is extremely difficult, so this would be the last option to consider.'

Sure, here's the translation:

'Thank you in advance.'
Regards,
Jean.

Sorry, that was the wrong log file. Here and above is the edited and viewable version.

Hmm, it actually looks pretty good.

MWB 008.2 should be greater than 50 mg/dose.
MWB 008.3 should be between 42 and 47 mg per dose.
MWB 008.4 should be above 48 mg/dose.

You're also doing well if the boost pressure is between 1900 and 2300.
And LMM also looks great.

I understand what you mean; I know of a similar BDG that seemed 'sluggish' in some way, but there was nothing really measurable to explain where that 'feeling' came from.

Sure, here's the translation:

'Note:'
There was an issue with the 401 control unit related to decreasing fuel injection amounts (specifically affecting warm engine idle, where MWB 1 field 2 dropped below 3 mg/H). In that case, a new control unit, part number 8E0997401 JX, was used. But I think you probably no longer have a warranty that would cover something like that?

First of all, thank you for the quick reply.

As I mentioned, I find the delayed turbo boost build-up to be somewhat noticeable. The maximum torque, and therefore full boost, should be achieved at 1600 rpm. I haven't observed any excessive smoke under full load, which would be expected with a stuck EGR valve. However, could this still be a possible cause?

If your AGR valve is like the one at AKN, I don't see a problem with closing it for testing purposes. However, you can try the following: Use a vacuum pump (by mouth) to draw air through the AGR valve and let it snap open. You should hear a click. If you do, then close the vacuum hose and test it. "In my case, the emergency mode isn't activating..."

I see that the boost pressure isn't reached until around 1900-2000 rpm, and the air mass and smoke limit follow the boost pressure. I also find this to be happening late. Also, you're driving in 3rd gear (which isn't optimal, even going uphill; my logs are much longer with smaller RPM jumps). Could you try driving the same log in 4th gear? It seems like you have a relatively small number of data points. Maybe you could increase the sampling frequency. I find your log to be too short. Perhaps the turbocharger isn't able to reach the required pressure due to the short duration.

Everything seems fine up top. If the light is on again, you can read the error codes or try to capture a "golden moment" (a specific event). The AGR (Exhaust Gas Recirculation) being blocked likely triggers an error code on some vehicles. You can live with it, but it shouldn't be a problem to insert a shim (a thin piece of metal) into the connection to the intake manifold (at the front). You can also hear a "clack" even at idle if you simply disconnect the hose. Of course, it's not guaranteed that the AGR is always clear. That's why the shim is used. It could also be a blocked exhaust system (if you often drive fast or just once for testing), or clogged injector nozzles. Or even air in the diesel fuel.

Okay, so the AGR valve makes a metallic clicking sound when it's activated. It seems that, unfortunately, this isn't the cause of the problem. Driving with the EGR disconnected (hose removed) doesn't improve anything, but strangely enough, it doesn't trigger any error messages.

Could it also be a blocked exhaust manifold, do you often drive fast, or just once for testing, or blocked injector nozzles?

He actually gets enough long, full-throttle stretches to let himself really go. You can tell afterwards that he's holding the throttle a bit better, but that's not the main problem.

If there were air in the diesel fuel, I would think the problem would manifest in some way (e.g., jerking), but the engine's performance has been consistently weak throughout the entire period. I'm going to replace a section of the tubing leading to the filter with a transparent hose so I can see if there are any air bubbles.

Is it possible for the pre-lift pump to be defective without generating an error code? How could this be tested using VAGCOM?

Once the road conditions are a bit better, I'll try logging again in a higher gear. It's also possible that the diesel temperature sensor is faulty, so I'll check it again when the car has been parked overnight and the diesel should be at ambient temperature. Otherwise, I am open to all suggestions.

Regards,
Jean.

Kruegerjean wrote:

"I'm going to replace a section of the tubing leading to the filter with a transparent hose so I can see any air bubbles." "Could it be possible for the pre-charge pump to be defective without generating an error code?" How could this be tested using VAGCOM?

1) The flow from the filter to the pump must be monitored. Just say, he doesn't have a clear hose anymore .
2) He needs to vacuum out the intermediate container. I think you can control it using an actuator test or something similar. I'll leave it to the experts now.

Is the VTG system fully engaged? Is the vacuum okay? Otherwise, try driving without the catalytic converter. I'm going to remove mine (for testing purposes). It's quite fast for its mileage.

Just saying, he doesn't have a clear hose anymore icon_eek.gif

However, he did. I remember encountering a similar problem back when I was in high school, but that kind of thing didn't exist back then. No bubbles are visible.

I logged the data again in the 4th and 5th gears. The pressure buildup occurs a bit earlier, as expected. I'm not sure if it's early enough. How is it with your V6?

As I mentioned, a slightly delayed pressure build-up should only be considered a symptom; the weak engine characteristics are evident across the entire RPM range. Even from the top, the engine looks somewhat strained.

I find it a bit strange that, in the 4th gear, the air mass readings decrease, despite sufficient turbo boost, starting around 3000 RPM. It might not be related to the power loss. (The smoke reduction hasn't been achieved yet.)

Otherwise, the logs appear to be fine, with the exception of a single outlier in the 5th entry. Walk normal.

Where is the fuel temperature sensor located? In the engine compartment or directly in the tank?

Kruegerjean wrote:

Where is the fuel temperature sensor located? In the engine compartment or directly in the tank?

In the fuel injection pump.

Kruegerjean wrote:

I logged the data again in the 4th and 5th gears. The pressure buildup occurs a bit earlier, as expected. I'm not sure if it's early enough. How is it with your V6?

My engine power isn't right, but I'm working on it. My boost pressure is not available early enough; with your log, I would already be satisfied .

No joke, my log in 5th gear looks even worse than yours in 1st. Log.

The LD-soll (target load) is reached well below 2000 rpm, which I consider acceptable. I don't see anything alarming in the log. The sluggish behavior at higher RPMs must be caused by something else.

Please provide a reaction from a professional regarding the following 2 points:
1) The one about the air mass should also be normal (it drops at high RPM), at least that's what I think I read somewhere.

"It would be interesting to investigate the potential cause of performance loss if the soot reduction system isn't engaged. Perhaps the turbocharger is creating too much resistance in the exhaust path for the desired load, even though the log indicates it's achieving its target value. Or there might be another cause for increased exhaust backpressure (e.g., a catalytic converter or a bend in the exhaust pipe). This could also explain the decreasing air mass."

I'm bringing this up again and wondering if anyone has any new insights.

These BDGs are probably the most sluggish 2.5-liter engines I know, even a 150 hp version performs significantly better.

Hello,
'Even though I have an AFB engine, in my case, it was a faulty boost pressure sensor (my mass airflow sensor readings were (too) low, and I was lacking power).'

Regards,
Dieter.

@vagtuning: Have you driven multiple versions of these? On paper, they should perform better than the 150PS models, even with Euro4 regulations. Does the throttle valve of the BDG/BDH engine actually play a significant role in the EGR system, or is it just controlled via a map, with the mass airflow still being regulated solely by the EGR valve and MAF sensor?

Yes, I've driven a few, and I found them all to be absolutely tough, sluggish, and incredibly slow in the lower RPM range... But they start to pull above 3200 RPM, although that's not really the RPM range where you typically drive a 2.5 TDI in general... But if you put your foot down at 1500 or 1800 RPM... Wow, 21... 22... 23... oh, things are happening .

What also bothers me about the BDG is the extreme difference in values in the MWB 08.
All the other 2.5-liter engines are quite close together in performance... but the BDG stands out with its values in the high 60s to low 70s, listed as a 'driver's preference,' only to be brought down by values around 38-40 in field 3 (rotational speed). What's the point of that? Is the software programmed in a really stupid way?

I mean that a 'good' running 2.5-liter AFB, AKN, or AKE carburetor typically has a difference of around 10 mg/H between fields 2 and 3. However, some BDG carburetors show differences of up to over 25 mg/H, or sometimes just under 30 mg/H, between the two values. I have one here with field 2 at 64 mg/H, field 3 at 36 mg/H, and field 4 at 52 mg/H (at 3000 RPM).

That means there's a significant delay when you suddenly press the accelerator.

I've also noticed the thing about the driver's request.