Slow Pressure Build-up? Troubleshooting Guide.

Hello TDI community!
Using VAG-COM, I recorded the following data. The measurement was taken in 3rd gear, with full throttle, while driving steeply uphill, so the acceleration was not excessively high (please consider this when evaluating the boost pressure).

The columns, from left to right, are: Engine speed, Mass airflow sensor value, Air pressure, Target boost pressure, Actual boost pressure, Wastegate valve duty cycle.

n LMM p_air p_setpoint p_actual WG
1/min mg/R mbar mbar mbar %

1365 475 979 1306 1000 34.7
1470 575 979 1785 1122 34.7
1680 715 979 1887 1285 35.1,
1932 855 979 1918 1581 35.5,
2184 905 979 1897 1826 35.1
2436 915 979 1887 1979 41.4
2688 835 979 1877 2030 59
2940 815 979 1867 1867 62.2
3150 790 979 1867 1897 64.1
3381 765 979 1867 1877 65.7
3612 745 979 1856 1867 67.3
3801 730 979 1846 1856 68.5
4011 695 979 1826 1846 69.7
4179 660 979 1805 1785 68.1
4347 635 979 1775 1744 65.3

Apart from the sharply decreasing mass airflow values above 3000 1/min (a new mass airflow sensor has already been purchased on eBay ), I'm not particularly happy with the boost pressure build-up of my TDI. In my opinion, the maximum boost is reached much too late, at approximately 2500 RPM. I would have expected the initial pressure surge to occur at 1900-2000 RPM. Overall, the entire boost build-up in the lower RPM range seems too sluggish (the deviations from the target value are sometimes up to 0.6 bar!). Although the car was accelerating in 3rd gear, it was on an incline of approximately 8%, so the engine should have had enough time to regulate the boost pressure.
Are there also problems with the wastegate chargers regarding the difficulty of controlling the valve or leaks around the valve? It looks like the wastegate might not be closing completely, which means it requires a higher exhaust gas flow to generate enough back pressure to get the turbocharger working. Above 2600 RPM, the boost pressure control seems to be working perfectly.
Or could it be due to the solenoid valve that controls the operation?

I am eagerly awaiting expert advice!

Regards,
Alex.

I just took my 1996 Golf3 TDI (AFN) for a test drive yesterday and encountered error code 1551.
Currently, I'm experiencing an issue where the engine takes approximately 5-7 seconds for the actual boost pressure to reach the target boost pressure. This results in a significant turbo lag.

I usually had a target value of 1948 mbar. Your target value seems a bit too low to me, or is this for the 90 hp engine?

What could be the reason for such a significant turbo lag in my car? This behavior has been present for about 3000 km. The car has approximately 250,000 km on the odometer. My VW mechanic suggested it might be due to a clogged EGR valve. However, that doesn't seem plausible to me, because after the described 5-7 seconds, the full power is available.

Holgi.

Holgi wrote:

What could be the reason for the extremely large turbo lag I'm experiencing? It has been behaving this way for about 3000 km. The mileage is approximately 250,000 km. My VW mechanic suggested it could be due to a clogged EGR valve. However, that doesn't seem plausible to me, because after the described 5-7 seconds, the full power is available.

Hi Holgi,

If the engine can increase its speed during that time, the amount of power delivered to the turbocharger also increases.
In some cases, a VTG (Variable Turbine Geometry) setting of "partial pressure build-up in the low-rpm range" (above which the system could be blocked by soot) may be sufficient to achieve almost full pressure build-up.

That would also be my assumption, especially since the 1996 AFN model, with its standard loader, likely also has the design flaw known as the "small loader container."

Okay, I'm ready. Please provide the German text you want me to translate.

Hello Ulf,

Thank you for your helpful answer. I will check that out on Saturday.
As far as I can see, in the worst-case scenario, a new charger will be needed.

@Alex: Sorry if I've repurposed your thread for my own purpose.

Holgi.

Hi Holgi,

In the worst-case scenario, yes, but let's not jump to conclusions right away. It might also be enough to reactivate the VTG (Variable Turbine Geometry) by moving it (see the Zlfs frequency generator or Bilnker method), or, as Ulf suggested, simply install the large pressure vessel. He has provided information on where to obtain them in another post, and that is SIGNIFICANTLY cheaper than a new charger.

Best regards,

Jan.

Hello Turbo Pros!
What do you think about my excessive turbo lag, which is about 5 to 6 seconds, and the delayed peak boost pressure, which occurs around 2500-2600 RPM?
Is my wastegate leaking?
It's unlikely that the problem is with the solenoid valve used for control, because the diaphragm in the wastegate is actually pressurized. If there are leaks in the area of the valve, the wastegate should open too late, resulting in too steep a pressure increase and a pressure level that is too high (or a limp mode), right?
How about a faulty EGR valve? When accelerating, the exhaust pressure should initially rise above the boost pressure before the pressure differential reverses, roughly around 1700-2000 RPM, right? This seems like a plausible explanation to me, as the leakage through the faulty EGR valve reduces the mass flow through the turbocharger, which contributes to a slower build-up of boost pressure. What do you think of this theory?
How would you proceed in my situation?

Thank you for your helpful tips!

Regards,
Alex.

donalexo wrote:

What do you think about my extended turbo lag of approximately 5 to 6 seconds, and the delayed peak boost pressure at around 2500-2600 RPM? Is my wastegate leaking?

Hi Alex,

That seems plausible... you can find some more tips, including information about the wastegate system, in the pneumatics technical article.

Quote:

It's my opinion that it probably can't be located at the solenoid valve used for control, because the diaphragm chamber on the wastegate is actually pressurized. If there are leaks in the area of the valve, the wastegate should then open too late, causing a too steep pressure increase and a too high pressure level (or a limp mode), right?

Basically, yes...

Quote:

What about a faulty EGR valve? When accelerating, the exhaust pressure should initially rise above the boost pressure before the pressure difference reverses, approximately at 1700-2000 RPM, right? This seems like a plausible possibility to me, as the leakage flow through the faulty EGR valve reduces the mass flow through the turbocharger, thus contributing to a slower build-up of boost pressure. What do you think of this theory?

It might be, but a severely leaking EGR valve often also results in a significant loss of peak performance, and the engine will also produce a lot of black smoke.

Quote:

How would you proceed in my place?

First, connect the turbocharger outlet directly to the wastegate actuator... -> This is a topic for pneumatic specialists, and we'll control the actual boost pressure with a lambda sensor (LDA).

Thank you, Ulf, for your tips!
Hier noch ein kleiner Nachtrag: Ich habe heute einen neuen LMM eingebaut, in der Hoffnung, dass zumindest der starke Abfall des Signals oberhalb 3000 Touren behoben wird. Think again!
Either this part is junk, just like my old mass airflow sensor, or there's a more serious problem with my engine that's related to the slow build-up of boost pressure.

If the new mass airflow sensor (MAF) is displaying correct values, then the engine isn't getting enough air at high RPMs (of course, I've already checked the air filter!).
If the EGR valve were leaking, wouldn't the measured air mass at least be correct at high engine speeds? I haven't been able to detect any significant full-throttle surging either.
Even if the wastegate doesn't close completely, the full amount of air should still be forced through at high engine speeds. Leaks in the pneumatic hoses would more likely cause excessively high boost pressure with a steeper increase in a wastegate turbocharger, rather than the opposite.
I'm really quite at a loss!
What is the likelihood of increased exhaust back pressure, for example, due to a broken catalytic converter? Could that be the solution to the problem?
Then, shouldn't you hear a rattling sound from the fragments when there are uneven surfaces on the ground, or not?
Has anyone else experienced this? How did the car behave in that situation?

Can anyone come up with a rhyme about my problem?

Thank you for your help!

Regards,
Alex.

Deleted upon request.

Hello diesel.gustav!

Quote:

Based on my experience, the engine also loses power (or, more precisely, torque in the higher RPM range). Is that not the case for you?

Yes, that is indeed the case. The engine feels weak at low speeds when starting, because the turbo lag is so significant. In the mid-range, the power is normal . And above that, the engine feels restricted again, so it's definitely not rev-happy! Also, the top speed is no longer quite reached.

Quote:

One of them would be which car you drive. I haven't found any information about that in this thread.

Please provide the text you would like me to translate from German to English. I will only provide the translation, without any explanations. Also, please provide the signature you mentioned so I can include it in the translation if necessary.
But here it is again, in great detail and with absolute clarity, to eliminate any possible misunderstanding:
Audi A3, 1.9 TDI, 66kW, engine code AGR, first registration 12/96, mileage approximately 131,000 km.

I hope this clarifies all the conditions. If not, please just ask, because this issue is really bothering me!

Sure, here's the translation:

"Thank you in advance."

Regards,
Alex[/quote]

donalexo wrote:

The engine feels weak at low speeds, because the turbo lag is so significant. In the mid-range, the power is normal . And above that, the engine feels restricted again , so it's definitely not rev-happy! Also, the top speed is no longer quite reached.

Hi Alex,

Have you ever gone through the technical article on systematic troubleshooting using VAGCOM?
I suspect that in your case, it would be a good idea to do a comprehensive check-up.

Hello Ulf!
You are likely referring to the article "Systematic troubleshooting of VAG TDIs with a distributor injection pump (VP37)," correct?
Of course, I have read this article several times and have largely verified the points described within it.
In the ESP (Electronic Stability Program) area, there appear to be no issues, as all values are within the normal range, both statically and dynamically. The injection timing and control dynamics are perfectly fine!

I still have a feeling, though I can't quite shake it, that the engine's reduced performance since the major service at 120,000 km (which included a timing belt replacement, of course) is related (as I said, it's just a gut feeling). I don't want to imply anything negative about my workshop.

With my usual driving style, which consists of over 85% driving on country roads, I almost always operate in the middle RPM range, where the engine's power is normally adequate. The lack of engine responsiveness only becomes noticeable when overtaking, where the necessary power is missing. And especially on the highway, the car feels quite sluggish at higher speeds. I noticed this during my short vacation over Easter, when I drove about 1400 km on the highway.

Okay, yesterday I checked the temperature sensors using VAG-COM while the engine was completely cold. The car had been parked in the underground garage for more than 24 hours. The coolant sensor showed a value of approximately 16°C, the air temperature sensor showed approximately 17°C, and the fuel temperature sensor showed approximately 18°C. While there are deviations, the values should still be within the tolerance range, right?

Well, it seems the only system I haven't checked yet is the pneumatic system around the engine control unit.
Should I purchase additional (spare) hoses and clamps (or whatever those things are called) for this job, in order to, for example, easily create cross-connections between the turbocharger outlet and the wastegate reservoir, etc.? Or are the hose lengths already present in the engine bay sufficient for these tasks?
How about those mini hose clamps? Can you inflate them to remove the hose, and then deflate them again to create a tight seal?

Is there another way to determine if a bone is broken, besides tapping on it?

Thank you very much for your help!

Regards,
Alex.

donalexo wrote:

Of course, I have read this article several times and have largely verified the points described within it. In the ESP (Electronic Stability Program) area, there appear to be no issues, as all values are within the normal range, both statically and dynamically. The injection timing and control dynamics are perfectly fine! I somehow can't shake the suspicion that the poorer engine performance since the last major service at 120,000 km (of course, including a timing belt replacement) has appeared (as I said, it's just a feeling). I don't want to imply anything negative about my workshop.

Hi Alex,

"Mostly" does not mean "all." For example, when replacing the timing belt, the camshaft could have been installed one tooth out of alignment.

Quote:

With my "normal" driving style, which consists of more than 85% country roads, I almost always operate in the middle RPM range, where the engine power is actually normal. The lack of engine responsiveness only becomes noticeable when overtaking, where the necessary power is missing. And especially on the highway, the car is quite sluggish in the higher speed range. I noticed this during my short vacation over Easter, when I drove about 1400 km on the highway.

I would initially suspect that the accelerator pedal isn't reaching 100%, or that there are restrictions in the air or fuel lines.

Quote:

Oh, by the way, yesterday I checked the temperature sensors using VAG-COM while the engine was completely cold. There are indeed some deviations, but the values should still be within the tolerance range, right?

That's perfectly fine.

Quote:

Well, it seems that the pneumatic system around the engine control is the only system that I haven't checked yet! Should I purchase additional (spare) hoses and clamps (or whatever those things are called) for this job, in order to, for example, easily create cross-connections between the turbocharger outlet and the wastegate reservoir, etc.? Or are the hose lengths already present in the engine bay sufficient for these operations?

"I have no idea; I don't know what your engine bay looks like - I haven't worked on an AGR system before either."
Reserve clamps, especially for hoses that are under pressure, are definitely a good idea.

Quote:

What about the mini hose clamps? Can you inflate them to remove the hose, and then deflate them again to create a tight seal?

I haven't managed to do that yet (without the appropriate specialized tools).

Quote:

Besides tapping, are there any other methods to determine if the cat's leg is broken?

In extreme cases, the pulsations of the exhaust flow that can be felt and heard at the outlet may be weaker or completely absent, if the exhaust gases are only allowed to pass through the remaining constriction as a "steady stream."

AFAIK, there should be some free space in the catalytic converter housing, behind the monolith, up to the outlet pipe.
So, if you insert a broom handle or something similar from the back and feel an obstruction right at the beginning of the tube (which might be able to be pushed forward), it probably strongly indicates a blockage caused by cat hair.

Hello Ulf!
Thank you for your quick response and your helpful advice.
I've also already checked the accelerator pedal using VAG-COM, and it's 100% okay.
The intake air path doesn't show any obvious bottlenecks. Perhaps I should disconnect the air intake snorkel from the radiator grille just to be sure, to see if anything might be lodged inside. It's very unlikely, but not impossible.
There are no air bubbles visible in the fuel system, and the filter was also changed according to the maintenance schedule during the last inspection (or at least, it was recorded as such!).
I should probably start by removing the timing belt cover and checking the timing, or what do you think?
How does a phase shift of one tooth affect the amount of air flowing through and the pressure build-up?

I think I'll definitely try disabling the AGR (Abgasrückführung - Exhaust Gas Recirculation) system sometime this week, just to rule out a defect on that side. I've already gotten a new flange gasket to use as a template for a "seal plate"!!

Thank you for the additional suggestions, advice, and potential solutions!

Regards,
Alex.

donalexo wrote:

I should probably start by removing the timing belt cover and checking the timing, or what do you think?

Exactly

Quote:

How does a phase shift of one tooth affect the amount of air passing through and the build-up of boost pressure?

No idea, due to a lack of practical experience.

Hello Ulf!
Please don't be annoyed, but I have a few more questions regarding the timing belt check.
Earlier, I carefully read the technical article about replacing the timing belt on a TDI (as I hope). While reading, I noticed that checking the timing is not as straightforward as I thought.
I remember seeing this on various vehicles (including an Opel Kadett 1.6i, a VW Bully 1.7d, a Mercedes-Benz 100 D, etc.). There are markings on the camshaft pulley and the cylinder head that need to be aligned when the first cylinder is at Top Dead Center (TDC). Checking this is quite simple.
Unfortunately, with the TDI engine, you likely have to remove the valve cover to access the notch on the camshaft where the timing tool should be inserted, right? Is this notch located on the side of the vacuum pump (which is why it's necessary to remove it during a timing belt replacement)? If so, does it also need to be removed when checking the timing?
Is a visual inspection sufficient for checking the valve timing, or is a timing tool required?

Wenn ich den Fachartikel richtig interpretiert habe, dann müsste ich theoretisch die Zahnriemenabdeckung für die Kontrolle der Steuerzeiten gar nicht abnehmen, sondern müsste stattdessen den Ventildeckel abnehmen, um die Nockenwellenposition zu erkennen. Auf der Getriebeseite noch den Gummistopfen entfernen und Kurbelwelle of OT-Markierung drehen. S"Is that correct?" or "Does that work?"

Can the valve cover gasket be reused (in terms of sealing), or should I get a replacement before doing this? What torque should be used when tightening the valve cover screws?

Thank you for the practical advice!

Regards,
Alex.