Accuracy of VAGCOM measurement values | Posts 16+

Jan6K wrote:

The values from VAG-COM are very accurate, as they are the ones that the engine computer also uses for its calculations. Bitte gib den Text an, den du übersetzt haben möchtest. Jan, who also wants to install an LDA at some point

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Hi Jan,

minor correction: the VAGCOM measurement values are not necessarily extremely accurate.

According to the "How it's done" guide, the tolerance range for EDC temperature sensors (all with the same value) is quite wide.
For example, if one sensor has a resistance of 250 ohms at 85°C, and another sensor has the same resistance of 250 ohms at 95°C, both are still (just barely) within the specified range.
Assuming that the ADCs (Analog-to-Digital Converters) in the engine control unit are programmed to a mid-point within the tolerance range, the actual temperature can deviate from the VAGCOM reading by up to +/- 5°C within a range of 90°C.

Conclusion: As is often the case in digital technology, the "high" resolution of the measured values (e.g., 0.1 °C) creates a false impression of absolute accuracy, which is not actually present.
The only thing one can normally rely on is that slight changes in the recorded values will be noticeable in the output.

Regarding the LDA installation: hopefully you'll get it done before your charger control system malfunctions (though, of course, I don't wish that on you).
When my car was temporarily losing power, the built-in LDA (Low-Pressure Diagnosis) was very helpful in the immediate diagnosis (--> turbocharger problem instead of mass airflow sensor or something else) without having to read out complicated data. Therefore, I recommend that every FDI driver have such a device as a mandatory piece of equipment .
Especially if you (like me) don't have a VAGCOM and therefore would have to rely on a workshop for diagnosis .

Hi Ulf,

You're absolutely right. What I meant was more that they show the same data that the main processing unit also receives; in that respect, they are actually identical because they are the same data.

I've also observed the issue with tolerance. After the car has been parked overnight, the values for charge air, fuel, and coolant *should* be roughly the same when the engine is off (unless the sun is shining directly on it), but they can actually differ by as much as a degree. While that's not actually so bad, there are other explanations for the temperature distribution inside a stationary car.

Regarding the LDA, I'm not as worried anymore since I have VAG-COM. If the control system malfunctions, I'll re-run the diagram I posted yesterday (engine speed, boost pressure, valve control) and put both versions side-by-side for comparison. With this information, the diagnosis should actually be quite simple, because you can see what the control system is trying to do and how the loader is reacting.

It's certainly much better to keep an eye on it constantly while driving, but since that's also hardly an effort with VAG-COM - just plug in the laptop and go - I'll postpone that for now until I finish my dissertation .

Best regards,

Jan.

Hello!

Thanks for all the answers! So, the whistling sound definitely comes from the turbo area. I checked under the hood. Could it still indicate a leak outside of the turbocharger? It's also strange that it whistles for about 3 seconds after I let off the gas. I don't think there's anything else to do. I'll probably have to carefully inspect the pressure hoses, millimeter by millimeter, using a mirror. That should definitely clarify things. Regarding the tapped hose, it's the only braided hose that leads to the control unit. The Y-connector is also very tight on my car without any securing. I'll check the things you suggested and get back to you.

Best regards, Dirk.

Sorry, I have to bring up this topic again:

I installed an LDA (from Conrad) today and connected it between the intake manifold, just before the inlet manifold flange, and the control unit using a Y-connector (At VW, they wanted €10.99 + tax for a meter of vacuum hose ).
Okay, the indicator is working (the pointer vibrates quite strongly even when idling...).

During my first test drive, I observed the following: The boost pressure increases proportionally to the engine speed, meaning that at 4000 RPM, I'm getting approximately 1 bar of boost. It's similar to what you'd find in a 'TDI-1Z' engine, except mine doesn't whistle. The path between the charger and the valves is sealed; I've checked it. --> Passat '94 1Z.

My solenoid valve makes a sound (but only sometimes) like when you put small pebbles into one of those yellow Kinder Surprise eggs and then shake it. It's a weird noise. I measured the resistance (28 ohms, which is okay...).

You'll probably tell me to replace the valve (which I'll probably do soon anyway...), but maybe one of you knows what the problem actually is.

Thank you.

David.

Hi,

So, you're saying that at 2000 RPM, you only have 0.5 bar of boost? I don't know how that's supposed to work with a single turbo, but it seems significantly too low. Usually, TDI engines already have their maximum boost pressure at around 2000 RPM, where they produce maximum torque. I would be surprised if it were different for wastegate chargers.

The only condition is to go full throttle; anything less is meaningless. Take a look at my full-throttle diagrams; you can clearly see how the pressure shoots up to 1 bar (or even slightly more) below 2000 RPM, and then, when the regulator kicks in, it remains relatively constant.

0.5 bar at 2000 RPM and full throttle is, in my opinion, also too low for the 1Z engine.

Overall, it might be a good idea for someone with a properly functioning 1Z engine and VAG-COM to perform the same diagnostic test as I did with the ASV. This would allow for a good comparison, and it would also provide the opportunity to better assess any errors related to the turbocharger control.

Best regards,

Jan.

I just took another test drive.

Okay, so it was a long, straight road, in third gear: at approximately 1500-1600 RPM, I pressed the accelerator pedal fully, and the boost pressure reached 0.5 bar at 2000 RPM. It continued to increase until about 3000 RPM, where a boost pressure of 0.9 to 1.0 bar was reached, which then decreased again by about 0.1 bar after a short moment...

So, everything seems to be reacting slowly, which might also explain my performance issues! I've already been to the workshop about this, and they gave me a different mass airflow sensor to test, but it didn't make any difference...

What else could it be?
Okay, the error memory was clear, the car isn't making any unusual noises, and the fuel consumption is also normal.

Best regards.
David.

Hi,

As I mentioned earlier, and as Jan correctly suspected, you should definitely have more than 0.5 bar of boost pressure at 2,000 RPM with your 1Z engine!
A pressure of 0.5 bar should be achieved just above 1,500 RPM.
And 0.85-1.0 bar (which will typically fluctuate within that range) should be constantly maintained from approximately 2,000 RPM up to at least 4,000 RPM.
I don't know about anything above 4,000 - I haven't tested it.
Of course, full speed ahead!!!

P.S.: The vibration of the display can be effectively reduced by using foam inside the tube. This dampens the resonances. You have to experiment with the amount and the firmness.

P.P.S: I can't perform a diagnostic test using VAG-Com. I'm not familiar with that kind of equipment. I can only perform one measurement run using 'eyes, pen, and paper' – meaning I have to write down the data...

@ Markus:

Thank you for your post! I've already tried the foam method, and it works great (great idea!).

One more question: Could the solenoid valve be the cause of my problem? I'm not really sure about that part; it rattles all the time...
Or should I consider other potential sources of error?

Thank you & Best regards.
David.

Hi,

Do you mean by 'rattling' a constant 'clicking' sound?
(I love the description of tones ).
Upon further reflection, I had the same thought. For me, I always heard it when the ignition was on - just before the engine started.
And then, shortly after that, the solenoid valve also malfunctioned. It's been quiet since the new installment came out.

All things considered, this probably isn't a great diagnosis, but try replacing the solenoid valve.

I'm just surprised that you have TOO LITTLE boost pressure. If the solenoid valve is defective, there is often an unhealthy INCREASE in boost pressure, which is then either prevented by reducing the injection quantity (in my case), leading to a sudden loss of power, or by activating the emergency program.

Okay, that clicks all the time, which then sounds like a rattling noise overall .

Could the insufficient boost pressure also be caused by a bypass that isn't closing properly? Is there a way to check this? (It's really hard to access.) The pressure sensor must still be working, otherwise I would have gone into emergency mode...

Hi,

Unfortunately, I'm a little unsure about that.

But here's at least the procedure according to the 'Help Yourself' guide:

Fault: Turbocharger boost pressure too low.
Inspection: Check the pressure sensor for boost pressure limitation. Was this intact?
Replacing a turbocharger .
Procedure:
- Warm up the engine (oil temperature must be at least 80°C).
- Disconnect the connector from the magnetic valve for boost pressure limitation.
- Have the assistant start the engine and, with a sudden (!) press of the accelerator, bring it up to...
Bring it to its maximum speed.
- The actuation rod of the pressure sensor for the turbocharger control valve must...
involved.
- If it doesn't move, adjust the lever for the boost control valve to the 'easy' setting.
Check for accessibility.
- Is this lever seized? Replace the turbocharger (see above).
- However, can this lever be moved, the actuation rod of the...
Pressure sensor, but NOT working; in this case, the pressure sensor must be replaced.

Wow, quite a lot of effort (and noise) - maybe it's better to just replace the solenoid valve first.
The clicking sound is a strong indicator!

MARKUS.

Thank you for the effort in transcribing this.

I will definitely check that. I now also think it's the solenoid valve, because I took it out yesterday (I shook it a bit and tapped on it ), and at least the clicking sound was gone (I could only check this while the engine was idling), and the boost pressure was around 0.9 to 1.0 at 2200 RPM, which is at least already better.
I'll definitely try swapping them out and see what happens.

Best regards and thank you again!
David.

The meticulous search of the pressure hoses yielded nothing. But now, something else. There's a slight, vibrating noise coming from the LDA (likely referring to a sensor or module) at approximately 2000 RPM. It occurs regardless of whether I accelerate or release the gas pedal at that RPM. I previously thought it was coming from the lighting connector, and I didn't really care until it started to annoy me. So, I removed the LDA from its mounting and drove it. That's when I realized the noise is coming directly from the display itself. Is the LDA defective? Is it leaking? I then inquired today about where I obtained the LDA. They immediately responded to me:

The vibrations that you hear are usually...
Airflow fluctuations caused by the turbocharger, or alternatively...
'Sounds of turbocharger wear. In the first case, a solution can be found by sealing the pressure hose that leads to the boost pressure gauge with hot glue or a similar adhesive. Then, create a new opening with a hot and very thin needle.' That should make the noise disappear.

If he's right, in my case it would be wear noises from the turbocharger. The question is whether he's right. By the way, the LDA is from KW-Systems. I'll definitely have a diagnostic test done after my vacation, as I have to go to the workshop anyway. Maybe it will be helpful. I will report back later.

Best regards, Dirk.

Hi,

@TDI-1Z:
As I explained earlier in this post, the noise produced by the LDA (generated by air columns/waves and resonances) can also be reduced by using foam inside the tube.
Just try it out and see how much is needed.
The method using adhesive isn't bad either.
It definitely works with foam - for example, it works for me!
Don't immediately assume the worst – the charger will probably be fine.

Okay, so! Since the workshops have previously denied me the opportunity to have a diagnostic test run , I have finally found a way to read my own data. Could someone please take a look at this and tell me...?
Does this suggest a turbo problem? Thank you!


Time -- Upstream -- Boost Pressure (Target/Actual) -- Duty Cycle (Valve) -- Boost Pressure Control.

178.6 - 903 - 898 - 979 - 93
180.43 -- 1701 -- 1346 -- 1102 -- 43.4
182.26 - 1638 - 1438 - 1214 - 39.8
184.07 -- 2541 -- 1754 -- 1785 -- 39.8
185.9 -- 3633 -- 1805 -- 1856 -- 53.1
187.76 - 4494 - 1734 - 1683 - 61.7
189.57 -- 3318 -- 1703 -- 1856 -- 65.2
191.4 - 3738 - 1795 - 1805 - 50.4
193.26 -- 4158 -- 1775 -- 1826 -- 59
195.12 -- 4452 -- 1714 -- 1683 -- 62.1
196.95 -- 4641 -- 1673 -- 1673 -- 64.1
198.82 - 3360 - 1683 - 1765 - 58.2
200.62 -- 3360 -- 1316 -- 1285 -- 87.5
202.41 -- 3255 -- 1081 -- 1244 -- 94.1
204.25 -- 3150 -- 1051 -- 1234 -- 94.1
206.08 -- 3129 -- 1469 -- 1826 -- 63.7
207.94 -- 3129 -- 1255 -- 1244 -- 94.1
209.74 - 3003 - 1071 - 1214 - 94.1
211.57 -- 2877 -- 1040 -- 1204 -- 94.1

... looks pretty good, as far as I can remember from my 1Z engine. What makes me a little suspicious are the maximum values: just above 0.8 bar seems a bit low. Slight pressure fluctuations up to a reading of 2000 mbar (which is 1 bar of boost pressure) should be normal, but at least the maximum boost pressure observed should be 0.9 bar (1900 mbar as indicated in VAGCom).
Check the LLK (low-loss coolant) system for leaks. Something might be fizzing away!