VAG-COM: Measure "Blow by" Gases

Hello,

First: By 'blow-by gases,' I mean the exhaust gases that leak between the cylinder wall and the piston rings and enter the crankcase.

That's normal; it can also happen with new engines. However, the higher the wear and tear/mileage, the greater the volume flow. A crack in the cylinder head can also cause exhaust gases to leak into the camshaft area.

The blow-by gases are separated from the oil (using a cyclone/filter), and then these gases are fed into the intake manifold, behind the mass airflow sensor, and in front of the turbocharger (a small amount of oil remains in the gases, so the entire intake path becomes oiled).

So, that means increased air mass flow after the mass airflow sensor => potential power loss.

I disconnected the hose on my side, and the intake manifold is blocked (to prevent vacuum leaks). Even at idle, there's a massive airflow coming from the engine side! If you block the outlet (on top of the valve cover) with your thumb, pressure builds up quickly, which pushes the oil dipstick out of its place...

Okay, it would be great to know what this 'blow-by' volumetric flow rate is. It could be a real alternative to the unreliable compression test, and even more informative.

Here's a possible way to measure it: Start the VAG-COM system, let the engine idle. Then, disconnect the breather hose from the valve cover and IMMEDIATELY seal it with your hand.

'Now, the 'blow-by' gases are escaping directly to the atmosphere instead of being recirculated into the intake manifold, and the mass airflow sensor should register a difference (more air, because the airflow that was previously being diverted by the blow-by is now being measured by the MAF sensor).'

Okay, I won't have the equipment for another two weeks, and I'll post the results (hopefully at multiple speeds) then.

If you have any more ideas, please share them! If we actually go ahead with this, we'll need as many comparison measurements from you as possible.

Regards,
Perwoll.

Please provide the text you would like me to translate from German to English.

The reason is that my car (A4 B6 1.9TDI, AVB, all stock) is not reaching its top speed. Even according to the speedometer, it's 10 km/h lower. Since all possible error sources have been ruled out, these more daring theories, and natural tolerances, remain as potential causes.

High load and high engine speed increase the blow-by volume.

Hello Perwollf,
If you're talking about "equipment" like a VAGCom diagnostic tool, you should check the boost pressure. There might be an issue there. Also, the top speed depends on several factors (tires, weight and equipment!, environmental conditions, etc.).

Best regards, Robert.

Why don't you just open the oil filler cap with the engine running and see if a massive blast of air comes out? I'm already claiming that the missing 10 km/h isn't due to a lack of compression. If that were the case, you would also have other problems, such as difficulty starting the engine.

Hi there.
The lack of engine power likely doesn't stem from escaping combustion pressure, as you suspect. As long as the engine starts well when cold, the compression is still good, and therefore, no power loss would be expected from that. Even vehicles with cold start problems due to low compression usually still have their full engine power. I suggest you simply perform a power test to see how much power you're lacking or how much torque you're missing. The maximum power for your vehicle should probably be around 4000 RPM. Has the catalytic converter been checked? That could be the source of the problem.
Best regards,
Rumpi.

Hello, thank you for the answers!

@LZ: The turbo boost pressure is fine; I've already checked it with LDA. Unfortunately, the -10 km/h speed reduction is consistently reproducible on a flat road, regardless of whether I'm using winter or summer tires, the temperature, etc.

@Marco: Removing the oil filler cap (or dipstick) is only half the truth. A large portion of the gases are still being sucked into the turbocharger through the crankcase ventilation system.

'@Rumpi: Yes, I also suspected my cat. But I don't know how I could check it, other than by taking it for a test drive without the cat... However, it doesn't make any suspicious noises, and the visual inspection is also okay.'
I don't want to invest any money in performance tests as long as the problem is only -10 km/h. It's only a theoretical issue anyway...

Generally, I wasn't necessarily thinking about my performance issue here, but purely from a technical perspective, about the blow-by volume. If I have anything new to report, I'll let you know.

Wishing you a lovely weekend!

Hello,

I'm not quite sure I understand your concern. At VAG, the crankshaft pressure is also measured using the oil dipstick. What's inaccurate about that? Just go to a VAG dealership, put a 5-euro note in the coffee jar, and borrow the tool or have the workshop measure it. The whole thing takes less than 5 minutes. However, I would rather investigate the reason for the missing 10 km/h.

Hello Marco,

falls in der Entlueftungstrecke zwischen KG und Ansaugrohr alles sauber und nicht verstopft ist (Oelabscheider, Schlauche, usw), dann findet man wahrscheinlich nie ein signifikantes KG Ueberdruck, weil die Gase sofort in der Turbolader gesaugt werden. Or?

Therefore, I would prefer to explore other methods for measuring the bypass volumetric flow rate.

Regards,
Perwollf.

Hi,

I was talking about the oil dipstick, not the crankcase ventilation system!
Assuming you were trying to use the method I mentioned earlier, how would you convert the LMM (mass flow meter) values into the corresponding mBar value for the kg pressure? You shouldn't rely on empirical values with this method either, because each LMM will react slightly differently. In my opinion, this is not a viable approach. However, I am open to being corrected.

What's wrong with Marcos' method? It's the simplest way. Believe him. What you're trying to calculate and convert has an overwhelming number of influencing variables, so the result is likely to be just a rough estimate. If you run the engine and carefully remove the filler cap, and you feel the wind blowing towards you, you'll know what's going on. The pressure always takes the path of least resistance. Therefore, compare the diameter of the KWG vent with the diameter of the oil filler cap.
That, in itself, provides a fairly consistent result.

Hello everyone!

I have to say, I don't think the idea of Perwoll is that bad.
However, I must point out that the difference (LMM value) must definitely be measured with the EGR system deactivated (mechanically locked), because otherwise, you will not be able to detect any difference. The EGR system is a control loop that regulates the programmed air mass flow, and it compensates for the disturbance caused by the blow-by gas volume .

For measurement purposes, first temporarily disable the EGR system and then determine the air mass values for specific operating points. Subsequently, temporarily move the catalytic converter outside the vehicle (this is prohibited during normal operation, only for measurement purposes!) and ensure that the opening in the intake manifold is properly sealed.
Now, it might be useful to distinguish between a "clamped" KGE (presumably meaning a KGE that is physically attached or connected) and a "deactivated" KGE.
I'm really looking forward to seeing the results!

It only makes sense to discuss how meaningful this measurement method is, or isn't, once we have results here.
In a standard 1.9 TDI engine, the amount of fresh air drawn in without the EGR system is approximately 470 mg/hour. The key factor is the ratio between the amount of blow-by gas and the amount of fresh air:wink:.

Regards,
Alex.

Okay, so you're saying that for each vehicle, the mass airflow sensor (MAF sensor) used to establish the reference values is installed? I still maintain that this method is absolutely inaccurate and, above all, cumbersome. But let's wait and see what comes of it.

EDIT: I would like to point out that an LLM also ages over time, which can further distort the results.

Marco wrote:

I would like to point out that an LLM also ages, which can further distort the results.

Yes, but differences between the KGE series and the KGR models, even when used outdoors, can still be measured :wink:even with aging LMMs.

Hello everyone,

I really don't want to hurt anyone here, and I appreciate every response. But I have the impression that donalexo was the first one to take the trouble to actually read and think about what I wrote. His point about the AGR is completely correct. The AGR hasn't been bothering me for a long time (it caused quite a bit of dirt in the intake manifold).

Many people only focus on one point, and without thinking deeply. Then comes a short, simplified answer, see https://community.dieselschrauber.org/en/viewtopic.php?t=6230.
I'm not a liar, and even if I perhaps express myself unclearly, the probability that the basic idea is complete nonsense is not particularly high.

Marco asks:
'How, or using what conversion method, do you calculate the corresponding mbar value for the kg pressure from the LMM (mass flow rate) values?'

I don't want to create KG pressure values from LLM values. I hope to directly obtain the blow-by volume from the differences in LLM values. If there is no significant relative difference, or if it 'spits out' something else, then, as donalexo also writes, we can forget about the whole thing.

Again: The pressure reading provides very little information about blow-by, as long as the ventilation system in the crankcase is functioning correctly. To calculate the flow velocity from the pressure reading, you need to know the exact flow resistance in the crankcase ventilation system, specifically in the section of the ducting between the crankcase, the oil separator, and the intake manifold.

@Matthias says:
'If you keep the engine running and carefully unscrew the filler neck, you'll feel the wind blowing towards you – you'll understand. The pressure always takes the path of least resistance. Therefore, compare the diameter of the KWG vent with the diameter of the oil filler neck.'

Comparing diameters is a rather crude approach. On the one hand, the slight vacuum before the compressor, behind the air filter, etc., helps with air evacuation. On the other hand, the oil separator also acts as a restriction.

But let's assume that everything comes out of the oil fuel nozzle. Estimating this volumetric flow rate is very subjective.


Regards,
Perwollf.

Hello,


So, that means increased air mass flow after the mass airflow sensor => potential power loss.


And that's precisely what can be measured using KGE pressure.
Furthermore, a diagnostic test using VAG-COM can also provide clarity on the aforementioned point. It allows you to see whether the mass airflow sensor (MAF) values are within the acceptable range.

EDIT: I admit that I haven't yet recognized the practical benefits of this.

Hi!

Perlwolf's thoughts give me another idea: Blow-by gases contain a relatively high proportion of exhaust gases. This exhaust gas is added to the intake air, which consequently displaces some of the oxygen. In a way, blow-by is a kind of mini-EGR (Exhaust Gas Recirculation).

To increase the cylinder fill level by a small amount, one could directly vent the exhaust gases at full load, which would further raise the soot limit.

Of course, only minimally. And only for motorsports purposes, naturally.

@perwollf

ich streite gar nicht ab das dein Projekt ein interessantes Ergebnis liefern wird zumal du dir ja wirklich weitgehende Gedanken zu dem Thema machst. Ich finde es sogar gut. Da ich aber las das du das nötige Equipment erst in zwei Wochen hast dachte ich dir wäre ersteinmal damit geholfen. Normalerweise bei einem neuen Motor ( auch noch bei meinem <50tkm gelaufen ) kann man den Öleinfülldeckel abnehmen und muss dann nur aufpassen das einem der Ölnebel nicht alles versaut. Der entweichende Luftstrom ist sehr gering. Ich weiß das das Messen so nur subjektiv ist. Auch weiß ich das diese Methode zu keinem konkreten Zahlenwert führt. Wie aI'm sorry, but I need more context to provide an accurate translation. Please provide the text you would like me to translate from German to English.
"Personally, I blocked the AGR valve in my car. However, even after doing that, I still felt that there was too much oil mist in the intake manifold, so I installed a ProVent (a filter that separates oil mist from the blow-by gases that escape from the crankcase ventilation system). Through this modification, I was able to observe that there is likely a slight back pressure in the original configuration. Of course, I know that a vacuum is created on the intake side, and I believe that this vacuum likely more than compensates for the resistance in the pipe. However, if a larger opening is created, any potential overpressure is more likely to find a way to escape through that opening. Alternatively, one could simply block the crankcase ventilation system."

Well, what can I say? I thought I could suggest a quick and relatively simple method, although it won't provide any specific numbers.

Meanwhile, I fully understand the value of your project and find it interesting, and I'm sure it's also useful.

I apologize for misunderstanding you.