Vacuum in Cold Conditions

Hi,

'I have an LDA (Lean Direct Injection) system in my ALH Leon, and I've recently noticed a slight vacuum (approximately 40 mBar). However, this behavior only occurs when the engine is cold (at idle or under load). When the engine is warm, the gauge reads zero. If something were clogged, shouldn't this vacuum always be present?'

Hi,

It's the same for me. The turbo runs less strongly at idle when the engine is cold compared to when the bearings are warm.

I would initially doubt that there would be a vacuum issue with a blockage, because even when idling, exhaust gases flow through the turbo and should provide enough boost to supply the necessary amount of air.

Best regards,

Jan.

Whether there would be a vacuum due to a blockage... I would first doubt that...

However, Ulf writes the following in his expert article:

In case of blockages on the suction side of the turbocharger (due to sucked-in leaves, completely dirty air filters, etc.), the turbocharger intake pressure can drop far below the ambient pressure while the engine is running. However, on an LDA with a suction range, blockages in the intake area reveal themselves through a 'peculiar vacuum' during acceleration and/or idle.

SeatLeon wrote:

.... the loader's inlet pressure ....

Those who can read have a clear advantage!
"Lader-Inlet pressure" does not refer to the boost pressure. The boost pressure is the "lader-outlet pressure".

Hi,

Hmm... In my opinion, neither of them really fits.

The charger's INPUT pressure (which you won't see on the LDA) should always be lower than the normal pressure when the engine is running, while the output pressure behaves as described above.

In cases of extreme blockages, I can imagine that the described scenario might occur (i.e., a vacuum on the outlet side as well), but the vacuum (at the inlet) would then reach extreme values under load, which would either clear the blockage or become noticeable in the driving behavior.

Best regards,

Jan.

Whoever can read has a clear advantage!
'Lader-Inlet pressure' does not refer to the boost pressure. The boost pressure is the 'lader-outlet pressure'.

I understand that already, but Ulf also writes... On an LDA with a suction area, blockages in the suction area become apparent.

And I highly doubt that Ulf, in his technical article about installing an LDA (/viewtopic.php?t=3224), assumes that anyone is measuring the boost pressure at the charger inlet - because there is no boost pressure there. The tapping point is described as being in close proximity to the EGR valve, and the above quotes are intended for troubleshooting.

Hi,

That's fine. / That will be okay. / It's alright.

"Before the turbocharger, the pressure will always be negative, depending on the flow resistance of the intake path (e.g., the air filter). This value should be quite low at idle. After the turbocharger, the pressure can only be negative if the turbocharger is not working, meaning the engine is operating as a naturally aspirated engine."
The rest can be inferred from Jan's description:

Jan6K wrote:

When the turbo is cold, it runs less powerfully in idle compared to when the bearings are warm.

SeatLeon wrote:

I understand that already, but Ulf is also writing... On an LDA with a suction area, blockages in the suction area become apparent ...

That is completely correct. However, you cited the technical article incorrectly and incompletely.

A negative pressure on the LDA (Laser Doppler Anemometer) can be perfectly normal under certain load conditions. Especially when the system is cold, I would not attribute much significance to the negative portion of the LDA reading.