Odd boost behaviour noticed in vag-com

[muddyboots]

I was playing with vag-com the other night, logging various things to see if I could spot anything unhealthy that might explain my recent big decrease in mpg.

Anyway, what I noticed was the following:

Sat in the car, not moving, at idle. Car stationary throughout.
Watching requested boost vs actual boost in vag-scope.
Very sllloooowwwwwllllly increase revs with accelerator pedal.
Engine revs rise slowly (but steadily) until around 1800rpm; up to this point requested boost and actual boost almost match (as you'd expect).
Then the engine starts to feel slightly uneven.
At approx 1900rpm the revs jump up by 3-400rpm, and the actual boost increases noticeably above requested (about 0.3-0.4 bar higher IIRC). This is without me pressing the accelerator any harder.

Any ideas why this happens ?
It's very repeatable.

 

[craig-pd130]

I'd guess it's because the VNT mechanism moves significantly at the ~1900rpm point.

Because it's vac-controlled the movements won't be precise, they will pulse back and forth as the N75 pulses to modulate movement. Perhaps the slight overboost here helps to get the compressor REALLY spinning ....?

 

[muddyboots]

Do you want to see if yours does the same ?

Next time you're sat still somewhere, just increase the revs realllly slowly and see if there's a jump around 1900 rpm

 

[sssstew]

im not sure this is the best way to check it though, cause the turbo doesnt properly actuate unless under load is my understanding.

 

[muddyboots]

Well I could understand if the "actual" boost was less than "requested" if there maybe wasn't enough exhaust gas to spin it - but in this case the "actual" boost is noticeably higher than "requested".

Maybe I have a sticky/jerking VNT mechanism. Perhaps I'll cycle it with vag-com and see if it makes any difference.

 

[sssstew]

requested is low cause it doesnt want any as its not under load... would that make sense?

but yeah, loads of wd40 on it to

 

[basstard]

requested is low cause it doesnt want any as its not under load... would that make sense?

boost pressure is related [quite strictly I'd add] to load,,, requested is low cos there's light load, or better, no load at all yet the turbo is spinning and it does produce pressure anyway, can't do any other way,,,

 

[micky 32]

I done the rev thing today. I edge mine slowly up to around 1900rpm and then it suddenly revs higher without pressing the peddle anymore. So must be normal.

 

[muddyboots]

Ah cheers for that, nothing untoward then.