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Discussion Starter #1
I connected vacuum dial gauge to the source port at intake, at ilde speed, the gauge needle is hitting the maximum and minmum readings, at frequency of engine rpm, like 15 times per second. Only when I open the throttle a little bit, does the fluctuations decrease gradually.
Even at 2000 rpm, it is still vibrating, say between 10-13 reading, but much faster...

Vaccum reading after the vacuum canister (buffer) is steady though - at 15.
Is this normal to the M5 eninge?

Thanks,
Karl
 

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Discussion Starter #3
I think I know why it is so unstable in E34 M5 vacumm reading:

BTW, I was trying to debug an engine problem and decided to test if there is problem in vacuum (leaking, etc...) and run into this findings

My explanation:
In most of the cars, the vacuume port is the "one and only" big chamber, usually on the throttle body with all cylinders contribute to the total vacuum pressure.
In E34 M5, there are six vacuum ports, one for each cylinder, and the the port I was reading from is the #3 cylinder. So it will have long pause of zero vaccum, as it will take two complete revoultions to have one short intake cycle... BTW, I also took reading of the others five cylinder vaccum ports, they are all the same as #3, very unstable.

That's also why my vaccum reading is stable after the "one way check valve", where it is also connected to the vacuum canister, which also control the resonance flap's open/close.

But then how do I monitor the condition of each cylinders at the same time? You will need six vaccum gauges to watch them all at the same time.

Or make a tube that connected to all six cylinders vacuum ports...
 

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Discussion Starter #4
I think I know why it is so unstable in E34 M5 vacumm reading:

BTW, I was trying to debug an engine problem and decided to test if there is problem in vacuum (leaking, etc...) and run into this findings

My explanation:
In most of the cars, the vacuume port is the "one and only" big chamber, usually on the throttle body with all cylinders contribute to the total vacuum pressure.
In E34 M5, there are six vacuum ports, one for each cylinder, and the the port I was reading from is the #3 cylinder. So it will have long pause of zero vaccum, as it will take two complete revoultions to have one short intake cycle... BTW, I also took reading of the others five cylinder vaccum ports, they are all the same as #3, very unstable.

That's also why my vaccum reading is stable after the "one way check valve", where it is also connected to the vacuum canister, which also control the resonance flap's open/close.

But then how do I monitor the condition of each cylinders at the same time? You will need six vaccum gauges to watch them all at the same time.

Or make a tube that connected to all six cylinders vacuum ports...
I found the source of my intermitten engine acceleration probelm: It is the spark plugs.
Somehow this time, the spark plugs did not last long, only 10,000 miles.
 

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Discussion Starter #6 (Edited)
No, It should not change the behavior - the wide vacuum fluctuations, especially at low engine rpm, is normal for E34 M5.
BTW, this makes debug vaccum leak by reading from this port location NOT very practical and useful. Use "smoking device" is a better way to see if there is leaking somewhere...
such as this:
 
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