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I'm not sure which hose you've tapped into, but the gauge is showing about 19 inches of vacuum in the picture. That sounds pretty normal for idling, although I haven't measured my Mazda's vacuum.

Most vehicles I've had vacuum gauges on seem to idle at or above 20 inches depending on such things as the elevation, whether it's in gear or the AC is on, etc.

At full throttle in gear on the road, the vacuum should fall to about zero.

Cruising at 60 mph, it should be between 10 and 15 inches or so, depending on the elevation and the engine load (hills, wind, etc).

It would take a supercharger or turbocharger to read a "boost" on the other side of the dial.
 

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but it has a turbocharger. is it not the skyactiv D which is a turbocharged diesel?
 

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While I can't comment on which line is right if you have the turbo diesel, what I can say is that seeing a vacuum at idle is completely normal. It will only show boost when the turbo is spooled and you're under load.

If you don't get into boost when accelerating briskly it's connected incorrectly.
 

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Oops, I'm in the US and completely missed the part about this being a diesel Mazda3, so disregard Post #3 . We don't have Mazda3 diesels here in the States.

If the Mazda diesel is like my Ford F250 diesel and almost all others, it has no manifold vacuum because of the design and operation of the engine. So, vacuum gauges are generally only used for gasoline engines with throttle plates, not diesels.

The vacuum reading shown in the Post #1 may be from a vacuum pump supplying vacuum to the brake booster or something like that. So, I'd say the vacuum gauge is installed on the "wrong" hose; however, on a diesel, there is no "right" hose anyway.

I happen to have a vacuum/boost gauge like yours on my F250. The needle does not go to the vacuum side for reasons noted above. The needle on the boost side moves as a load is put on the engine and the turbocharger begins pumping. When the engine is idling, the needle is on "0."
 
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