Well, at 70 - 75 mph the turbo is running and compressing the air. This is due to the load on the engine. A better test would be to drive the car at a lower speed and check the boost.

You might also check the wastegate control hose at the compressor sousinh. It's a 1/4" hose which goes from the housing to the wastegate diaphram under the turbo and close to the block. If this hose is damaged, you will get uncontrolled boost, the engine will shut itself off momentarily and can lead to engine destruction. (worst case)

Also, ckeck the vacuum hose from the boost gauge to the manifold for breaks or splits, loose connections and even oil in the line.

Check all the vacuum lines at the intake manifold including any of the large ones, they can split too. Lastly, check the turbo inlet hose. The rubber deteriorates at the bottom by the turbo due to heat and oil from the engine, the long hose from the intake side accross the top of the engine to the inlit hose. I'm on my third one so far.
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Warren Bain - '86 744Ti M46 >290Kmi, '99 V70Glt > 45K mi, '96 965 >100Kmi Wifemobile near Manassas Va.. Check the 700/900 FAQ via the 'features' pull down menu.