by Fredjohn » Sat 09.02.2019, 15:19
The boost gauge is electrically operated, not via a hose. The likely problem is a poor earth connection, probably at the transducer. It's the orange thing under the bonnet, driver's (UK) side. See WSM electric section: Instruments page 24 of circuit diagrams.
Access to the gauge itself is via the removable panel on the top of the dashboard, but you probably don't need to do this if the problem is under the bonnet. Remember the boost only shows when driving, not when revving on the drive.
If there is a hissing noise from under the dash, then this is likely to be a vacuum hose for the heating/ventilation system. With the engine running and the fan on max, turn the air distribution knob to each position in turn, checking that the air flow does indeed go to where it should. Check 2: separately turn the hot/cold knob to check the flow changes temp. This works not by cutting off water from the heater matrix, but by re-directing airflow to the matrix or not. If either of these tests fail, then you will need to check the manual for the routing and location of all the vacuum hoses and reconnect as necessary. You will also need to be incredibly flexible with tiny hands and long fingers to do this!!!!
Hope this helps and good luck.
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