Soapy water is used for finding pressured air leaks, look for the bubbles? A vacuum leak is air being sucked in, no bubbles. A spray bottle with plain water can be used, then when the water reaches the intake manifold your idle will drop slightly. But that only works with a large crack.
Generally, vacuum hoses over 10 years old will start to develop cracks, especially where they make bends around engine componants and where they connect to various nipples. These areas will appear to be softer than the rest of the line.
Some on this board swear by new silicone vacuum lines. I haven't done that yet. I just replaced the elbows when they appeared to get soft.

Usually, the front O2 sensor gets a code that the engine is running lean, adds more gasoline to the mix, and your gas mileage goes down. Or the O2 sensor, front, cannot keep up with the fluctuations of a leaky vacuum line and causes the ratio of the rear O2 sensor to also post a code.

Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)