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Q: low and rough idle on 1996 Volvo 960

My Volvo 960 idles low and rough. The diognostic code is PO172. (fuel too rich) it's either too much fuel or too much air, according to the auto part store. I think it might be the intake manifold gasket. Is this accurate, or am I missing something?
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A leaking intake manifold gasket is very common on this engine. The way I find it is to get a water bottle and spray a stream of water around the intake runners where they attach to the head. They normally leak at the ends,, near the #1 or #6 cylinder intake runner. If the gasket is leaking, the engine will stumble when the water is drawn into the cylinder from the vacuum leak.
An intake manifold gasket implies a vacuum leak. It is possible that there is a vacuum leak, but there are some other things that could be wrong as well.
1) TPSensor
2) O2 Sensor
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