Dishwasher was filling with dirty water, so I raised the outlet pipe above the level of the kitchen sink trap. In doing so I noticed that the sink outlet pipe slopes slightly UPWARD into the grey soil stack from the upstairs bathroom... do I have a major problem... or am I letting my imagination take a walk on the vile side?

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sink outlet pipe slopes slightly UPWARD

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Assuming that means the trap arm is pitched slightly upward. Trap arm should be level or sloped slightly downward. However, that is unlikely to be the source of the dirty water in the dishwasher.

The dishwasher discharge hose should either go to an air gap on the sink flange or should be have a "high loop" in it by attaching it to the under side of the counter top. Otherwise dirty water from the sink will flow into the discharge hose and into the dishwasher.