The fuel pressure regulator can be checked just by sniffing the vacuum line that goes from the FPR to the vacuum tree. If it smells like gasoline, there is a problem. It is easy to replace. but I forgot if the fuel rail needs to come off or if the holding screws can be accessed from the front.

While you are playing with the 850, check the elbows and other vacuum lines. They do get old and brittle. Check that orphan vaccuum line on the passenger sidde of the manifold. And at least take out the copper plugs, clean them and check the gaps -.028

I doubt the O2 sensors are bad yet, but continued running rich will make them worse.

Sorry about the berating, but sometimes I get a little frustrated with the questions. I guess you get a response equal to the question :)

Klaus
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Why are Volvos so endearing? Its just a car.