Welcome, and I hope we can help.

It is not your fuel pump, yet.

Yes, your pressure regulator could be bad. What happens then is that fuel will go through the FPR, into the vacuum line to the vacuum tree at the throttle body, and into the manifold. When this raw fuel is dumped into the manifold, you will run rich and start fouling plugs.

The FPR is under the fuel rail, easily located by its vacuum line. There should be no gasoline in the vacuum line. The line on my 1995 850 was clear so it was easy to spot gas droplets.

Also, when my FPR failed, starting the car took a lot of cranking, and then there qas a puff of black smoke when the engine finally caught.

Other probable causes require you to inspect ALL of the vacuum lines, they are getting a little old. Any unmetered air entering the manifold will cause a rich condition.

Pull a couple of spark plugs and check their condition. If they are light grey, you are running too lean - time to check the fuel filter (less than $20).

Klaus
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Two great Volvos, a 1967 220 and a 1998 V70R