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Car had plenty of time to learn, but I needed more time to learn what was really happening. I tested the fuel pressure using the prescribed testing protocol. Condition 1: key on motor not running - fuel pump primes - can hear it running - fuel pressure goes to 40-42 lbs pressure at initial prime - pressure quickly dissipates Condition 2: key on motor running - fuel pump primes - fuel pressure goes to 40-42 lbs - fuel pressure remains constant regardless of throttle increase or decrease. - nasty black exhaust upon starting after (accelerator pedal depressed to get engine to start) - engine runs but not very well - good acceleration from idle to high rpm but rough idles I have noticed that the exhaust has been particularly foul smelling and not typical of Buick motors that I have had previously in 3.8l - black exhaust typically means additional fuel entering system - pulled vacuum line from the fuel pressure regulator - Eureka - GAS dripping from the vacuum line. This means fuel was being vacuumed into the intake via a bad seal in the pressure regulator causing multiple running issues. I have ordered a new fuel pressure regulator and am confident this will correct the running issue and the exhaust.

HI, I've done some more research and it appears that ACDelco 15-2490 is the correct part. At the very bottom of one listing on ebay for "High side pressure switch" it states that it is, in fact, the low side switch. Raining again here in England so no practical work today. Thank you all again for your input. Regards, Paul.

No senior moments, it really is sometimes obscure. After I used the part number from our British friend, it helped narrow the search area and then I found they have stuff in a sub-menu that I had not encountered before. If you click on the heat and a/c, you must look under a/c compressor and components, which is just a bunch of compressors and then on the "related parts" menu to find the switches. There are several pressure switches listed, including the Delco one, but I never would have found that without the suggested part number. Not only are some of the locations different, the temperature sensors are not interchangeable between some years either. Thread size and electrical connections are completely different, but it appears the pressure switch is the same for all years.

Good detective work on tracking down the problem. ?