I just recently bought a 2000 Bonneville with the supercharged 3800 engine. I was on a road trip and the oil pressure gauge indicated that I had lost oil pressure. I naturally freaked and had it towed the rest of the trip home to my mechanic. He did a bunch of electrical tests to it, cleaned and replaced some leads, charged me about 300 bucks and said it wasnt doing it anymore so it must be fixed. I was, needless to say, righteously pissed off when almost immediately after I took it out of the shop the pressure started to jump wildly and would fall to zero. When idling the pressure is at zero but when on the highway it seems to maintain right around 50-60 when I have my foot on the pedal, but when I let off the gas at all it falls to zero or close to it. Sometimes it will read 120 psi solid then just drop to zero again. When I first start the car it will appear normal for about 3-5 miles then start acting weird again. Anyone else know what this could be? Any advice or guidance would be appreciated.

Your first order of business should be to confirm that the oil pressure is, in fact, all over the place. Just simply verify the oil pressure with a mechanical gauge.

The oil pressure senders are a known problem area, but when they fail, they usually peg at max, or can flip from 0-max. This isn't the typical way they fail, but it could be the likely culprit.

I'm leaning towards a possible faulty instrument cluster. But like Randall said, get a mechanical gauge on the engine and verify first hand exactly what'* going on.

I have had two OEM oil pressure sensors fail. The first one pegged the gauge at max pressure, the second one that failed showed the same symptoms you are describing.

A course of action you should consider is to replace the oil pressure sensor. Your post does not indicate that your mechanic replaced it.

I think you have is a bad oil pressure sensor.

I think you also need a new mechanic.

Yeah That'* what Im thinking. I've been going to him for years and he has always taken care of me but this is souring me for him.

Im going to see if I can get him to swap out the sensor for me for the cost of the part seeing as he did 300 dollars worth of fruitless diagnosis.

Sensor replacement is under 15 minutes.

I had an almost identical problem with my 2002 SSEi. My "mechanic" wanted to change instrument clusters, instead I changed the oil pressure and temp sensors.

Oh, and the mechanic got changed too.

Like eveyone is saying, it most likely is the sensor. If that does not correct the problem, take a look at the oil pressure relief valve located just above the oil filter. There is a spring loaded plunger that can get stuck that will mess with oil pressures.