I have a 92 300D 2.5 the oil pressure gauge goes to 3 or all the way up as quick as I turn on the switch. It does on change as I idle or idle with the a.c. on high.But when I turn off the engine it slowly drops down as the pressure goes away. any idea?

Read this....

w124 oil presure gauge. Noraml to hit 3 bar?

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92 300D 2.5L OM602 OBK #59

I recently tapped a 1/8 inch pipe hole into the top cover of my filter partly to install a oil pressure alarm and partly to check what type of oil pressure I am rally getting. (I had also been told several times by one of the members that the pressure goes over 100 psi. I did not believe this.). At cold idle the pressure went up to 90 psi (aprox. 6.2 bar) and did not increase when brought up to 3000 rpm. I drove around and got the engine hot and did the same test and found the hot idle to be about 18 psi (aprox 1.5 bar showing on the gauge). The hot pressure at 3000 rpm dropped below 90psi some where more than 85psi but less than 88psi. So what I learned is that if my car was new the max oil pressure could be above 100 psi just as the other member said. The next thing I learned is that a 1-3 bar pressure gauge is not so accurate.
Look in the Official maual where it tells you how the oil pump worked and the part where they speak about the oil pump pressure relief valve. This will give you an idea what type of pressue the pump is supposed to put out. When I was doing the hot oil test at 3000 rpm you could see the gauge needle would rise up to 90 psi than drop down to the upper 80s psi. I believe this was the relief valve opening.
Sound like your car has really good oil pressure but I would read the manual.
It would be nice if the had a gauge that showed the whole range of the actual oil pressure.
I took the oil pressure reading where I plugged the oil pressure sending unit in.

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84 300D, 82 Volvo 244Gl Diesel