Q&A

car has travelled 105000 miles. the oil pressure operates and pressurises as normal when the vehicle is cold and when it gets to normanl operating temperature the oil preesure will increase when the engine revs but when you get to 2000+ rpm the oil pressure completely drops and the red light comes on. then when you take foot off accelerator the pressure comes back to normal.

The mileage on your engine seems very low to expect any engine problems to do with oil pressure. Have you put in a mechanical gauge to verify actual oil pressure? It's not a good idea to drive the car until the problem is figured out. Cold oil will have higher oil pressure readings all right as the oil gets hot the viscosity drops as does the oil pressure. Is there any rod knock on start up from cold indicating bearing damage?

motor runs sweet until it gets hot. I personally think the oil pump is shot. car isnt losing oil or leaking. it runs fine when cold but slug performance when hot.

Your car has low mileage for it to have worn out the oil pump. Oil pump wear is due to dirt or debris in the engine oil, which would also have worn the con rod bearings and main bearings. My hope would be is that the by pass valve has a little debris under the seat and is bleeding off pressure. Worse is something is cracked in the engine and as the engine expands with heat the crack expands and bleeds off oil pressure. I am not familiar enough with Mercedes, call a few Mercedes independent repair shops ask them if they have seen thsi before and look on the internet for engine rebuilders that specialize in Mercedes and ask them if they have seen an oil pressure problem with these engines.

change the oil pressure sending unit. This is a common problem, the coil of copper wire inside the sending unit has a brake in it, when the engine is warm, so is the sending unit and the coil of wire expands and opens up the brake in the wire. When the pressure gets up to a particular range it will align with the brake in the wire and the pressure will drop all of a sudden due to the lake of signal to the gauge.