I work with HP GC/MSD (5890, 5971). I am having a big problem with the diffusion pump, it moves oil into the detector chamber. This results in extremely high background and a lot of extraneous peaks on the chromatogram. Has anyone experienced this kind of problems? Is there any remedial action possible or the pump must be replaced?

You have a very big problem, and it may not be with the diffusion pump. Check the rough pump to be sure it is functioning correctly and that there are no leaks between the rough pump and the diffusion pump.

Does the diffusion pump bleed oil into the chamber constantly, or was the problem someone venting the vacuum at the rough pump instead of at the interface?

Once you get a large quantity of oil in the chamber everything must be removed and cleaned very thoroughly or the contamination will not go down.

You basically have two problems. The first is finding and fixing the oil leak into the vacuum chamber. The bigger problem will to be adequately cleaning the system. You probably will need professional help to get this system back to proper operating condition.