Thread: 300Tdi vac pump leaking oil

I have a 300Tdi vacuum pump that looks like it's leaking oil out of one of the Welch plugs.
It seems some folk have repaired similar.
Is it the Welch plug that's leaking? Or is there a bigger failure inside?

Tony
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[QUOTE=TonyC;3233793 It would also seem that if there's that much oil there, it's likely that the piston seal is damaged.Tony[/QUOTE]

The cap seats on the main body via a seal. Leaks usually occur through this seal when the rivets holding the cap stretch. Seal may not be damaged. Replace rivets with small tensile bolts. The bolts will need to be re-tightened over time.

From memory welch plugs don't have a rubber seal, but rely on a metal on metal seal. I bought one new placement vacuum pump to find the welch plug rocking in its hole - could almost remove it with my finger.

One welch plug seal is subject to evacuation of air/vacuum (less likely to leak oil), The air is then expelled via the other welch plug seal into the engine block with little resistance via an open gallery. The oil comes from lubrication of the piston - I don't think the oil is under any real pressure, but rather has a lazy passage under a poorly sealing welch plug - if under real pressure you might expect a bit of a spray pattern when coming from under the welch plug, either that or the bulk of it (if that is the case) gets scooted into the engine block with the expelled air via the open gallery. The welch plug holes are drilled to allow, as you say, the insertion and crimping in of the valves. Pity they used the lid for part of the system. Better if all the valves were inside the body and the cap was a screw down lid on the whole lot.

I pulled the welch plugs, threaded the holes, machined a couple of threaded steel plugs, wrapped the plugs with plumbers tape before insertion - but that is a lot of work using a lathe, but seems very effective. The bolts replacing the rivets remain a problem over the very long term. Some people double the thickness of the gasket between the vacuum pump and the body of the engine, rationalizing that this extra thickness means the piston shaft rides with less pressure on the cam and therefore less pressure on the cap rivets - and presumably for the benefit of the welch plugs as well. Would be interesting if anyone has has a welch plug blow out, although without filming, it may have just fallen out.