Can I use Vaseline to lubricate a raw water pump impeller?
I am servicing my diesel engine and need to fit a new raw water impeller. I have lost the little packet of lube that came in the Jabsco box. Can I just use some Vaseline from the first aid kit to slide the impeller into the pump housing?
Answer
Do not use Vaseline or any other petroleum-based grease to lubricate a black neoprene raw water impeller. Petroleum breaks down the rubber compound, causing the vanes to swell and eventually tear off inside the engine cooling system.
Petroleum destroys neoprene rubber
Manufacturers like Jabsco, Johnson and Sherwood mould most raw water impellers from neoprene rubber. Neoprene flexes well to pump water but degrades rapidly when exposed to hydrocarbons. Smearing Vaseline inside the pump body coats the impeller vanes. The petroleum lingers there and softens the rubber over a few weeks.
Swollen rubber increases friction against the bronze pump housing. This drag can strip the splined brass hub out of the rubber centre or snap the vanes at the root. A replacement impeller costs around £30 to £50. If a vane snaps, the fragments travel downstream and block the heat exchanger tubes. You will spend hours dismantling the cooling system to pick out small chunks of rubber and cure an overheating engine.
Safe marine rubber lubricants
Genuine replacement impeller kits for Volvo Penta or Yanmar engines usually include a small sachet of lubricant. This contains silicone grease or glycerine gel. Both are safe for rubber. If you lose the sachet or reinstall an existing impeller after an inspection, you have three alternatives:
- Silicone grease: A tube of marine-grade silicone grease like Molykote 111 is inexpensive and safe for all rubber O-rings.
- Glycerine: Liquid glycerine costs a couple of pounds at a local pharmacy. It is slippery and water-soluble. Glycerine will not harm neoprene or nitrile rubber.
- Washing-up liquid: A squirt of Fairy Liquid works well as a temporary assembly lube. It washes away completely the moment the engine starts and draws in raw water.
Polyurethane run-dry impellers offer slightly more chemical resistance than neoprene. Manufacturers still recommend silicone grease or glycerine to prevent warranty invalidation.