Why does the vacuum pump on my inboard diesel keep running, and how can I diagnose it?
There's a small unit beside my inboard diesel that I've been told is the vacuum pump, and it seems to run whenever the engine is on. How do I identify it and check whether the problem is a leak, a faulty switch or the pump itself?
Answer
A demand-controlled electric vacuum pump usually keeps running because it cannot reach its cut-out vacuum, air is leaking into the system, or the switch or relay is faulty. Identify the component first: the cooling-water "vacuum valve" beside many marine diesels is a passive anti-siphon valve, while some fuel pumps run whenever the ignition is on.
Identify the pump or valve
With the engine stopped, trace the hoses and read the label. A component's position beside the diesel does not tell you what it serves; a toilet vacuum pump, for example, may share the engine compartment.
- Cooling-water anti-siphon valve: usually mounted high above the engine and connected to the seawater cooling circuit, sometimes with a small vent hose. It has no electric motor.
- Electric fuel lift pump: connected between the diesel tank and engine fuel system, with electrical wires. Continuous operation can be normal, depending on the installation.
- Electric vacuum pump: connected to a vacuum reservoir or appliance through air or sanitation pipework, commonly controlled by a vacuum switch.
A standard Volvo Penta MD2020 installation does not normally need a separate electric vacuum pump to run the engine. On boats such as a Hallberg-Rassy 31, the component described as a vacuum valve is often the cooling-water siphon break.
Stop the engine and isolate the relevant electrical supply before disconnecting wiring or hoses. Close the seawater seacock before opening cooling-water plumbing, and reopen it before restarting.
Water from the vacuum valve
On a valve-type anti-siphon device, cooling-water pressure should close the small valve while the engine runs. When the engine stops and pressure falls, the valve admits air to break the siphon, preventing seawater from reaching the exhaust and potentially the cylinders.
Water escaping through the small vent hose while the engine runs commonly means salt, debris or a damaged seal is preventing the valve from closing. Some installations, however, use a deliberate continuous bleed through a small hose to a proper discharge point. Establish which design you have before treating the flow as a fault.
- Remove the valve cap or insert following the manufacturer's instructions, noting the position and orientation of the spring and seal.
- Rinse away salt deposits with fresh water and inspect the rubber element and seating surface.
- Replace the seal if it has hardened, split or lost its shape. Replace the complete valve if its seat is damaged.
- Reassemble, reopen the seacock and run the engine while checking the valve and connections.