Impellor problems

Occasionally my fresh water impeller gets an airlock, and fails to pump water through the heat exchanger. This is on a Universal 22 deisel in my 1983 Morgan Nelson/Marek 36R.

I've already replaced the entire impeller assembly, the intake strainer, and all the hoses, and even added a back-flow check valve before the strainer.

The airlock pops up randomly, either when the engine is first turned on, or when we idle for a while. Not all the time, though, and usually it starts up just fine. Never had a problem with this when the engine was running with power, only on start up or after idling for a while.

We can fix it by turning off the motor, and filling the strainer cup with water, and re-starting.

We can still see air accumulating in the strainer basket, and flowing as bubbles through the intake hose. But we cannot figure out where the air is coming from. The intake system does not leak... at least no water leaks out of it. The intake is well below the waterline and well forward of the stern. The strainer is slightly above the waterline, as the manufacturer recommended. We've greased the strainer lid to prevent any leaks there. It seems unlikely there would be that much air dissolved in the water, but if there is, why is it just a problem on my boat?

Again, it runs just fine most of the time with some air in the intake system. Even with the basket mostly filled with air, it still draws water in and pumps it through the exchanger. I have never had it develop an airlock while running above an idle, even when motoring all day. The airlock, when it happens, is usually at start-up or occasionally when idling for a while. It does NOT appear to be related to wave action.

Anyone got any experience with this???

 

Do you have a gear or an impeller pump?

 

Don't think I understand your question. I have an impeller. Its a separate mechanism that bolts on to the front of the engine block, and houses a five-lobe star-shaped flexible propeller-like pump that rotates in an eccentric housing. The pump is driven by a shaft that keys into something inside the timing gear housing.

 

The advantage of such pumps over most other designs is that they do not need priming: they move air quite good an can pull a considerable vacuum, albeit not very long because rotating dry quickly wears the impeller.

Normally there can be no air visible anywhere in the circuit because the intake is under the hull. So you have a leak in an unexpected place between the intake scoop and the pump, beyond that point there would be water leaking at rest and you would have noticed it.

So your main suspects are the strainer and the pump. If you see air in the strainer already, the housing or plumbing on the intake side is leaking. It may be a seal, a cracked hose or a hose clamp applying uneven pressure to the hose under it, creating a gap.

In my own boat I use no check valves, no strainers and no hose clamps on the intake side of the Jabsco impeller pumps. There is no positive pressure there, so a tightly fitting hose cannot leak or pop off.

 

If you are seeing air in the strainer, that is the source of your problem. Check the strainer carefully for an cracks. Remove the hose barbs and reset with teflon tape of pipe dope. Double clamp the hoses, make sure the clamps are tight. Most strainers have a gasket, they get hard and brittle, change it or replace if missing. The cause of the air leak can be so small that you won't see the water won't leak out. You don't see it under power because the pump is pulling sufficient water. At idle or rest the pump isn't pumping enough to overcome it.

 

Is there a vented loop installed in the water system? Deep draft installations usually require one and it should be installed between the raw water outlet from the engine and exhaust elbow. If it is installed between the inlet and raw water pump it would allow air into the loop and at low speeds the pump may likely not be able to pump enough to overcome the air leak. Otherwise, depending on its location it may be allowing the pump to lose its prime when sitting. Also the height of the siphon break might be putting too much head on the pump at low speeds if it is too high.

Also look for a kinked or collapsed hose somewhere in the system causing back pressure. A kink you should be able to see. A collapsed hose may not be visible externally. You need to check the remainder of the circuit from the pump outlet to the point where it dumps into the exhaust.