Onan generator - raw water not flowing

b_arrington

b_arrington

Well-Known Member
GOLD Sponsor
Feb 21, 2007
3,485
Setauket, NY
Boat Info
Back Cove 37
AB Ventus 9VL
Engines
Cummins QSC 8.3 600
Need some suggestions on resolving an issue with my 9kW Cummins Onan generator.

Raw water is not discharging from the boat, but I have good flow from the pump. I got a no water flow alarm and shut down, and found the raw water hose had disconnected at the heat exchanger.

Setup details:
  • Cummins Onan 9kw generator
  • Sound enclosure
  • Water-separating muffler. Cummins muffler with Centek water separator.
  • Groco sea strainer.
Here's what I've done/checked.
  • Strainer is clear, sea cock open. Also using a fresh water flush cap to supply water.
  • New impeller. Old one was intact - no missing bits.
  • When the generator starts, the raw water hose from the pump to the HE pressurizes. It swells, and pulses about every 6 seconds. Seems like this is excess pressure, and why it blew off the HE.
  • Removed raw water hose from the pump: strong flow.
  • Removed the zinc from the HE. Strong raw water flow from that hole, raw hose does not pressurize.
  • Removed the HE end caps and flushed - clear and no apparent debris. Running generator with HE end cap removed produced strong flow.
  • Closed the sea cock for the exhaust water to divert it out the side of the both with the exhaust gases. No water was discharging. Exhaust gases ran fine.
  • At last run, the generator ran with no alarms but was getting hotter than normal so I shut it down.
The only hose I haven't checked yet is the raw water from the HE to the exhaust elbow. It runs behind the generator and is very hard to access.

Seems like there is some sort of blockage in the raw water path between the heat exchanger and the mixing elbow.

Anyone have suggestions or words of wisdom on this one?
 
b_arrington said:
Need some suggestions on resolving an issue with my 9kW Cummins Onan generator.

Raw water is not discharging from the boat, but I have good flow from the pump. I got a no water flow alarm and shut down, and found the raw water hose had disconnected at the heat exchanger.

Setup details:
  • Cummins Onan 9kw generator
  • Sound enclosure
  • Water-separating muffler. Cummins muffler with Centek water separator.
  • Groco sea strainer.
Here's what I've done/checked.
  • Strainer is clear, sea cock open. Also using a fresh water flush cap to supply water.
  • New impeller. Old one was intact - no missing bits.
  • When the generator starts, the raw water hose from the pump to the HE pressurizes. It swells, and pulses about every 6 seconds. Seems like this is excess pressure, and why it blew off the HE.
  • Removed raw water hose from the pump: strong flow.
  • Removed the zinc from the HE. Strong raw water flow from that hole, raw hose does not pressurize.
  • Removed the HE end caps and flushed - clear and no apparent debris. Running generator with HE end cap removed produced strong flow.
  • Closed the sea cock for the exhaust water to divert it out the side of the both with the exhaust gases. No water was discharging. Exhaust gases ran fine.
  • At last run, the generator ran with no alarms but was getting hotter than normal so I shut it down.
The only hose I haven't checked yet is the raw water from the HE to the exhaust elbow. It runs behind the generator and is very hard to access.

Seems like there is some sort of blockage in the raw water path between the heat exchanger and the mixing elbow.

Anyone have suggestions or words of wisdom on this one?
Click to expand...
Brad, I think I would look at the syphon break valve. It should be mounted higher than the generator so perhaps you can get at it. Most have a removable cap to check for obstructions.
 
I think I found the problem. Looks like a plugged heat exchanger. Soaking it in some barnacle buster.
 
A fouled heat exchanger was indeed the culprit. Soaking in barnacle buster plus a little rodding with a wire did the trick. All back together and working running nicely.
 
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