Generator Cooling

I have an Onan 7.5KW diesel genset. I've run it for probably 3+ hours total. Friday morning it ran great for over an hour, cooked breakfast on the electric stove, heated up the hot water and charged the house bank some.

Friday evening I fired it up again and started to load it up. After a few minutes I decided to check the coolant flow out the exhaust. Barely a trickle!

I shut it down quickly and opened the engine compartment. It smelled hot and there was a bit of smoke in the air. Assuming a coolant flow issue, I left it off until I could troubleshoot some more.

Today I checked the strainer basket. No problem. Pulled the hose off the thru-hull, full flow of water. Pulled the hose off the raw water pump and was able to easily blow air in and heard it bubbling up from the thru-hull on the outside. Pulled the other hose off and blew through the heat exchanger and aqua-lift muffler, felt no restriction at all. Put the hose from the intake side of the pump down in the bilge, opened the sea cock and verified it flowed freely.

The belt looked a little loose, but it didn't appear to be slipping.

Took the pump apart, checked the impeller. Looked great. I found the box it came in, mfg date of July 2012, which confirms what the PO told me that it had been replaced as part of general maintenance late last year.

Put everything back together, including tightening the belt. Ran for 5 minutes. A little dribble out the exhaust, but no more. Shut it down.

Came back an hour or two later, checked all the hoses, made sure the supply hose was full of water, and fired it up again. Still just a dribble after 5 minutes.

Could it be I need to let it run longer to fill up the muffler? I'm running out of things that could be restricting the flow.

my waterlift takes a long time to fill...not sure in seconds total...but seems an eternity....not 5 minutes though!

if you are getting a steady trickle, that doesn't sound right.

I would rig a garden hose downstream of the pump and see if the system is restricted....or almost pull off the discharge side of the pump to see if the pump is working.

psneeld said:

I would rig a garden hose downstream of the pump and see if the system is restricted....or almost pull off the discharge side of the pump to see if the pump is working.

Click to expand...

Bingo! That's what I did right after posting. Only I went in before the pump. Plenty of water coming out the exhaust after that, so I knew there was no restriction downstream.

But in the process, I noticed water coming out from around the intake fitting on the pump. The fitting comes out through a hole in the bracket, and it's hard to get the host clamp on right. I suspect the PO had over-tightened the clamp too far out and cracked the end of the fitting.

I removed the cracked-off piece and there's still enough left to get a good clamp on. No water coming out now.

Re-connected to the thru-hull and ran it for almost a half-hour under a 30A load. Looks good!

So, if water could leak OUT, then I assume the pump was sucking air.

CaptTom said:

I removed the cracked-off piece and there's still enough left to get a good clamp on.

Click to expand...

Sounds like you found the problem. Does the genset have a working high temperature kickout? Don't forget to get a new hose installed ASAP. And all the others on the genset while you are at it - they are likely in similar shape.

I would think the pump should have restricted a lot of the flow.....when you say a lot...hopefully there was some restriction but it it's good to go...then so be it. Just watch that pump as it sounds like there may be interior wear issues.

The high-temp shutoff never happened, but it does have one. Hopefully I noticed the reduced flow it before it got too hot. I may get one of those exhaust hose temp alarms for it. I thought it was odd that there's no remote temp gauge. No hour meter either.

I expected a higher flow rate with the engine running and the water going in under pressure. Before starting, it didn't really run in at all, after the first second or two, so I expect the (stopped) impeller was restricting the flow quite a bit.

I had enough hose that I was able to cut off the damaged end. So I think I'm good for now. I'll order a new impeller, pencil zinc, oil and fuel filters, and probably hoses, just for spares, but I'm now confident a good PM was done one it last fall.

While your doing all that I would pull off the heat exchanger and blow it out. There may be a lot of rubber impeller vanes lodged in there from a previous impeller failure. I did this when I had a failure and blew out enough rubber to make three impellers.

It's baaaaaaaack...

After inspecting the impeller and finding it in brand-new condition, running on a garden hose for a while, then running through the thru-hull for about an hour, I was confident that the problem had been a poorly reconnected hose that was letting air in just in front of the impeller housing.

Then last night, after just a few minutes, the cooling water coming out the exhaust slowed to a trickle, and the exhaust hose into the aqua-lift started getting hot to the touch. During the test run this had stayed warm, but cool enough to keep my hand on indefinitely.

I shut down the genset.

I verified that the strainer basket was clean and there was full flow from the through-hull.

Here's my question: The strainer is not full to the rim. With the cover off and the through-hull open, it's only full up to the top of the basket, and BELOW the outlet where the hose up to the impeller is connected. I'm used to seeing the through-hull completely below the waterline, so this got me wondering.

I assume the whole hose contains nothing but air, all the way down to the strainer, too.

I've never thought about how much suction an impeller pump can pull, or about the need to prime one. In my experience the strainer is usually below the waterline, so this can't happen.

I'm going to test my theory that priming the hose may help, once I know everyone on shore and in the mooring field is already awake. Meanwhile, any opinions on my theory?

Some strainers the manufacturer recommends to be installed above the waterline...no that's not it. The impeller should fill the strainer even well above (up to several feet) the waterline.

BUT....it can't have any air leaks. If you can run the genset on a hose all day long (directly into the impeller intake hose after the strainer) then you'll have to check for an air leak as it can't be a restriction if you have full flow through the seacock and strainer with basket in.

Have or can you fit a hose from the pump outlet to a bucket to check flow there? that will test the strainer and pump.

Windmills point of checking the heat exchanger for vane pieces or other crud is a good one.

Sometimes a visual of the impeller is not enough. They can deceive you. Replace it with a new one and then you can mark that off the list.

And the winner is...

psneeld said:

BUT....it can't have any air leaks.

Click to expand...

This time I hooked the garden hose up to the hose I pulled off the seacock. Sure enough, I'd torqued down the strainer cover unevenly, or maybe there was a small piece of debris on the seal. Either way, water squirted out from under the strainer cover, so that means air could get in.

I shut off the hose, cleaned and re-seated the cover, and hooked everything back up. No leaks. Ran fine for 10 minutes on potable water. Re-connected the hose to the seacock and she's been running (and cooling) fine for an hour now.

This is the 2nd time I've had an air leak on the seawater side of the raw water pump in as many weeks. Also the 2nd time in my life. Go figure.

Batteries are back up to 80%, water is hot, fridge is cold and I can stay out as long as I want now

From now on the Honda 2000i comes with me though!