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Old 08-10-2019, 09:25 AM   #1
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Prime raw water after zinc pencil replacement Perkins 6352

Folks:

Changed pencil zincs on main heat exchanger and exhaust mixer. But seems no water is now pumping through system, i.e., dry exhaust and no flow at the intake strainer.

Impeller is new, and wet when I open the housing. So it seems primed between the strainer/intake and the pump.

The zincs I changed are downstream of the impeller pump. Can there be an air lock there? DO I need to prime downstream somehow?

It's a Perkins 6.354
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Old 08-10-2019, 09:33 AM   #2
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Seacock open?

How worn is the inside of the water pump? The end plates may allowing water to flow out of the compression section of the pump.

Plugged heat exchanger, transmission cooler, or wet exhaust elbow?

Plugged external strainer or through hull?

Wrong impeller?

Try taking off the pump discharge hose and blow into it. If you can blow through the rest of the exhaust system, the pump should be able to pass water through it.

Ted
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Old 08-10-2019, 01:53 PM   #3
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You might have an air pocket, but that should go away on its own. A real air lock is difficult to achieve IF the water pump is making good supply.
First verify that, then
Backfeed the raw water system from the turbo water outlet with the water pump hose removed. If no flow, disassemble and clean all components.
You could have inadvertently dislodged debris that have been lurking in the system from a previous water pump impellor shredding.
Remember that impellor debris are not always found downstream of the pump, they can also hide upstream between the strainer and the pump.
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Old 08-10-2019, 02:27 PM   #4
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yes, kapnd, good suggestion. i did recently shred an impeller, but it had been pumping fine until i pulled and replaced the zincs. hoping to find a solution that didn't involve pulling out a bunch of hoses, but that hope is diminishing...

can't quite imagine how impeller shreds get upstream, but i've heard that before, so that's where i'm gonna start. should i replace hoses while i'm at it? what's the life expectancy of such things?
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Old 08-10-2019, 03:21 PM   #5
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The dreaded shredded impeller syndrome. I'm afraid there is no cure. Pull all the hoses and shine a light into all exchangers/coolers. Hoses don't have an end date, but a close look with LED light and mirror to look for cracking should be on your annual maintenance list. Soft spongy hoses are also ready for replacement. Water, fuel, hydraulic, and exhaust hoses should be checked beginning to end along with fan belt.

If I notice one cracked hose I'm apt to replace them all.
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Old 08-10-2019, 03:30 PM   #6
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The shredded impellor bits are forced through both of the gratings by the remaining rotating intact blades, both intake and output sides.
The intake side is tricky, because the bits move down and away from the gratings when the engine is switched off, so you won’t see them when looking inside the pump.
Disconnect the hose at the raw water strainer and flush from the pump side, also take the strainer basket out and look in there.
Good luck with the hunt!

Edit:
When changing out a shredded impellor, lay all the bits on the bench and reassemble, if all is recovered, no problem, but if you’re missing bits, keep looking, it’s highly unlikely that they have passed through.
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Old 08-10-2019, 03:32 PM   #7
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On the Perkins, impeller pieces tend to get trapped at the end of the hose going back into the block after the pump as there is a small metal pin like thing across the pipe spud that tends to catch the broken vanes. Pull that hose end off and see what you find
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Old 08-10-2019, 04:41 PM   #8
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"When changing out a shredded impellor, lay all the bits on the bench and reassemble, if all is recovered, no problem, but if youíre missing bits, keep looking, itís highly unlikely that they have passed through."

Then I could get a job in a museum repairing egyptian pottery (I was going to weigh the pieces and compare the weight against a new one, but decided to cast off the dock lines instead.)
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Old 08-10-2019, 07:14 PM   #9
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N

Quote:
Originally Posted by SoWhat View Post
Then I could get a job in a museum repairing egyptian pottery (I was going to weigh the pieces and compare the weight against a new one, but decided to cast off the dock lines instead.)
My boat has one engine, and it takes me to some really remote places, so I take great pains to make sure everything is up to snuff.
Wandering bits of trash in the raw water system would make me crazy!
Thereís a strainer just downstream of the pump that insures no impeller pieces can get beyond that point.
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Old 08-24-2019, 02:20 PM   #10
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So hereís the upshot. Plenty of impeller bits downstream at the oil cooler intake. But that was not the problem. There was an airlock upstream of the water pump, downstream of the trans oil cooler. Primed that 18 inch copper pipe, problem solved.
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Old 08-24-2019, 04:11 PM   #11
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Fussbudget,

I don't want to say that an airlock wasn't the problem. You are saying it is, and I sure have no reason to disbelieve you. And, I sure don't know how your plumbing runs.

But, I had two 6.354MGTs in my last boat and a 4.107 powering the genset, and have two 6.354Ms in my present boat. I have serviced all sorts of parts of those systems, getting a ton of air into it. It has never had a problem blowing it all out. I just try to make sure the impellers don't run dry without lubrication, because I'm afraid of wrecking them. When I asked my mechanic about air locking when he was working on things, he told me to go check the water in the back -- but they normally just blew most of the air right out and all of it within a few minutes.

A lot of friends have boats that "run hot" if they run at a high throttle. In these situation their temp gauges will slowly climb until they back down. One thing that struck me about my old boat (I haven't done an extended, extended test on my new boat) is that it could cruise WOT indefinitely without running any hotter.

I like to run very heavy to WOT for a few minutes each trip on the way back just to see how it does and stress test things. On the old boat with the turbos, I wanted them to get to run hot. One day on my old boat I was coming back and went WOT for the last few minutes, then we decided to extend the trip and go somewhere for lunch. In all of the discussion and turning around, &c, I forgot I was at WOT. I ran almost an hour that way. The engines were still at the same temperature as at normal cruise (I can't say the same for the EGT....or even the engine room)

Admittedly, on that boat I had all new coolers and exchangers and hoses and rebuilt raw water pumps. But, in well designed and built systems, when things are all clean and running well, they don't run hot. I know that wasn't a complaint of yours, just an observation of mine.

I guess what I'm saying is you might want to keep an eye on things. There might be some other inefficiency in the system that result in it not having as much ummf to drive out that air, e.g. more impeller or zinc bits or trash or scaling, somewhere else in the system that is cutting into its efficiency. So, there might be more going on than you fixed and, even if it doesn't affect you at a gentle cruise right now -- it might affect you if you try to run harder get some other junk building up, maybe in the same place.
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