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Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
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Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
Last edited by spartonboat1; 10-18-2017 at 09:19 PM.
50 Years on the Great Lakes...
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10-23-2017 02:15 PM #13
Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
My MDJE started doing it again, with only about 3 hours since I would say the pump was completely rebuilt and definitely not run dry during that time. I think it's the back pressure from the exhaust at the mixing elbow pushing the water prime out of pump. For winter project I'm going to take everything from exhaust elbow to exhaust thru-hull out and inspect. If I undo pump outlet hose and turn over for 5 seconds I get prime back. It then works like a champ once I put hose back into heat exchanger. I already replaced all the hose from raw water intake to heat exchanger and removed the heat exchanger and flushed it, on top of rebuilding pump. The water lift muffler and exhaust hoses are probably 8 years old but hardly used, but betting obstruction somewhere in there causing high back pressure. Unfortunately won't know for sure until Spring time at this point but will advise. Funny, I look for answers on the internet and not uncommon I find my threads on Sams from up to 10 years ago with the answer.
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Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
I will watch with interest. My observation is that the slightest loss of good fit of the impeller to the bore, plus good fit of the two primer plugs can affect the prime. So since you have, I believe, a new wear plate and new cover, plus I assume a dimensionally correct gasket, prime ought be be good. If possible, you might try replacement of the cover gasket with the use of Permatex Ultra Copper, i.e. omit the gasket. My tech used the Permatex on my Heat Exchanger, when he could not secure gaskets and it has held up for a long time. I has never failed, when I use it.
Have you had a chance to check the bore depth all around, with the wear plate in place. Maybe a small amount of material is under the wear plated.
Otherwise, again, will watch with interest, as I am kibitzing at a far distance.50 Years on the Great Lakes...
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10-25-2017 03:32 PM #15Senior Member
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- Jun 2007
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Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
Positive displacement pumps should be self priming. You probably have an airleak somewhere.
FTFD... i drive a slow 1968 41c381
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10-25-2017 04:12 PM #16
Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
Pump primes no problem when I take outlet hose off heat exchanger and let run into bucket with generator running. There is allot of pressure on it to if I try to hold it back with my thumb. Turn generator off, put hose back on generator and everything good again for at least the next time I run generator, which is usually immediately there after. As far as I can figure, raw water will not make it out the exhaust if generator sits unused for week or so OR I run the boat without generator running, then try to start at later time. If I go to start after either of these instances, no raw water out exhaust until I take hose off heat exchanger and prime it again. Given simplicity of system and ruling out pump is out of spec (about completely rebuilt), no obstructions in heat exchanger (cleaned out), raw water hose (replaced), thru hull (strainer clean and inspected mid season when hauled for bottom cleaning) or mixing elbow (inspected), seems highly probable issue with exhuast hose or muffler. This same system, untouched for at least 5 years, never had a prime problem before. It started day one after launching back in April, perhaps water froze in muffler and cracked a baffle loose causing an obstruction. Bitch to get at that muffler and exhaust hose, but only place left I haven't inspected and/or replaced since this problem started.
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Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
I haven't read the thread in detail in a while. Any possibility of the anti-siphon valve sticking and raw water filling the muffler up when the unit is not in use?
Semper Siesta
Robert Clarkson
ASLAN, 1983 55C #343
Charleston, SC
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10-26-2017 08:29 AM #18
Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
There is no anti-siphon device in this batch of plumbing. The generator exhaust is above waterline and probably 2' loop of exhaust hose from that thru-hull to muffler, can't see water coming in that way and definitely not coming in the other direction from generator.
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10-26-2017 09:16 AM #19
Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
If the pump was rebuilt and the problem persist you have a air leak ond the suction part of the system. Take a look a the strainer if you have one, you may have an air leak in the cap or in any of the connections. I hope this help
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11-27-2017 09:05 AM #20
Re: Onan MDJE Raw Water Pump Not Pushing Water/ Cure
So it turns out there was an internal tear in the outgoing 1 5/8" exhaust hose! I have to give Boatsb some credit here - I read on other post somewhere on sams that he found inner lining separated and blocked flow. I may not have checked this without reading it there first as possibility. I believe exhaust gas caused so much back pressure from this hidden inner hose restriction that exhaust gas went through the mixing elbow, heat exchanger, and to outlet side of raw water pump where it could not overcome the additional head pressure needed. It's worth noting that tear was at a 70 degree bend in hose that was not part of original installation. You know how your supposed to loop the hose high above the waterlift muffler to exhaust thru hull? Well it looks great when you first do it (note this was good quality Trident 252 wire reinforced exhaust hose), but it apparently sagged over the years and made this bend down low, which was well hidden from any normal view. I'm going to zip tie that exhaust hose loop up so it stays on next install
I also took the end caps of the generator heat exchanger (it's an aftermarket Sendure) and noticed that half the tubes were clogged on the zinc end. The zinc pencils, which have been replaced fairly regularly, souped up into a big mess blocking half the tubes. Nothing a scraping out with a screw driver, wire coat hanger, and hose didn't fix.
As an aside, picked up a $30 endoscope on amazon (see below) that you can view thru your iphone. I scoped down exhaust tube and found the flap of delaminated tube. Measuring how far down I put endoscope is where I cut exhaust below. While endoscope has led lights on end, this one was worthless in illuminating view. However, I shone a flashlight in there at same time and that got it done. I also used to scope out my main engine heat exchangers thru the zinc plugs, what a mess inside! I'm going to remove the end caps and start cleaning that way over winter and follow up with phosphoric acid cycling in spring.
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