Onan MDJE Raw Water Pump Not Pushing Water/ Cure

[spartonboat1]

My Onan MDJE raw water pump stopped pumping water this year. I would help it get prime and it would pump for awhile, but then act erratically. Finally, I was out for a short day cruise, running the genny underway, and she lost prime again, overheating, losing internal coolant and the overheat switch shut her down; thank heavens the overheat switch works!

So I finally began to take a closer look, since I have seen this type of post before of MDJE raw water pumps failing to pump.

My poor mans micrometer found that the bore that the impeller sits in seems to be uneven in depth and esp. that it is deeper now that the width of the impeller. So instead of a tight/close fit for the impeller, there is a very slight gap between the end/cover plate and the impeller; about 1/32" or smaller.

I also notice that when I poured water into the upper priming port that it ran out fairly quickly from the lower port; that is with the cover plate and gasket installed. So that is another indicator of a tiny gap between the impeller and cover plate.

My Cure (Temporary or Permaent- not sure):

I just so happen to have a small sheet of 1/32" Copper Sheet Stock. I cut out a profile that is a good fit for the bore of the impeller cavity and fits also to the cam in the bore. I fine tuned the fit with my Dremel and chain saw sharpener grinder bit, for a fairly smooth edge.

When I had a good fit, I installed the impeller, with the ubiquitous Dawn detergent, and inserted the Copper insert into bore, which sat against the impeller at the the very outer edge of the bore. I installed the cover plate and gasket, including inserting the upper hold down screw fairly firmly, and rotated the plate into place, so that the copper insert would remain in the bore. I finally installed the other two hold down screws to the factory 16" #'s.

I filled up the Genny internal coolant reservoir with DD pre-mix coolant; it was low from the overheat.

Sorry, since this was a proof of concept, aka first time try, I did not take pictures; those will follow when can.

I did prime the pump upper port and found far less leaking out the lower port, which I felt was a sign of a tighter fit between the impeller and cover plate. I re-installed both the port plugs.

I have been getting no prime, up to now, so with fingers crossed, I went through the start up procedure, and it fired right away.

After about 10 seconds, spurts of water began appearing and at about 15 seconds, major flow began. This was a huge breakthrough, since I did not help the priming process, above the upper port. I then loaded the Genny by running the AC's and she ran fine. No leaks at the raw water pump and no unusual noises or vibration, which might have been due to my Copper insert. So all seemed well.

I stopped and started the Genny several times, always getting good water flow. I also ran her with heavier loads and she ran well, with no smoking. I fact, in the past, under heavy load she would smoke a little. I am wondering if she was not always getting enough raw water flow.

I shot various areas of the cooling system, with my temperature gun and all seemed be in normal ranges of 175-190 degrees, depending on where shot. That was also good.

I will go back in a few days and try her again. I will check the coolant reservoir to ensure she is up.

So if this fix holds up, I will continue to use it. Eventually I will attempt to have my pump bore re-worked to the correct depth and uniform dimensions.

So if anyone else starts to have raw water pumping issues, it could be your pump is worn beyond serviceable specs, esp. the end play between the impeller and Cover Plate, primarily due to wear at the end/bottom of the bore. I don't think these pumps are available anymore, e.g. from Cummins. If I am able to rework the pump, I will post that. I will also post some pictures of the internals, i.e. the Copper insert.

 

[sgharford]

Thanks for Post! If nothing else, nice to know others out there having the same problem (which I am). There is a wear plate on back end of pump impeller housing. I ordered one few weeks ago, along with new cam and cover plate, and just waiting for some time to put in (maybe today after the solar eclipse). Hoping that solves my prime problem. I rebuilt the pump about 5 years ago and probably have only 50 hours on it since then (have an inverter I use 95% of time). Figured I would have replaced wear plate then, but can't find receipt for it so hoping that's it. The problem of losing prime only started this season. Think I'll bring machinist gauges down and take some measurements while I have apart when I do replace parts per your findings to help quantify. Hope we can solve it! All and all my MDJE still runs well.

​

 

[spartonboat1]

Thanks for Post! If nothing else, nice to know others out there having the same problem (which I am). There is a wear plate on back end of pump impeller housing. I ordered one few weeks ago, along with new cam and cover plate, and just waiting for some time to put in (maybe today after the solar eclipse). Hoping that solves my prime problem. I rebuilt the pump about 5 years ago and probably have only 50 hours on it since then (have an inverter I use 95% of time). Figured I would have replaced wear plate then, but can't find receipt for it so hoping that's it. The problem of losing prime only started this season. Think I'll bring machinist gauges down and take some measurements while I have apart when I do replace parts per your findings to help quantify. Hope we can solve it! All and all my MDJE still runs well.

​

Thanks for the note, esp. that you are experiencing the same issues with pump losing prime. As for whether they replaced the wearplate, during the rebuild, wouldn't be the first time someone "saved you money, etc." and did not install a new one.

I earlier researched this morning and found a source for the 131-0158 wearplate. I then saw your reply, so the wearplate sounds like a good option. I ordered one at $6, so will install that, at some point. Apparently "very little" wear will cause loss of prime. Very little means not much wear to the naked eye, but enough to prevent a good seal for the impeller.

The measurements I made were of the impeller bore depth around the periphery of the bore. If you pull the cover plate and make some measurements, I would be curious if you found the bore depth to be greater than the impeller width. Since the cover plate is held slightly away from the body, by the width of the gasket, that would indicate a slight amount of clearance is needed, but wearplate wear exceeds that tolerance.

Thanks for your reply.

P.s., I have gotten frustrated with UPS and FX, so now use USPS. I understand that USPS is delivering "the last mile" for either UPS or FX anyway, in some cases. Also, if the item will fit in the mailbox, that protects the item. By law, UPS and FX cannot use the mailbox, or anyone else for that matter.

 

[sgharford]

As cover plate is not notched to compensate for gasket thickness, you would think the impeller would have to stick out that same thickness to get good seal, right? I know mine currently does not.

 

[stimmy]

I had the opposite problem with my Onan. When I bought the boat 10 months ago, it had a 20kw Northern Lights genny that was working fine, but the 20kw Onan (backup?) was not running; the broker told me the previous owner would not talk to me (and still refuses to do so), so I had to discover all the idiosyncrasies of the boat myself.
When I finally got to the Onan, I primed the fuel, and cranked it and it started easily after a few seconds (I have no idea how long it had been since it had run; it could have been years) but it seemed fine except the fan belt slipped completely while squealing loudly. Upon inspection of the raw water pump, it became obvious the impeller was about 1/8" too wide and was binding badly in the pump housing. A new impeller completely fixed the problem.
The old impeller was clearly used and worn (the "bulbs" on the vanes were somewhat flattened); could the impeller have somehow swelled from disuse? It made no sense.

 

[t109a60]

Excellent, and timely, discussion on Onan raw water pumps. I have a diesel MDJE 7.5KW.
I too am experiencing new low flow/loss of prime issues. I have checked the obvious things
(impeller, strainer, belt tension, back-flush lines) to no avail.

Question for the forum: How common is it to have these pump parts 'wear out' (wear plate, cam)?
My boat is approaching 40 years of age. I have owned for 9 years and up to now regular impeller
replacement has sufficed. I do not know the prior maintenance history of this pump.

 

[spartonboat1]

Update on the new Onan Pump impeller "wear plate". Pics to follow, after transfer from phone.

1) I pulled the old wear plate, which is very easy to do.
2) Remove the pump Cover Plate and gasket
-- 2a) In my case, next pull out (zero effort) my copper sheet "adapter"; about 1/32" dimension
3) Pull the impeller
-- 3a) Inspect impeller- no cracks or wear
4) Pull the screw holding the internal body/cavity "cam"
-- 4a) The hex head machine screw that holds down the "cam" inside the pump cavity, is the same hex head size (1/4") as the cover plate screws; it is an identical screw/bolt
-- 4b) Pull the cam; it sits loose, so comes outs easily with a finger.
5) Remove the Wear Plate.
-- 5a) It lays against a flange or ridge machined in the cavity base.
-- 5b) The plate is "loose", after the Cam is pulled.
-- 5c) Any small "pick" will pull it, as it will almost fall out.
-- 5d) 1/16" thick all points measured old plate, so really no "wear" as such, but did have grooves worn.
6) I pulled/check dimensions from the pump body face, where the Cover Plate/Gasket sits, down to the machined ridge in the cavity base.
-- 6a) Seemed to be uniform depth all around
7) Installed the new Wear Plate; It has a small notch on the OD, so the orientation is easy to set, per the notch
-- 7a) Cummins/Onan Genuine Parts package "0131-0158". WEAR PLATE - WATER PUMP".
8) Again checked depths around from body face to where the new wear plate sat.
-- 8a) Confusing readings did not seem uniform
-- b) I decided to again use my adapter sheet, which I installed below;
8) Installed the "cam" in the cavity,
-- 8a) Push Cam to sit faced up to/against the Wear Plate; check that plate notch is sitting correctly
-- 8b) Cam was a little scummy, where it sits in cavity, so I scraped it off a little; will use find grit paper next time
-- 8b) Cam sits loose on bottom of cavity, so shift until aligns with screw hole
-- 8c) Installed screw, set to 15-17 "#'s
9) Re-Installed the impeller
-- 9a) Since I remembered to bring it, I used my impeller lube instead of Dawn Detergent
-- 9b) Impeller lube is "Magic Lube", #630 by Alladin a PTFE based lube/sealant
-- 9c) I sourced Magic Lube, at a swimming pool outlet, as they use it for all their pump work and recommended it
-- 9d) Coated the cavity and impeller with the lube
-- 9e) Installed the impeller
10) Installed my adapter sheet/plate (1/32")
10a) Temp installed the Cover Plate/Gasket with one screw and swung it to hold the adapter in place
11) Finished installing the Cover Plate/Gasket, setting to 15-17"#'s
-- 11a) Did not want to torque up, so set low at 15" #'s; maybe got some lube on the screw threads
12) Topped off the coolant Reservoir
13) Checked oil- is right up
-- 13a) This GenSet was bench rebuilt, by the best man in Chicago, before I bought the boat in 1995

Went through the start cycle and started immediately and spit Water quickly with good flow. Exercised GenSet with loads, about 4.0Kw, and no issues. Seems to not smoke as much under load, so hoping am getting better flow than before.

Next year I will do better checking of dimensions with a better micrometer. I may buy a new Cover Plate, as this one had a little wear. I will clean up all surfaces with some Crocus Cloth and try without my adapter plate.

 

[sgharford]

I got busy yesterday too and tackled problem! So I put in new wear plate and cam. Did not solve the problem, still no self prime and no exhaust water. I then put in new impeller, new cover plate, and new gasket. Water starting flowing quickly and steadily. Tried 3 different times yesterday and pump did not loose prime. Not sure what exact cause was. Note I did replace all these parts about 5 or so years ago when I completely rebuilt pump and only put 50 hours on it during that time with no issues. I replaced impeller few times since then, including this year and it did not solve this problem. For cost of semi-complete rebuild (wear plate, cam, impeller, cover plate, cover plate gasket, looking at about $150) perhaps best to simply replace everything if experiencing this problem and hope for best. Note I did replace raw water intake hose, checked all other raw water hoses and exhaust elbow, remove heat exchanger and flush it to make sure restrictions were not in there. All were clear - problem definitely somewhere in pump. Now for that dam voltage regulator.....

 

[spartonboat1]

I got busy yesterday too and tackled problem! So I put in new wear plate and cam. Did not solve the problem, still no self prime and no exhaust water. I then put in new impeller, new cover plate, and new gasket. Water starting flowing quickly and steadily. Tried 3 different times yesterday and pump did not loose prime. Not sure what exact cause was. Note I did replace all these parts about 5 or so years ago when I completely rebuilt pump and only put 50 hours on it during that time with no issues. I replaced impeller few times since then, including this year and it did not solve this problem. For cost of semi-complete rebuild (wear plate, cam, impeller, cover plate, cover plate gasket, looking at about $150) perhaps best to simply replace everything if experiencing this problem and hope for best. Note I did replace raw water intake hose, checked all other raw water hoses and exhaust elbow, remove heat exchanger and flush it to make sure restrictions were not in there. All were clear - problem definitely somewhere in pump. Now for that dam voltage regulator.....

Thanks for the note. I can see me replacing the Cover Plate next. I am beginning to think these raw water pumps are well dialed in, so that slight wear causes loss of suction. I will hold off on the cam, as total parts cost, not including the impeller could stay below $50. The impellers are the high expense item. I bought a couple of special run-dry impellers from Amazon, but they are a tight fit on the shaft, so I need to hone out the shaft hole and "fit" them. I am hoping they may be more durable.

The Onan impellers seem fragile, compared to say, the Jabsco Toilets impellers, which have a very long life. I do not even pull the Toilet impellers between seasons, and they hold up well!

 

[t109a60]

Excellent, and timely, discussion on Onan raw water pumps. I have a diesel MDJE 7.5KW.
I too am experiencing new low flow/loss of prime issues. I have checked the obvious things
(impeller, strainer, belt tension, back-flush lines) to no avail.

Question for the forum: How common is it to have these pump parts 'wear out' (wear plate, cam)?
My boat is approaching 40 years of age. I have owned for 9 years and up to now regular impeller
replacement has sufficed. I do not know the prior maintenance history of this pump.

Ended up replacing my wear plate, cam, and impeller. Problem solved. Wish I knew exactly which
part did the trick as none had any visible issues.

 

[bobk]

Thanks for the note. I can see me replacing the Cover Plate next. I am beginning to think these raw water pumps are well dialed in, so that slight wear causes loss of suction. I will hold off on the cam, as total parts cost, not including the impeller could stay below $50. The impellers are the high expense item. I bought a couple of special run-dry impellers from Amazon, but they are a tight fit on the shaft, so I need to hone out the shaft hole and "fit" them. I am hoping they may be more durable.

The Onan impellers seem fragile, compared to say, the Jabsco Toilets impellers, which have a very long life. I do not even pull the Toilet impellers between seasons, and they hold up well!

I tried the blue run dry impellers on my Onan years ago. They wore out very fast, maybe 50 hours or so. The manufacturer replaced them, but the same thing happened. Now I stick with Johnson or Jabsco.

Bobk

 

[spartonboat1]

I tried the blue run dry impellers on my Onan years ago. They wore out very fast, maybe 50 hours or so. The manufacturer replaced them, but the same thing happened. Now I stick with Johnson or Jabsco.

Bobk

Thanks very much for the observation and update, plus the mention that they are blue, which mine are as well! May have same me some heartache, esp. they don't fit on the shaft. I think they are headed for the impeller museum (lol)...

 

[sgharford]

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.

 

[spartonboat1]

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.

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.

 

[krush]

Positive displacement pumps should be self priming. You probably have an airleak somewhere.

 

[sgharford]

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.

 

[racclarkson@gmail.com]

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?

 

[sgharford]

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.

 

[piripucha]

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

 

[sgharford]

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.