Onan MDJE Raw Water Pump Not Pushing Water/ Cure

[krush]

Usually the root cause is very simple! Annoyingly, it's usually NOT SIMPLE to find it!

 

[bobk]

This exact same problem happened to a friends trawler shortly before he died. He bought a new SS mixing elbow or shower (not sure which now) and I asked my mechanic to look at it before he did the install. Yep, it was a collapsed hose. They can be easily overlooked.

Bobk

 

[sgharford]

I'm hoping (for me anyway) that this gives final closure to this thread. So far I have replaced every component of the raw water cooling system and did phosphoric acid flush of heat exchanger. This included replacing the muffler, the exhaust hose (which had an inner liner tear), and new raw water pump a fellow member had as a spare. This all worked for about a season, then it came back. I would get it going again by disconnecting the output hose from pump and re-priming it by manually starting the generator with my finger over the heat exchanger raw water intake (it spits residual water back out from exhaust otherwise) and letting output hose run into the bilge, then stopping and reconnecting to heat exchanger. So last time I did this it would not do it first time around, which got me thinking maybe an intake air leak. I checked out the strainer and noticed the water level and that the output (intake to raw water pump) was only about half way. This is natural water level when boat at rest. So again I throw kitchen sink at it by completely rebuilding the strainer with new gaskets and proper wing nut (about $40 total) as well as turning it about 30 degrees so the input and output are now completely submerged. I also found a place that sells ‘New’ onan mdje water pump impellers (see below). I think the originals have been discontinued for some time (been buying spares on ebay) and that they perhaps even shrink over time? I noticed comparing old one to new one that the new one seemed just a few hairs wider. Anyway, rebuilt strainer as well as new impeller done at same time so can’t tell for sure what actually fixed it, but so far so good after 3 separate outings. Oh Lord, please let this be the end….at least for 2 seasons 

 

[t109a60]

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.
View attachment 23869View attachment 23870

Please clarify where this section of hose was located. Between the HE discharge & muffler?
Between the muffler and side hull discharge? I suspect, closer to the generator if it is an exhaust gas
deterioration issue.

I am getting good flow out of my raw water pump (disconnected) and no loss of prime. When I
reconnect the resulting overboard exhaust flow is a fraction of what the pump alone puts out.

 

[oscarvan]

I am getting good flow out of my raw water pump (disconnected) and no loss of prime. When I
reconnect the resulting overboard exhaust flow is a fraction of what the pump alone puts out.

Sounds like a bad hose too.

 

[sgharford]

It was the hose between the muffler and thru hull. Believe you are suppose to get that hose looping above the exhaust manifold for proper installation. This caused too tight a 180 degree bend in my hose and at about apex of loop is where the inner ply separated. Hope this helps.

 

[spartonboat1]

This is an update to this thread, with 98% success rate, at this time.

Note: Spring 2020, I ordered from Onan/Cummins and installed a new Cover Plate (had wear) and Wear Plate at the back of the bore (some wear), plus new gasket and impeller; retained the cam, which seemed "true"..

I still had loss of prime with the new parts installed. See remarks on my shim installation below.

I am going to maintain that the mfgr of the Onan/Cummins brand of impeller is molding them undersized at their width. They are slightly narrower than the bore is deep.

This year, Spring 2020, I permanently installed my 1/32" copper shim at the back of the bore, so that I do not have to re-fit it, each time I pull the cover plate.

I have had good water flow each and every start this year, since installing my shim, plus new parts; no fails. One time only, while running the genset under way, I lost prime. Otherwise, I have good water flow at all times.

So if the impellers by Onan are from China, I believe their dimensional configurations are suspect and they are slightly too narrow.

My next effort is to try and source a Jabsco impeller, which will fit, both for dimensional comparisons, plus the Onan's are fragile with a limited life. As mentioned before, my Jabso toilet impellers are very durable.

 

[racclarkson@gmail.com]

Have you replaced the cam? Sorry if you mentioned that somewhere.

 

[whoover]

Hello, you can try Onanparts.com for impellers. Some times they have them and worth a try. I agree with the limited life of generator impellers compared to others on boats.

Walt Hoover

 

[spartonboat1]

Have you replaced the cam? Sorry if you mentioned that somewhere.

I have not yet replaced the "cam". But I intend to acquire a new one from Onan, as it is the only part, which I have not replaced with a new part. I will pull my shim out, although it has been working, install the new cam and run tests!

If my tests prove less than 100%, I will retain the new cam, but re-install my shim, which seems to be helping.

Also, others have mentioned loss of suction. I have vacuum gauge, so I will try to run some of those tests as well.

 

[sgharford]

You may want to try ordering impeller from Stay Cool Pumps per my prior post #23. As I noted there, I believe their impeller is a bit wider then OEM one's you get these days from Onan.

 

[spartonboat1]

You may want to try ordering impeller from Stay Cool Pumps per my prior post #23. As I noted there, I believe their impeller is a bit wider then OEM one's you get these days from Onan.

That is of great interest! I will follow up with them. Thanks! I will hit them both with a mic, on their width, when I get the Stay Cool impeller!

 

[t109a60]

That is of great interest! I will follow up with them. Thanks! I will hit them both with a mic, on their width, when I get the Stay Cool impeller!

Sparton,

Please do let us know your width measurement result. My new Onan brand impeller lands flush with the top
of the housing bore. Installing the gasket results in a slight gap between the impeller and cover (by
the thickness of gasket).

I strongly suspect this is the problem. I have ruled out everything else (process of elimination)!

Very frustrating......