Thursday, April 18, 2013

The Onan MDJE is Putting Up a Fight

Yep.  I'm still battling with the Onan MDJE generator.

I pulled off the heat exchanger and found that it was clogged with scale.  After soaking it in a solution of vinegar and West Marine toilet bowl cleaner, I got it unclogged and pretty good lookin'.  I put it all back together yesterday and fired up the generator.

At first, things were looking pretty good.  Then I saw tell tale signs of wispy steam rising.  I hoped it was just residual coolant burning off, but within a short period of time it was overheating again.  That leaves only two more things.  It's either the centrifugal (fresh water) pump, or a blockage somewhere deep inside.

I stood there staring at the engine for a few minutes, looking for a sign, or inspiration or something.  The way I used to check the water pump on a car was to remove the radiator cap and start the engine.  Once it warmed up and the thermostat opened, you'd see water flowing by.  Unless the pump was bad, in which case it wouldn't.  I decided to try it on the generator.

Of course, a two cylinder diesel shakes a whole lot more than a V8, but I was pretty sure nothing was flowing.  It was hard to tell because it looked like a tempest in there, but I'm pretty sure it wasn't flowing.  Which means... it could be either the pump or a blockage.  Well, that accomplished nothing.  I shut the engine down.

I stared at the engine some more.  Too bad there was no way to check the pump... wait!  Of course there is!  Unlike a car, the centrifugal pump has water hoses connected to it.  I could just remove the exit hose from the pump and start the generator.  If coolant squirted out, the pump would be fine.  If not, bad pump?

So I pulled the hose off that runs from the pump to the block and started the generator.  Nothing from the pump.   Which means that... either the pump is bad, or there's a blockage.  Why a blockage?  Try sucking on a straw with your finger over the end.

The pump is that white gizmo at the bottom left of the photo below, and the plumbing fitting is the top right of the pump.  Above that you can see the hose I pulled off.

The engine was still hot from running it before, so it didn't take long for the coolant to overheat and squirt out the hose.

I called the local Cummins/Onan dealer to get a replacement pump.  He couldn't find a part number in his database, and the one I gave him from the parts manual didn't match anything.  With him on the phone, I went into the engine room and found a part number on a metal tag.

"It's a 50P-35", I said.

"Ah, got it.  It's an Oberdorfer pump.   Can't get it.  It's a marine unit.  Why don't you buy it online?"

Um... OK.

So I looked online, and found that these pumps are bronze, and cost about $300!  For $50, I would have just bought a new one, but for $300 I'm going to inspect it.  I had it off in just a few minutes and I put it on my workbench and popped the cover off.

It was as clean as a whistle.  I turned it, and wiggled the shaft to check the bearings.  Everything seems fine.

That leaves a blockage somewhere.  I know it's not in the heat exchanger or the pump, nor in any of the rubber hoses.  I'll bet it's gunk that's settlde at the bottom of the engine block.  Now what?  Today or tomorrow I'll look for drain plugs and hose fittings.


  1. A few years ago I created a blockage in my diesel's coolant system by switching to a different type antifreeze. I was under the belief that I had removed all the old coolant prior to adding the new. It worked fine throughout the season but the stuff coagulated somewhere in the cooling system over the winter. After going through all the same steps you have, Dave, inspect and flush the heat exchanger, disassemble and check the coolant pump etc. one of the boatyard mechanics took pity on me and gave it a look. He felt the return lines, confirming the readings taken with an infrared thermometer, and advised me to remove the lines going to and from the pump and flush with the dock water hose. After a few moments of flushing a wad of nasty gray sludge came blowing out the return line and the problem was cured.

    1. That's what I suspect the problem is, but in the block, not the rubber hoses. I removed the hoses when I removed the heat exchanger and water pump and coolant trickled out of each. I think it's in the engine block.

  2. Dave,
    Have you ruled out a blockage in the generator intake? Maybe a plastic bag got sucked in, or a buildup of barnacles?

    1. Hey Jim. I haven't looked on the raw water side, only because it's pumping water like crazy.

  3. Dave, Seems like you're covering the possibilities; raw water flows, heat exchanger cleaned, water pump moves water. Yet over heat. Might want to check the thermostat. Is it opening? Corroded? Can you hookup a dock side hose to the line coming from the circulation pump and flush the engine that way? though you would have to remove the thermostat to get full circulation. Good Luck. We're all waiting to hear.
    Rick, Deb & Izzy

  4. When all else fails just hang out and watch/Listen to this..

    Albert King & Stevie Ray Vaughan — In Session 2010 1983