Friday, April 19, 2013

The Onan MDJE Strikes Back

Sometimes I can be a real dumbass.  That's just to set the tone.

So I was back dealing with the small MDJE Onan generator's overheating problem yesterday.  I figured that I could flush the entire cooling system by pulling the two hoses off the centrifugal water pump and connected an inlet water hose and a drain hose.  So I merrily hied myself to our local hardware store and purchased the appropriate hose fittings and skipped all the way back to the engine room.    I connected pulled the hoses off the pump and connected the inlet hose to a faucet in the engine room and put the  drain hose in a bucket.

I turned on the water and looked in the bucket.  Sure enough, it was working.   Working as in a tiny stream of water worthy of an old man with prostate problems.

I waited, expected a glob of goo to suddenly plop in the bucket.   No.  So I swapped the hoses, changing it from a reverse flush to a regular flush.   Same thing.

Dejected, I turned everything off, washed up, and sat at my laptop, staring at the Onan Master Service Bible, looking for a clue.  And there it was...

To flush the engine, remove the thermostat...


Of course I wasn't getting anything.  The thermostat was closed, and the only circulation I was getting was through the small bypass in the thermostat housing.   So now that I'm an expert in removing the thermostat from an MDJE, I had the thermostat out in 38 seconds.   I turned on the water and got a powerful stream of water, worthy of a rodeo horse in a parade.

I let it run for several minutes.  I took the opportunity to wash away 33 years of crud under the generator.  That's worth an extra victory beer.

I was convinced that everything was flowing normally, so I shut off the water, reconnected the hoses, replaced the thermostat, fired up the generator, and watched as it slowly overheated.


  1. Ok, so now the block is flushed out, unless there is still a blockage somewhere in the block. Did you test the thermostat? Place in pan of water on stove/heater, bring to boil. You should see the thermostat open. Or you could run the genset without the thermostat to see if it still overheats. Also, may want to check vanes on raw water pump and back flush the thru-hull. It may be blocked or restricted. Has the genset run for you without overheating?

    1. I did test the thermostat, and it opened, although I don't know at what temperature.

      My next move is to check the impeller, and as you said, backflush the thru-hull. I'll then run the generator to see if it overheats. If it does, I'll try running it without the thermostat.

      The generator ran two years ago without overheating.

  2. Dave,
    Look closely at the tubes of the heat exchanger, there maybe a thin layer of scale in the tubes that prevents the heat exchange.

    Bill Kelleher

  3. Dave,
    I copied this from Boatdiesel

    My Onan 15kw was shutting down as previosly described.
    I thought I'd replace the thermostat and tested the old one; which started to open around 145 degrees - fully open at about 165 degrees.
    As I reinstalled the old thermostat I refilled the coolant, opening the bleed valve until coolant came out (as described in the service manual). This time, though, different than before, I had the coolant feed from the thermostat housing off and waited until coolant returned on this line before I reattached the hose.
    I think I created an air block when refilling the system previously (I pulled the heat exchanger last winter) by not allowing coolant to fill the hose from the thermostat.
    It ran great for at 30 at full load yesterday and I had to shut it down to do home.
    Looks like no more overheating (knock wood).

    Bill Kelleher

  4. Dave: I think Bill K has hit on the answer--an air bubble in the block that defeats the in-block antifreeze circulating pump. I had a similar bafflement with a 3-cyl diesel. After the gen cools, remove the thermostat and look into the block. It should be full to the brim with antifreeze. If not, fill it up. Bump starter a few time to burp the air, refill, victory beer x 3.