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Onan 3.0 MDJA Resurrection (or lets fix this junk)


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Old 10-10-2017, 10:43:50 AM
Ranchero50 Ranchero50 is offline
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Question Onan 3.0 MDJA Resurrection (or lets fix this junk)

So, another project. To avoid confusion I should probably start this story from it's sad beginning. My dad, with his strange sense of nautical curiosity decided that he must have another boat. A larger boat than the 1984 28' Chris Craft Catalina 280 he's battled with the past ten years or so that's seen multiple engines, 3/4 of an interior redo and even an hand built aluminum hard top. Oh my.

So with the ever present complaints of not enough room to swing a cat and 10 gallon per hour fuel consumption on the 350 Chevy powered Catalina he talked my mom into buying a '86 Mainship 34 III with a big old Perkins 6-354 and no genset. While the paperwork was flowing back and forth between the owner, the broker and the prospective new owners a diesel Onan genset popped up on Craigslist for $150 down by the boat (two hours from home). Hey, it's diesel so it should run as long as it turns over and if the generator side passes the smell test we should be able to get it to make some volts. So with the thoughts of having AC while underway he went for it. Picked it up from a sketchy marina for $100 with no ID tags, no injector, and did I mention stuck...

So since it was my idea it's now sitting in my shop awaiting tender loving care. All is not lost. The oil looks good, the coolant is very green and whats left of the injection system has clean fuel in it. The bad is it's missing the injector, the heat exchanger ends are blown up and the front belt cover is MIA. Googling shows that it's a 3.0 MTJA and I've found the owners manual for it. Spent some time getting it apart while letting the chamber soak in diesel fuel and found that the piston popped loose with a couple taps and once free it spins over very smoothly. There is some corrosion on top of the piston which I expect since Onan's poor choice to keep the exhaust valve open as a shut down strategy exposes the engine to salt condensation.









So, moving forward I'm looking for parts source for an injector and thoughts about replacing vs. annealing the copper head gasket? I'll machine new ends to the heat exchanger and of course need to replace the water pump impellers. This should be a fun project, especially getting it in the boat.
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Old 10-10-2017, 11:11:14 AM
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Billy J Shafer Billy J Shafer is offline
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Default Re: Onan 3.0 MDJA ressurection (or lets fix this junk)

OnanParts on this site should be able to get you what you need. I would try and reuse the head gasket. If it looks in good shape. Parts are out there.

The best day of my field service time. Was when they told me. You will no longer work boats.
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Old 10-10-2017, 02:28:06 PM
Ranchero50 Ranchero50 is offline
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Default Re: Onan 3.0 MDJA ressurection (or lets fix this junk)

Spent some more time browsing different part numbers and it looks like 147-0136 is correct for the MDJA. Funny how pricing is all over the place for them but at least they are available.

Looks like it's time to pull the rest of the cooling stuff off and see how hard it'll be getting parts for it. Once I know what I need I'll order everything at once. Hopefully that way I'll be able to avoid an 'Oh crap' moment. I'd love to deck the block but I don't think it'll fit in my CNC mill.
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Old 10-10-2017, 07:19:22 PM
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Default Re: Onan 3.0 MDJA ressurection (or lets fix this junk)

The hardest part to find for the MDJA is the decompression solenoid so hopefully you got a good one, it will start without it with a very good battery but you wont have any way of stopping it. The block is big and meaty and unlikely to need decking. The injector is I believe common to the other J series diesels , there is a sticky on how to overhaul it. Your control box looks quite good compared to most I have seen. Cheers Dan
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Old 10-10-2017, 09:14:24 PM
Ranchero50 Ranchero50 is offline
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Default Re: Onan 3.0 MDJA ressurection (or lets fix this junk)

A little further along. Pulled the rest of the top hamper off and dressed the deck with my 12" file. Dark spots show low. I really wish the fire ring was clean but... Torn between file / stone or pull it apart to machine. A lot of work for a little distortion. I haven't looked at the head surface yet.




I also pulled the brush cover off. Smells a little bit burnt and the commutators are either heat or salt discolored. Brushes are in good shape and the commutator looks pretty good where the brushes were touching it. Hour meter only shows 1500 hours.



Finally a shot of the boat the genset is going into if it proves out.



Oh yeah, pics are clickable for full size images.
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Old 10-17-2017, 09:05:45 PM
Ranchero50 Ranchero50 is offline
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Question Re: Onan 3.0 MDJA Resurrection (or lets fix this junk)

So, some progress has been made. Spent some time dressing up the head gasket mating surfaces. Filed and stoned pretty flat, less than .002" across both surfaces.



Head gasket is multi layered copper and composite so I couldn't get it too hot. Silver paint should help keep the liquids where they belong. Old hot rod trick...




We also picked up one of the injectors off eBay. It has a '67 date code, came in a box with the Studebaker corp logo and came from Hawaii but looks brand new.


Next mission is to find the heat shield and flange dimensions for the injector pocket. Or chuck another $50 into the kitty for the parts and see if it's worth it. I'd almost rather make them on the mill just because that's how I'm wired...
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