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Onan 7.5MDJE out of service for over 10 years, need to bring it online


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  #1  
Old 09-22-2019, 05:04:36 PM
IslandMountainFarm IslandMountainFarm is offline
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Talking Onan 7.5MDJE out of service for over 10 years, need to bring it online

Received this diesel as payment (not the original deal but something is better than nothing) for some custom farm work I did. Has not been run for a long time but a friend of mine remembers it and says it did run well back in the day.

It has an airfilter that was added by the previous owner so that's not an issue. He was going to run it in a shed and had purchased a muffler, exhaust pipe, and fittings and they came with it.

I will be running it in a shed so I need any thoughts on the best way to set up the exhaust system. Don't even want to try and start it until that's set up.

I'm at best a YouTube/shade tree mechanic but I know them what are much better than me. Problem is I'm being given advice that ranges all the way from "Get a new battery and crank it up" to "Pull the injectors, put oil in to make sure the cylinders are lubed then crank it over by hand". This is coming from guys who take their own semis and heavy equipment engines apart and but them back together again.

Found this forum via a search engine and thought I'd throw myself on your mercy. I'd like to get this thing up and running before the snow flies. I'm off the grid and the days get pretty short up here 20 miles from the Canadian border. I've got 2 Nissan Leaf batteries for my solar storage system and that generator would make a great charging system for them.

Thanks in advance.
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Old 09-22-2019, 05:09:44 PM
EICBob EICBob is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

Welcome to the Stak, there are many smart guys here that will be more than happy to answer any questions you may have.
Please post the full model and serial number, that way we can be sure of the details of your set and the answers will flow.
Pictures really help!!

The only “dumb” question around here is the one that is NOT asked!

Good luck and the real smart guys will be by soon!

-BobC
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  #3  
Old 09-22-2019, 05:40:05 PM
IslandMountainFarm IslandMountainFarm is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

Thanks EICBob, plate is pretty hard to read but this is what I think it says:

7.5MDJE-30R/226SA

L750991867

120/240 ph1

I will take pictures to answer anyone's request.
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Old 09-22-2019, 06:25:26 PM
YellowLister YellowLister is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

Does it look to be water cooled? Is it white, have a radiator? The M stands for marine.. which will usually have a water cooled exhaust manifold.

If your not comfortable pulling injectors, squirt some wd40 down the intake once you removed the air cleaner..

But first.. is the old mechanical fuel pump still hooked up or is it electric?.

You want to make sure you have fuel, preferably fresh fuel going to the injection pump, maybe rig up a gravity fed lawn mower fuel tank and get some fresh fuel to the injection pump before you try turning it over if it has sat a long time..

Crack open the fuel lines at the injectors and see if fuel is present then turn it over by hand.

You will not get any fuel to the injectors till you manually pull up the fuel stop solenoid, which h is attached to the injection pump on the right side... you can remove the solenoid, but be careful they are hard to find and $$$ then pull up on the arm to allow fuel to flow when you turn it over by hand, or wrench.

Turn it over slowly, incase you have a stuck injector plunger..

Tell us what happens..

Welcome to the stack, your in the right place for onan help

Your set was made December of 75. Spec a should be yd generator end with a voltage regulator in the top box above the generator section
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Old 09-22-2019, 06:39:45 PM
Birken Vogt Birken Vogt is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

Look up exhaust thimbles to get it out of the wall. It is a big project. You also need a stainless accordion flex connector as the first step off the exhaust manifold.

Before you do that make sure it runs. I would bar it over a couple turns with a wrench. Then fire it off.
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Old 09-22-2019, 06:53:56 PM
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Billy J Shafer Billy J Shafer is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

You need to go to our manual site. Download manual 940-0750. You will need to supply cooling water to it. The maual will explain it to you. Other wise you will destroy the engine.
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Old 09-22-2019, 10:07:24 PM
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

By the serial number, it was built in December, 1975.
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Old 09-22-2019, 11:17:38 PM
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

I have done a bunch of conversions on these sets. With any luck you can unthread the nipple from the exhaust manifold and replace it with an 1 1/2" short nipple , then head down to your local parts store and get an exhaust bushing to match up with standard 1 1/2" exhaust piping , Walker makes a bunch of different ones to fit as well as a selection of exhaust flex pipes in various lengths. I like these radiators "2-Row Dual Core 42mm Racing Radiator+12 inches Black Fan " off Amazon ( has an 12V electric fan) . You can remove the heat exchanger and re route the lines into the radiator with 3/4" to 1 1/4" rad hose adaptors. The raw water system is superfluous for a land based system , you can remove the pump and block off the mounting hole with a piece of aluminum plate.
They make a nice relatively quiet set being water cooled. Post some pics and report back on how you are making out , lots of guys willing to help here. Cheers Dan

Just up the road on the Canadian side.
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Old 09-23-2019, 01:48:36 AM
VelocityDave VelocityDave is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

If the generator has been sitting for ten years before I did anything I would remove the injection pump and make sure I could turn it with my fingers. This is assuming it has an Ambac pump. If it is stuck you will break the pump when you turn the engine. Big $$$ mistake. If it is stuck it can be cleaned and saved for little or nothing. The pumps can lock up from setting and the fuel going bad. The pump should turn easy with your fingers. You will have to retime the pump but that is easy with the service manual that you can download.

David
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Old 09-23-2019, 02:00:48 AM
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

Good advice Dave!!
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Old 09-23-2019, 08:13:29 AM
PopsGarage PopsGarage is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

Quote:
Originally Posted by BigBlockChev View Post
I have done a bunch of conversions on these sets. With any luck you can unthread the nipple from the exhaust manifold and replace it with an 1 1/2" short nipple , then head down to your local parts store and get an exhaust bushing to match up with standard 1 1/2" exhaust piping , Walker makes a bunch of different ones to fit as well as a selection of exhaust flex pipes in various lengths. I like these radiators "2-Row Dual Core 42mm Racing Radiator+12 inches Black Fan " off Amazon ( has an 12V electric fan) . You can remove the heat exchanger and re route the lines into the radiator with 3/4" to 1 1/4" rad hose adaptors. The raw water system is superfluous for a land based system , you can remove the pump and block off the mounting hole with a piece of aluminum plate.
They make a nice relatively quiet set being water cooled. Post some pics and report back on how you are making out , lots of guys willing to help here. Cheers Dan

Just up the road on the Canadian side.


I installed my 1966 MDJB at home back in 1999 still runs perfect

Bob
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Old 09-23-2019, 10:48:45 AM
IslandMountainFarm IslandMountainFarm is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

I am going to respond to each of you as time allows, it is fall, harvest time and our first freeze is due sometime this week so we are in overdrive.

I did want to let you all know how much I appreciate your information and the time you have taken to share it with me.

Tomatoes, peppers, eggplants, and squash are on the agenda for today.
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Old 09-23-2019, 12:07:23 PM
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

Take care of your stuff. We all understand. Wanna trade weather. 72/95 down here today. I am ready for some cooler weather.
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Old 09-23-2019, 12:42:26 PM
Dale Russell Dale Russell is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

57* here when I got up this morning Billy. I am sending it your way, I think Summer is over for us up here.
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Old 09-23-2019, 01:02:22 PM
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

Thanks Dale. Doc says I have to go inside when it hits 90. I get out about two hours in the morning. Sure be glad when it gets cooler.
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Old 09-23-2019, 01:40:05 PM
J.B. Castagnos J.B. Castagnos is offline
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

I would check-lube-or replace the rear bearing, a friend put one in service, the rear bearing went out and destroyed the magniciter.
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Old 09-23-2019, 02:08:46 PM
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Default Re: Onan 7.5MDJE out of service for over 10 years, need to bring it online

I would also look at the water pump inpellers.
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