Onan Mdl4 Generator

Any numbers still readable on the old seal?

My notes show 509-0203 as the original OhNo rear seal part number (spec A).
That includes a wear sleeve.

Sadly, these engines are way past end of life and parts are near non-existing in North America.
The last time I ordered MDL* parts, they came in from South America.
Verify this part number, call your Cummins rep back up and ask for a world wide search.
Maybe Cummins can offer the original vendors part number.

I do understand why your tech is in no hurry to jump in there.
It is a shame, when running, they are darn good engines but every time I work on one, it turns into a Pandora's Box. Splitting yours for a rear seal may turn into an ugly ordeal for you and your tech also.

How bad is the leak? Will it last till you get home or end of season?
Maybe some snake oil or thicker straight weight oil.

Ouch.
No backup gen-set?
Leaks in a tray, with a drain port?
Have you been gen-set shopping lately?
Where is your ship?
You probably have an ole Hatt, what model ship?

Previous boat had 8kw Onan. Turned out to be same Kubota motor as in Phasor Generator and the local dealership stocked parts priced way below Onan's.

These OhNo's had some nice at the time, Cummins engines.
The MDL3,4 & 6 just never took off like the Kubota, Yanmar, Shibaura, Mitsubishi & Isuzu. Interchangeable parts was a great idea but I figure Cummins was just late to the party.
Other than in gen-sets, I don't know of them anyplace else.
Now that Cummins & OhNo are in bed with other, nobody else using that engine line, I fear there is no place else to shop for parts other than Cummins/OhNo.

By the way, That Kubota D1105 used in your previous 8kw OhNo is the same engine model I'm using for my #2 gen-set. Over 4000 hours and was running perfect;
One of the intake valves stuck open during our vacation.
It was much cheaper to purchase a whole new head thru Amazon than have my head serviced.

My 20 year old, 5000+ hour #1 gen-set uses an Isuzu 4LE1. Since new, the only service was valve lash adjustment. I noticed Isuzu parts in Amazon also.

Before he went sailing around the world a neighbor had about 50,000. HOURS on his 8kw Onan. Said it used to smoke only after first starting. He made an anchor out of the circuit board; replaced with a couple relays similar to the one I used to have. Last I heard; bought home on mountain in Ha. & sold the boat.

This thread may have jinxed my 12kw NL. Water flow looks normal but it shuts down after about 5 min. Dipstick shows right on the full line but the oil color appears to be too tan plus I noticed dark oil in its pan. About 3300 hrs. and no clue how it can squeeze out short of disassembly so will be gen. shopping after the weekend.

Your probably going to drag it out in pieces.
If it is down for the count, start tearing it down now.
May find the issue with the exhaust manifold or head is serviceable and does not require a whole replacement.

How old a gen-set is it? JohnDeere or Shibaura Engine?

Next, If the electrical end is good, just find a long block.
Find a tractor or engine dealer in your hood.
With data from the valve cover sticker or casting number (on the block behind the alternator) you can shop the internet also.

I was truly amazed how many Kubota D1105 engines are on the net and cheap.

Good news Brother Lunderic
Who located the seal for you? Dealer, shopping the net? Magic?

Hour meter showing 3800; saw it working but no clue if it had ever been changed. Neighbor said dip stick oil color was normal for diesel and suspects the dark oil in the pan arrived from a lazy mechanic when changing the filter. Question that remains for now is why it shuts down, even with no load, after about 5 minutes. Suspect either a water temp or exhaust temp sensor but neither of them are visible and I saw 0 fuel solenoid linkage like the Onan on previous boat had.
After this saga is resolved I plan to install a set of gauges for it.

There are no delay thingies in the NL box so you have a flaky fault sender Or as Brother MBevens found, a pinched fault wire.
You have three fault senders on your NL.
I found most of the time, the temp sender on the (2 wire) exhaust mixer is at fault.

Simple (Bosch type) relays are in the box. Once in a blue (rare) moon they can cause some headaches.

Access to the exhaust temp sensor, I recall had gone bad on the previous boat, blocked by the top of the sound shield so need to get that thing off. Always heard their a PIA and now I see why so on the to do list and happy at home dock & not off in the islands. Too many things run off 110 & 220 so need to find room for back up gen or maybe an inverter.