Onan MDJE 7.5 KW Fuel Leak

[CaptTom]

This has been going on for a few years, and I've yet to find the source.

It's an old (1980?) 2-cylinder. It runs fine, but I sometimes notice there's fuel saturating the oil absorbing pads I keep in the pan. I've watched it running, and I've moved the pads around trying to pinpoint a leak, but never been able to.

This year I've only run the genset for 2-3 hours so far. Again the pads were saturated with fuel. I changed the pads and ran it for a while. Not much to see, just a few drops. So I wiped down everything in the fuel system; hoses, injectors, filters, lift pump, injector pump and all the lines.

I stuffed paper towels around and under all these parts and ran it, watching closely, for about 15-20 minutes. No sign of fuel on any of the paper towels or the pads.

Could it be that some gasket somewhere is drying out over the winter, then slowly sealing again after running (and leaking) for a while? Everything fuel related seems to be right on the front of the engine, easily visible and accessible. Is there some other place I should be looking?

 

[rslifkin]

If it's intermittent and not spraying, I'd suspect filters, injection pump and a lift pump if there is one.

 

[CaptTom]

If it's intermittent and not spraying, I'd suspect filters, injection pump and a lift pump if there is one.

I can't see any spray, or any signs that there was a spray hitting anything on the motor or the sound shield. I'd suspect the injector pump and connections first, but that side always seems dry. The lift pump always looks a little shiny, but I haven't seen any actual drops. The filters are below that, and again, a bit damp looking but no visible leaks. My best guess so far is the gaskets on the lift pump, but I couldn't get them to show any signs of leaking today during and after a test run. I've left all the paper towels in place and will check tomorrow morning to rule out a slow leak when it's not running.

 

[Alaskan Sea-Duction]

Fuel filter? There is a little plastic screw on to of the fuel filter housing, which has an o-ring. Then there is the filter itself. Feel under the filter with your hand.

 

[CaptTom]

Status: No signs of leaks anywhere after sitting for 24 hours, and running for 15 minutes. All the paper towels I stuffed under, next to and on top of all the potential leak sources mentioned above are clean.


I'm stumped. I'm leaning more toward my theory that some gasket somewhere had dried out over the winter, and after leaking for a while it soaked up enough to stop.

 

[O C Diver]

When was the last time you changed the fuel filter(s)?

You might consider spraying the engine down with carburetor cleaner or another engine cleaner. It's easier to detect a misting leak with a completely clean engine.

While time has passed, can you associate the rate of leakage with high or low generator loads?

Ted

 

[Tator]

I had a really fine mist that came out of a teeny hole in one of my injector lines. The mist would collect in the air and then drop to the engine pad. Both I and an Alaska Ferry mechanic tried to find it and failed. It wasn't until I was looking at the area while the engine was running using my LED headlamp that the light picked up the mist. Traced the hole to a spot where a screw on a hose clamp had abraded the metal injector line over the decades. Just a thought.

Tator

 

[jgwinks]

I have the same unit. Check the o-rings or gaskets under the filter mounting bolts, these might not have been changed. Last set of filters I bought had the big o-rings for the filter cartridges but not the little ones for the bolts. If they look "wet" , that's probably the source. You just didn't let it run long enough to show up better. And I suppose they could have swelled up enough to stop leaking if they are the fiber kind instead of rubber.
Mine doesn't leak fuel, it leaks lube oil. Maybe through the fill cap it it's not screwed down tight enough, I've found it loose a couple times. I'll try putting a new gasket in it.

 

[CaptTom]

Thanks all for the tips!!

Ted, I have wiped the engine and parts dry, no sign of leaks yet. I did load the generator up when testing by running the water heater. But I sometimes load it more when the battery charger kicks in, and we're cooking too. So it's possibly load related.

Tator, I've used a headlamp to check for mist, so far nothing. I'm going to keep checking, those things can be hard to spot if they're behind something. A mist like that near an exhaust manifold can be a very bad thing!

jg, I haven't pulled off anything to check gaskets yet. I was hoping to find the leak, first. But I suppose I could just order every gasket there is, replace them all, and hope for the best.

And yeah, I've had lube oil in the pan too. I also suspect not getting the fill cap tight may have contributed, but it's also possible that those engines just like to leak oil. Right now that's a secondary issue. The oil filler cap has a gasket too. I'll add that to the list.

 

[jgwinks]

While on the subject, does any one know of Onan built the engines themselves or used an existing industrial engine? The Universal Diesel in my old sailboat was really a Kubota and I got parts from them way cheaper. The Onan does not look like a Kubota. Cummins now owns Onan so parts should be available there, not Marine specific so hopefully not too expensive. So far I haven't done anything but filters and those are generic.

 

[rslifkin]

Newer Onans often use someone else's engines, as did many of their big units. But they also built a lot of their own engines over time. The J series engines (both gas and diesel) are an in house Onan design, as is the CCK series (like my MCCK) and a bunch of others. The J series came in 1, 2, and 4 cylinder versions with 2 different cylinder displacements for the 2 and 4 cyl versions. And most or all variants were built in both gas and diesel variants.

The xxJA engines were single cylinder, xxJB were 2 cylinder, xxJC were 4 cylinder, all small bore. The xxJE is 2 cylinder, xxJF is 4 cylinder, both large bore (3.5" bore vs 3.25", all are 3.625" stroke, so 30 vs 35 cubic inches per cylinder for the small and large bore versions). I think the large bore variants may have only come in diesel, not gas (so they'd only exist as xDJE and xDJF variants).

 

[Portage_Bay]

Another tip I think I got it from Ski in NC on another thread. Tighten the screws on top of the lift pump. That solved my hard to find leak.

 

[Redhook98]

While on the subject, does any one know of Onan built the engines themselves or used an existing industrial engine? The Universal Diesel in my old sailboat was really a Kubota and I got parts from them way cheaper. The Onan does not look like a Kubota. Cummins now owns Onan so parts should be available there, not Marine specific so hopefully not too expensive. So far I haven't done anything but filters and those are generic.

The MDJE were Cummins motors. Great place to get engine parts is through them.

 

[rslifkin]

The MDJE were Cummins motors. Great place to get engine parts is through them.

The J series engines did not come from Cummins by any account I've ever seen. As far as I know, it's an in-house Onan engine. The only relation to Cummins is that they now own Onan.

 

[Redhook98]

The J series engines did not come from Cummins by any account I've ever seen. As far as I know, it's an in-house Onan engine. The only relation to Cummins is that they now own Onan.

My mistake. I see Cummins did not buy Onan until 1992. I have bought a lot of NOS old MDJE stuff from the Cummins site. Figured it was originally a Cummins motor at that point.



Disregard my last.

 

[rslifkin]

My mistake. I see Cummins did not buy Onan until 1992. I have bought a lot of NOS old MDJE stuff from the Cummins site. Figured it was originally a Cummins motor at that point.



Disregard my last.

It's easy to get confused, as Onan did use engines from others (including Cummins I think) on some of their bigger units prior to the Cummins purchase. They only ever did their own in-house engines for fairly small stuff (under 20kw I think).

 

[Alaskan Sea-Duction]

My 1988, 8kw Onan has a 3 cylinder Kabota motor.

 

[TowLou]

Add a little uv dye to a small can of fuel and run her. Leaks get fun to find.

 

[kapnd]

Is the generator hard to start?
If so, the leak is on the supply side, if not it’s more likely between the high pressure pump and injectors, or on the fuel return.
Just replace any old hoses and clamps.
Get the whole engine squeaky clean and wrap and stuff paper towels at all connections and fittings. You may have to use some tape to keep them in place.

 

[CaptTom]

Add a little uv dye to a small can of fuel and run her. Leaks get fun to find.

I run dyed fuel, it shows up clearly red on the oil absorbent diapers or paper towels.

Is the generator hard to start?
If so, the leak is on the supply side, if not it’s more likely between the high pressure pump and injectors, or on the fuel return.
Just replace any old hoses and clamps.
Get the whole engine squeaky clean and wrap and stuff paper towels at all connections and fittings. You may have to use some tape to keep them in place.

Starts fine, that's a good point. Not sure how big a leak it would take on the high pressure side, but I think it would become pretty obvious if that were the case. So I think you're onto something there - I'll keep a close eye on the low pressure side.

I already did the clean and wrap with paper towels bit. No evidence of leaking running it for about 15 minutes at a stretch, twice, and leaving the paper towels in place for several days. I'll probably run it for an hour or so this weekend when I'm off the dock. If nothing shows up I'm going to assume a dried-out gasket somewhere.

 

[TowLou]

Not off road red fuel dye. I mean add UV dye to a gallon gas can and run it and then with a uv light go hunting.

 

[Jaybo]

Fiber screw

Fuel filter? There is a little plastic screw on to of the fuel filter housing, which has an o-ring. Then there is the filter itself. Feel under the filter with your hand.

Yes, it's a bleed screw with fiber threads. I was getting a weeping fuel leak there. Replaced with a metal screw. A likely suspect. And easy to check.

 

[CaptTom]

Ran the genset for 30 minutes over the weekend. Still no sign of any new leaking. It's still a mystery why it started, and stopped leaking.

 

[rgano]

I ran my 1972 MDJE for 29 years and had few issues with any fuel leak. It should run dry as a bone both lube and fuel. As you say, all the fuel-related stuff is right there and not hard to replace. In 1989 I had an oil sample with 7% fuel dilution from the lift pump. Another time I did not get one high pressure fuel line tightened sufficiently after an injector service. BTW, if you have no record of an injector service, do yourself a favor and pull them - mine ran so much better after that it was like a new engine.