Oil diluted

I have a 120hp Lehman, I sampled the oil after 200 hrs, all wear components were good. But there was some fuel dilution happening. The engine does run a bit cold 155-160 but it seems that it’s not crazy low for these motors. Anyone else have minor dilution issues? Or fixes?

I had it on my Stb 135. On a 120 you have the benefit of a separate oil supply for the injector pump. This takes the pump off of the list of possible causes. Typical fuel dilution on a 120 can come from any injector piping leaks under the valve cover (main line, fuel return line) or a poorly functioning or leaking injector. I did some pressure testing of the fuel return rail and found no leaks. In my case, my fuel dilution also showed up on a annual oil sample and slowly increased each year. I replaced the injectors with a freshly rebuilt set and subsequent oil samples proved the dilution was no longer happening.

Ken

3800 hrs and no dilution even though I typically ran around 1650-1700 RPMs.... usually my temp was up around 180.

The lift pump may be a source, and if it is then a sign it may be ready to fail.

Soo-Valley said:

The lift pump may be a source, and if it is then a sign it may be ready to fail.

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If it is a mechanical fuel pump, this is the culprit in the majority of cases.the diaphragm gets a pinhole in it, which results in a (usually) slight drip of diesel fuel into the crankcase. This is a serious issue not because of loss of lubrication due to dilution but it will raise the oil level in extreme cases to where the piston skirts dip into it and deposit it on the cylinder wall which can result in a runaway engine that you cannot shut off. You can often get an advance warning something is wrong when after setting a day or 3 the engine has to crank and crank to start the first time and afterwards starts normally. Consider replacing with an electrical fuel pump.

When I had the problem on my 120 it was because of a leak on the return rack under the valve cover. I had removed the rack to replace the injectors with re-builts and hadn't replaced the copper washers. Took it back off and put new washers in-problem solved.

Tator

Not necessarily related to fuel dilution, but 155-160 is more than a bit cold. This engine may not have a thermostat, but if it does, check it and probably replace it. You should run about 180.

What is the fuel dilution pct on the oil analysis?

David

Tator said:

When I had the problem on my 120 it was because of a leak on the return rack under the valve cover. I had removed the rack to replace the injectors with re-builts and hadn't replaced the copper washers. Took it back off and put new washers in-problem solved.

Tator

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Back in olden days we were taught to heat the copper washers dull red then drop them in some water and then file them until you have an unbroken circle of shiny copper. Nowadays I have a compartmented box with an assortment of new washers but I still toss the old ones into a small cardboard box in case I need an add sized one. Of course I never do so the box just sits around as a good luck charm to ensure I have a new one the proper size.

DavidM said:

Not necessarily related to fuel dilution, but 155-160 is more than a bit cold. This engine may not have a thermostat, but if it does, check it and probably replace it. You should run about 180.

What is the fuel dilution pct on the oil analysis?

David

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It was 5%, I do have replacement thermostat but the supplier recommends 170 degree thermostats….. I didn’t think it was worth switching and making a mess for 10 degrees.

These boats also have potential for poor coolant flow to cylinder #6 which cause excess heat and wear in #6. Which is why I assume the OEM replacement is now 10 degrees colder than the original OEM.

Who is propagating that these engines still, currently, widespread, etc, etc....have a no. 6 cylinder cooling problem?

When I asked American Diesel about the plug and flushing procedure...they said it was overblown and not to worry.

Believe less than 10% of what you read on the net and research the crap out of that 10%.

Why else would they recommend a 170 thermostat, coldest setting I’ve ever heard of. In the manual optimum temp is 195, however in the same manual it recommended 180 thermostat??

How many engine hours did it take to get to 5%?

As a long time caretaker of a couple of these fine engines, I concur on all of the above advice.

When I had a serious dilution problem on my port 120, I was told one methodt to troubleshoot would be to hook up a bicycle pump (one with a pressure gauge) to the return fuel connection at the back right side of the head and pump it up to 10-20 PSI. I didn't ever get to that sort of pressure because while trying to I noted fule bubbling up out of the top of the banjo connection at the number two injector. Turned out to be the injector itself. I always used new copper crushing washers on the return banjo and at the bottom of the injector well.

I had this issue and it was the copper washers near the banjo nuts in the fuel return. I replaced them and problem solved. I also learned that I could have annealed (heat up v hot with a torch, let cool slowly) them.

GoldenDawn said:

I had this issue and it was the copper washers near the banjo nuts in the fuel return. I replaced them and problem solved. I also learned that I could have annealed (heat up v hot with a torch, let cool slowly) them.

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Actually letting copper cool SLOWLY tempers it, it does not anneal copper. Copper acts the opposite of steel or iron in this respect. So, to SOFTEN (anneal) copper, you heat it up to the appropriate temp, then quench it quickly in either oil or water as discussed above.
Letting it cool slowly results in the copper being HARDER, not SOFTER.

Slowgoesit - thank you for the correction!

In tests I've seen, diesels run better at higher temps. OEM makers usually recommend lower temps to warranty avoid boil over issues.
I run my diesel pu at 205° and get much better mileage and more power.

Lepke said:

In tests I've seen, diesels run better at higher temps. OEM makers usually recommend lower temps to warranty avoid boil over issues.
I run my diesel pu at 205° and get much better mileage and more power.

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It's true of most engines, gas or diesel. Running them hotter means hotter cylinder walls and better thermal efficiency, plus better fuel vaporization. It's generally a balance of running it warmer vs when it starts to cause durability problems, or for a gas engine, pre-ignition. And on some engines, it's hard to run the coolant warmer without making the intake air too hot (which hurts power and on a gas engine further reduces pre-ignition margin).

A diesel ignites with heated air from the compression (no ignition spark) and the atomization of the fuel the closer to 180 for water temp you can get the better. You could have unburnt fuel you could have a leaking injector or fuel pump as well. I have seen all 3 contribute to dilluted oil. The leaky injector actually increased the standing level in the oil pan it was that bad.

kchace said:

I had it on my Stb 135. On a 120 you have the benefit of a separate oil supply for the injector pump. This takes the pump off of the list of possible causes. Typical fuel dilution on a 120 can come from any injector piping leaks under the valve cover (main line, fuel return line) or a poorly functioning or leaking injector. I did some pressure testing of the fuel return rail and found no leaks. In my case, my fuel dilution also showed up on a annual oil sample and slowly increased each year. I replaced the injectors with a freshly rebuilt set and subsequent oil samples proved the dilution was no longer happening.

Ken

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Good summary. Thx

Do not forget a leaking diaphrgm in the diesel LIFT PUMP as a possible source of fuel dilution in a FL120. Never had an issue with it in many years running a pair of them. I have heard of people replacing that pump with an electrical fuel pump avoiding the issue altogether.

rgano said:

Do not forget a leaking diaphrgm in the diesel LIFT PUMP as a possible source of fuel dilution in a FL120. Never had an issue with it in many years running a pair of them. I have heard of people replacing that pump with an electrical fuel pump avoiding the issue altogether.

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Replacing the mechanical pumps with electric has the side benefit of being able to bleed the system post-pump by simply energizing the pump.

Randomwake said:

I have a 120hp Lehman, I sampled the oil after 200 hrs, all wear components were good. But there was some fuel dilution happening. The engine does run a bit cold 155-160 but it seems that it’s not crazy low for these motors. Anyone else have minor dilution issues? Or fixes?

Click to expand...

For a variety of reasons, all related to combustion efficiency, you want to run a diesel as hot as is reasonably allowable. I would say 180 min to 210 max. I am not saying that this is the cause of your problem, I would check injectors first, but as a side note, the operating temp wants to be higher.