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Old 05-14-2018, 06:21 PM   #1
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Flushing Oil System with Diesel Fuel

I did a search of the forum and came up with nothing on this topic, so I figured I'd post here. Maybe someone has done it already.

I've got a converted 1998 Thomas MVP with a Cat 3116ta and an allison auto trans. I had a diesel mechanic come by to walk me through the steps for my first periodic maintenance on my engine today. I ordered some filters and he's going to return Thursday to perform the service with me. I've never serviced my engine before, got it in August, did the conversion, and have been living on her full time since January.
  • So the mechanic recommended flushing the oil system by:
  • draining the oil
  • leaving the oil filter in place
  • filling the engine back up with diesel fuel
  • running the engine for a few minutes with the diesel fuel in there instead of oil
  • draining the diesel fuel out
  • changing oil and filter as usual

He said he's always done this on his big engines and the oil stays cleaner for much much longer and his engine last forever. I liked the sound of it so we're planning on doing it on Thursday.

Any thoughts? Will my engine explode!! This thread may end up being an interesting idea for people to try, or a terrible cautionary tale of a kid listening to bad advise. Either way, maybe people will benefit. I'll take a picture of what the oil looks like before the change, after the flush and new oil, in a month, and then in 6 months.
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Old 05-14-2018, 06:28 PM   #2
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Running only diesel in the engine crankcase even for a short period of time does not sound like a good idea to me. Are you sure he didn't say to put a gallon of diesel in the engine oil and running it before draining it?

Personally, I would run a few quarts of ATF in the oil and then drain it if need be.
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Old 05-14-2018, 06:34 PM   #3
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Some times it's best to let sleeping dogs lie. Flushing with diesel will effect every seal in the motor negatively. It will also break free a bunch of crud that will now circulate through your motor before getting to the filter. Oil pump could clog.
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Old 05-14-2018, 06:46 PM   #4
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Sounds like a bad idea to me. Does he have a reason for this? Diesel has low viscosity and low lubricity, and running it as lube oil, even for a short period, could cause excessive wear on the thin layer of babbitt metal in your engine's bearings. Not worth the risk.

If you operated the engine with a significant amount of coolant in the oil, and are using the diesel to flush the gunk out, you don't need to worry about the bearings. They're gone already.
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Old 05-14-2018, 06:58 PM   #5
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Sounds like a bad idea to me too.
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Old 05-14-2018, 07:03 PM   #6
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ive heard of putting a gallon of diesel or kerosene in the crankcase before draining. and running the engine 30 seconds to 1 minute and thats it.. never longer.. then you drain.

my personal method of opil changes is to drive my bus and get it all well heated iup.. (oil temp at 175 or above)... then I shut it off.. let it sit about an hour.. drain and fill.. this allows the oil to be thin up i nthe engine and will drip to the pan... the oil temp will still be close to 100 or a bit above in the pan and drains out nicely...
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Old 05-14-2018, 07:11 PM   #7
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Diesel fuel IS kerosene (and home heating oil, and jet fuel). And kerosene is a good solvent for removing oil and grease. So I would be careful with that.
Diesel does have a tiny bit of lubricating oil in it, which is the only difference from kerosene, but it is precious little.

If I were to fool with a "cleansing project", I would drop the oil pan and clean it out also.
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Old 05-14-2018, 08:35 PM   #8
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Filling with diesel to flush is a very bad idea. Diesel has a much lower viscosity and could burn bearings and seize pistons. Even worse the low viscosity diesel will work its way past the rings and cause a runaway condition. Seen it. Use atf or add a few cans of seafoam and run a while before draining.
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Old 05-14-2018, 08:38 PM   #9
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Unless there is some imperative to do this differently, why not just drain the (warm) oil and replace it with fresh oil?
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Old 05-14-2018, 09:40 PM   #10
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Quote:
Originally Posted by Twigg View Post
Unless there is some imperative to do this differently, why not just drain the (warm) oil and replace it with fresh oil?
The diesel flush is suppose to loosen all the muck that doesn't come out with the warm oil. It may not all come out with the flush, but now that you have it loosened up and fresh oil breaking it down it gets to flow through your motor.
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