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Old 06-05-2016, 05:30 PM   #1
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Rear Main Oil Seal Leak 8.3 Cummins

Has anyone had to have the rear main oil seal replaced the their 8.3 Cummins? I have a small leak from mine but it is just enough that if I am towing for a few hundred miles it eventually makes a mess on the front of the toad. I am curious as to how much it cost.
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Old 06-05-2016, 05:45 PM   #2
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Might be your main seal but are you sure--hard to imagine an RV wearing out a main seal on a Cummins? Always get confused with front vs rear--I take rear to mean the end mated to the tranny???? My diesel fuel lift pump was leaking [not uncommon] and was washing dirt/oil down to the area between the engine and tranny bell housing--even Cummins said it was a rear main--not, couple turns of the screw caps on the lift pump and issue was solved....
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Old 06-05-2016, 08:31 PM   #3
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Quote:
Originally Posted by Old Scout View Post
Might be your main seal but are you sure--hard to imagine an RV wearing out a main seal on a Cummins? Always get confused with front vs rear--I take rear to mean the end mated to the tranny???? My diesel fuel lift pump was leaking [not uncommon] and was washing dirt/oil down to the area between the engine and tranny bell housing--even Cummins said it was a rear main--not, couple turns of the screw caps on the lift pump and issue was solved....
I agree with Old Scout, mine did the same. I installed a new fuel pump.

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Old 06-05-2016, 09:06 PM   #4
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Rear main seal leak in modern engines are pretty rare. Not saying it can't be but It should be double checked for any leaks. Best practices call for cleaning engine all over, use brake kleen to spread off common leak areas, with coach on clean parking area, run engine at high idle, say 1200 rpm with cruise and every so often slide under and look for leaks.

Does your engine have an open blow-by hose on side of block just by oil filter? There's a few threads here on how to deal with the slobber .....fuel lift pump is also a commn one

To answer your question, I'd say rear main (requires transmission R&R) would be a few K $
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Old 06-08-2016, 05:43 AM   #5
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Thanks everyone. I did look in that area but will take a much harder look at it again.
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Old 06-08-2016, 07:25 AM   #6
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We did have an oil leak and Cummins fixed it in a day.
Don't recall where it was, but because I was still under drive train warranty, the cost escapes me
Can look at paperwork tonight to see
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Old 06-08-2016, 10:10 PM   #7
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I thought I had a rear main seal leak. That is where the oil was accumulating. I cleaned everything up real good. Ran the engine and got under there and watched. Turned out to be a cracked dipstick tube. Cracked right at the block. Much easier to fix than rear main seal. Cummins C8.3 1998.
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Old 09-05-2016, 10:45 AM   #8
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Very odd, I purchased a 2008 Monaco Diplomat 35,000 miles 400 ISL had freightliner look at it they told me it was weeping oil, dripping from the bottom of the bell housing I wanted it fixed before purchasing so the owner agreed $ 2500.00 drove 100 miles huge leak cummins repaired it $ 4500.00 still leaking what is wrong
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Old 09-05-2016, 12:48 PM   #9
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Very odd, I purchased a 2008 Monaco Diplomat 35,000 miles 400 ISL had freightliner look at it they told me it was weeping oil, dripping from the bottom of the bell housing I wanted it fixed before purchasing so the owner agreed $ 2500.00 drove 100 miles huge leak cummins repaired it $ 4500.00 still leaking what is wrong

This sounds familiar. Is this unit 16? If so, I'd be happy to have a look.
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