LB7 Coolant in the Oil HELP!

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Rarely does the Duramax put coolant in the oil, they typically pump combustion gasses into the cooling system Since yours apparently leaked into the crankcase you are probably are going to need a teardown. Once you get coolant in the oil the bearings go FAST so keep that in mind. I would convert to straight distilled water in the cooling system provided you can keep it from freezing and do a thorough flush of the crankcase until you can locate the issue

We don't typically see injector cups leaking. It could be possible to leak at the water pump shaft I guess if the weep hole is plugged.

As for injector cup leaks it seems they are quite rare and typically man made.

Good advice!

The LB7's injector cups each use 2 synthetic O-rings near the top of each cup that prevent coolant from leaking into the top area of the cylinder heads where the valve springs and rocker assemblies are located. These O-rings look to be robust and up to the job. Unless they or the bores in the heads were damaged somehow during a prior dis/re-assembly, I can't imagine them leaking.

A coolant/oil leak at the oil cooler happens occasionally. You can read about them on Youtube. So, it remains a possibility for your engine' prob, but generally, a leaking oil cooler accompanies a head gasket problem brought on by chronic over-pressure in the cooling system (which also affects the water pump seals).