400 Cummins Big Cam Oil in Coolant

[TD25C]

I have a 79' M920 with the Cummins 400 Big Cam. After a recent haul I discovered a blackish gray liquid coming out of my radiator overflow. When running It seems that the truck is pumping oil into the cooling system somewhere. When the truck is shut off the oil/water settles back into the oil pan. I realize this could be caused by a number of issues. Any suggestions on how to go about trouble shooting the issue?

Thanks

 

[R Racing]

I'd first check your oil cooler . I have seen them do this.

 

[Ferroequinologist]

First, I would check the cooler. If there is a leak the engine is designed to pump oil into the coolant instead of coolant into the oil. I had an m818 with the same basic engine do exactly as you discribed and it was a leaking oil cooler.

Drain the coolant (you have to anyway to get the oil out) then take the lower hose off the cooler on the coolant side, then crank the engine. If oil comes out there is your leak. (Don't run the engine very long.)

 

[WillWagner]

Drop the pan, pressurize the cooling system with around 17-20 psi and watch where the coolant/water is coming from. if it comes from between the piston and liner, you have pin holes in the liner...poor cooling system maintenance. if it comes from a main bearing, usually #2, it's the oil cooler or the o-rings in the cooler housing. In rare occasions coolant from between the p/l can be a head gasket

 

[TD25C]

Thanks for all the advice. I haven't been able to get back in the shop to tear into it.

 

[TD25C]

How do I know which line is on the coolant side? Does anyone have a coolant flow chart?

I believe it is a Series I NTC400.