Cummins 290, oil in coolant

Remove both the Oil Filter & the Oil Pan.
Pressurise the Cooling system.
If Coolant drips out the Filter Head it's the Oil Cooler.
if Coolant drips from the Oil pan area you need to Dig Deeper. Much deeper.. (Most likely electrolysis or a Liner O ring)

 

Is there coolant in the oil? Pull a valve cover and see if there is sludge
building up in the top end. It usually shows up on the bottom of the valve covers.

 

The oil cooler is about the cheapest place to start if Inderjit's test is non conclusive. How old is the compressor?

 

If you have oil in the coolant, you've got an oil cooler issue.

Change the oil cooler. You're gonna have to flush 5 or 6 times to get the oil out. Don't worry when you noticeca bit more show up in the coolant, it's hard to get all the oil out.

 

No special knowledge needed. Pretty easy job, just need lots of Cacade for flushing

 

I thought that once. Two weeks later it was worse. Engine had a cracked liner.
You wanna fight? I do not just sharing.

 

And no oil coolant in the oil? Did a mechanic tell you this was the cause of a failure, or did you find it yourself?

The reason I ask, and I'm not trying to be condescending. On older engines, there is absolutely no where besides the oil cooler, where pressurized coolant and oil are together. There is no pressurized oil around an injector cup or liner. I'm not saying it didn't happen, but I've seen hundreds of cracked liners. There's almost no way for splashed oil to penetrate a crack in a liner, or to get past a bad liner o ring.. If the crack is above the piston, you'll get pressure in the cooling system, and likely some coolant out the exhaust, and below you'll get water in the pan.

Any other failure, say in an air compressor, could also put compression in the cooling system or coolant into the oil, but not oil into the coolant.