2016 Peterbilt 389 - ISX15 - Oil in Coolant Reservoir

Question

I've got a bad situation here. Opened my coolant reservoir this morning and it's filled with black sludge. Engine oil level is slightly low. The truck hasn't overheated yet and it's running fine, but I know oil in the water is a disaster waiting to happen.

I'm praying it's not a head gasket or a cracked liner. Is there anything else on these ISX15s that can let oil cross over into the cooling system?

OTR_Carlos · Answer

✅ Confirmed fix: It was the oil cooler element. Removed the oil cooler housing, replaced the element and all gaskets. The real trick was the cleanup; had to flush the system four times with Cascade dishwasher detergent to get all the oil out of the radiator and heater cores before refilling with Restore and then fresh ELC coolant.

RamRunnerRay · Answer

It's almost certainly your oil cooler element. The plates inside fail and since oil pressure is higher than coolant pressure, the oil pushes into the coolant. Stop driving it immediately because the oil will swell all your rubber coolant hoses and you'll end up replacing every hose on the truck.

2016 Peterbilt 389 - ISX15 - Oil in Coolant Reservoir

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