Cummins ISX 15 cm2350 Black Coolant

The bubbles certainly make it look like compression in the cooling system. A failed air compressor can behave the same way and dirty the coolant. While this isn't the culprit very often it certainly does happen.

One strategy we follow when troubleshooting this complaint is to first remove the belt from the water pump so the coolant doesn't circulate. Next, drain the coolant and remove the thermostat, re-install the empty thermostat housing and leave the upper hose off. Fill the cooling system through the thermostat housing. You can now operate the engine and if compression is entering the cooling system it will migrate to the to opening and you'll see bubbles. Next, if you do see bubbles you'll want to identify a cylinder. We will crank the engine over without starting it, hopefully you'll see bubbles during cranking. If you do then remove the injectors and pressurize the cylinders using a rubber tipped blow gun; make sure the valves are closed when you charge the cylinder with air. BIG CAUTION THOUGH. When you pressurize the cylinder the engine will likely spin over. Make sure you're clear of the fan or anything on the front of the engine or camshafts. Using the blowgun doesn't add a lot of pressure in the cylinder so really pay attention to the coolant level, you may only see a subtle rise of the fluid level and not a steady stream of bubbles.

If we do identify a cylinder we then rotate the engine so the piston is up top, again with the valves closed (TDC) and pressurize the cylinder using the blow gun. The purpose of this final step is to rule out the possibility of a pin-hole in a liner being the leak path. Newer engines don't typically fail this way (pin holes) but again, anything's possible. Hopefully this all makes sense, good luck.

 

Head gasket, cracked head, cracked cylinder. In NAPA you can buy coolant test kit to test it for combustion gases. If test positive drop oil pan and pressurize coolant system about 20 PSI for the night. Next day bare engine and check every cylinder for coolant.

 

Most likely the oil cooler