Truck has been low on coolant for a few weeks. Pressure tested it and it was slowly losing pressure, but we couldn't find any leaks. We used a combustion gas tester, and it showed that the coolant has combustion gases in it. Oil is also consistently low a quart or so a week, with no leaks found anywhere. I don't see any coolant in oil. I remember a while ago while looking around on Cummins quickserve I found troubleshooting for Combustion gases in cooling system, but now I can't find it. Could it be the EGR cooler? Air compressor? Or head gasket?
How many hours/miles? EGR cooler is a prime suspect.
340,000 mi, 16,000 hours
Right mileage for EGR. A head gasket or cracked head won't necessarily show coolant in oil.
How can I check for EGR cooler? Take off both end of cooler and look for coolant?