2011 Cummins ISC overheating problem

Customer complaint was truck keeps over heating and has to add coolant all the time. No noticeable leaks present and no white smoke from tailpipe. Pressure tested cooling system. System held 11 PSI never went down and no dropps formed. Water pump wasn't weeping. I removed the turbo down pipe while still pressurized, no coolant present and did the same to the EGR Intake side and still, no coolant present nor any residue. I ran truck till about 190 degrees, top rad hose still cool. Customer stated he had already replace the thermostat. Checked the operation of the thermostat. Changed water pump, added coolant and ran truck. At around 180 the thermostat opened and the top rad hose began to get hot. ran for awhile in shop, engine never got above 197 degrees. Test drove truck, gauge would read as high as 207 but then would go back down to around 195 197. Customer picked truck up the next day and complained that it had been overheating again about 8 times that day and still had to add coolant. Note that I did run engine with cap off, and no coolant was being pumped out from a bad head gasket or loose injector cup and the engine oil was clean. any Ideas? I did all I can think of doing maybe it just hit a air pocket and needs to be bled out some more?

 

The engine does have the light on. Being in a Dodge 5500 we are unable to scan. All we have are the datalink connectors for Heavy Duty trucks. the customer said that the light was on for a DEF level sensor or something. I imagine he will be brining in the truck tomorrow.

 

No leaks were found. left the pressure tester on for about 15 minutes, never dropped. Has a mechanical fan. was running constantly while in the shop. I hung a rag infront of the radiator to see if the fan was even pulling, sucked it right up, so I don't think the rad is clogged. Customer states it over heats while driving down the road.