Cummins ISX15 Air in Coolant Problem

Kenworth with a Cummins ISX15 arrived with a problem of coolant overflow. The expansion tank cap was broken (almost didn't hold any pressure).

We replaced:
- the expansion tank cap
- the water filter

We conducted the following tests:
- combustion leak test (twice with two different fluids). No combustion gases detected.
- cylinder cut-off test. All cylinders appear to be working.
- measured the pressure buildup time in the cooling system for every cylinder cut-off. No significant difference observed. The pressure builds up to 10 psi in 49-54 seconds.
- opened a cylinder head cover - everything appears normal.
Conclusion: there is no combustion gas leak.

Air continues to come out of the system beginning 2-3 minutes from the start (cold engine), even after:
- disconnecting the air compressor (two water lines and both air lines)
- disconnecting all water lines from the turbocharger and air lines as well
- disconnecting 5 water circuits to external devices (cabin heater, transmission heat exchanger, etc.), all lines in and out
- disconnecting water lines from the EGR fuel nozzle
- checking the smaller hoses connected to the expansion tank - clean, no clogs

We even disconnected the water pump for a short test - no change observed - air continues to be pushed out. We have run out of ideas. Need advice.

 

about 200k after inframe. I used two different combustion tests. I checked them by applying exhaust gasses from the same truck - turns yellow - so it works. I run the test with coolant gasses for about 15 mins - no color change.

There is also no smell which could be presented in the case of combustion leak.

How I used the test:
I connected the overflow tube from the coolant extension tank directly to the bottom of the test tube - so every piece of gas comes out into the testing fluid in that tube. See the video

 

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