Cummins ISX 7CEX emits white smoke, smelling coolant, then another day not?

[JimT]

Just for reference, I had the issue of my DPF blowing white-ish, sometimes blue-ish, smoke. It only occured during active regen, parked regen worked fine. The system wasn't getting hot enough while driving and that led to the injected fuel not burning properly. I never did figure out the exact cause, though an old DPF/DOC were likely a big factor. My doser injector (7th injector) was also getting clogged up fairly quickly, however even when freshly cleaned the problem did not change. I strongly suspect some sort of malfunction with the EGR system leading to incorrect exhaust temps, as this would explain why parked regens worked fine since they are not subject to the changing driving and operating conditions (that is, the EGR level is basically a constant during parked regen). Also in my case the engine was burning a fair amount of oil which in turn affected how the DOC/DPF performed.

I'm not trying to suggest that this is your problem. I'm simply providing information that might help you diagnose or eliminate potential problems.

[JimT]

Oh, one other thing. At one point I too smelled coolant during engine operation and was losing a small, but noticeable amount. It turned out that my pressure cap was bad and allowing coolant to come out the overflow tube. A new cap immediately resolved that problem.

The smell was deceiving since the engine fan would blow the air over all areas and make the source harder to determine. It's possible you have different problems each with different symptoms.