Freightliner Columbia 120 Reservoir Overflow Dumping Coolant

I have an 2008 Col 120 with Detroit. It has the plastic reservoir on the radiator. The reservoir has a plastic cap on top and a plastic cap on the side of it with the overflow tube sticking out of it.

The other night the coolant level dropped below the sensor and stopped the engine. Added a gallon. About 4 hours later, stopped again, added a gallon. In between the temp spikes up to 220 or so but then comes back down pretty quick when he slows down. It was pouring rain and we couldn't see any obvious leak. The mechanic drove it to the shop and on the way their the temp spiked twice but then came down.

The mechanic has had it all day today - pressure tested at 15 PSI, checked head gasket, checked EGR cooler, idled for hours, bob-tailed for hours, and found NOTHING! Coolant level and temp all steady all day.

My driver went out and 5 hours later the temp spikes so he slows down a bit and it gets back to normal. Now the coolant is down below the sensor line and he has to add a gallon. He noticed steam coming out of the overflow tube.

On my KW's I would just replace the pressure cap and solve the problem.

Does the Detroit Freightliner have a pressure cap, is it the side cap? Any other thoughts what this might be? Flaky thermostat?

 

Oil checked and looks good. Mechanic did the head test and the colors showed ok.

Is the side cap the pressure cap?

 

It has two caps, see picture attached. One on top, and one on the side middle that has the overflow tube out of it (middle of picture, left side). Is that the pressure relief cap? (On my KW's it's a metal cap)

 

Yes, I believe he did.

 

Welp, gonna put on a new cap in the morning and see what happens. I hate problems like this because it's not black and white... Thanks for the ideas.