coolant expansion tank 670

Don't know what the mech could or couldn't have done but if there isn't coolant anywhere else then I would check egr cooler and air compressor. The compressor is easily checked by taking off the radiator cap while running. Pump brakes until it starts building air. If coolant pushes out while air compressor is pumping then it typically is a bad cyl head on said compressor

 

i had same problem with my 07 d12, it was the injector cups, there is a test that can be done to confirm if it is cups, i had no other symptoms other than expansion side of tank filling up

 

Ouch......need to get it looked at. If it bubbles as soon as you start then I would suspect injector cups or head gasket

 

To check if injector cups are leaking; locate fuel return line on fuel filter housing, disconnect (plastic push in fitting) purchase another fitting from dealer and clear plastic tubing from hardware store that fits it long enough to go back to fuel tank (route out side of truck to view fuel flow). Start vehicle when cold and let warm up (rev engine up and down some) to temp while watching fuel in the new return line, you are looking for air bubbles in the fuel flow. No air bubbles should be present, if air bubbles are present you may have cups/o rings/injector issues. Another symptom for this would be prolonged cranking to start engine after sitting for a while, it acts the same as fuel check valve is bleeding off, this may not happen right at first but will start to happen as things will start to leak more, would also check fuel filter and housing for leaks first as these housings have been known to get hairline cracks in them.

As far as the coolant tank it has 2 sides to it with baffling in it, a coolant side with a coolant level sensor and black fill cap and a overflow side with blue pressure cap which should not have coolant in it (overfilling coolant or a problem somewhere could cause this). This system should be pressurized and coolant will rise some when engine is up to temp and coolant will drop some when engine is cold. However you should not have to add coolant on a regular basis, if you do than there is a leak somewhere. Pressurize the system and check for leaks, if you are loosing pressure check to see where coolant going. There are a lot of places where it could go, external leaks anywhere (metal coolant tubes or bunk heater-core connection are bad for this). Internal heat exchangers; EGR cooler, transmission cooler, power-steering cooler or contact areas like; air compressor, fuel pre-heater, injectors. Locate where it is going and trace back to your source.

The "sudden pressure" brings me back to post #2 for most likely cause. When compressor shuts off its built up pressure inside it feeding into coolant system, may also have some coolant in air tanks.

Not really some thing the mechanic did but may now be more noticeable due to a more sealed cooling system.

I'm not a mechanic, just trying to help. If someone knows more or better please post!