Coolant overflows

I know nothing about your engine; does it have EGR? If so, it could be starting to fail. It would not lose much while running empty, but work it and it will leak more. It would be exhaust gas leaking into the coolant via the EGR cooler, and naturally you would have more exhaust pressure under a heavy load.

I know you said that the dealership did the coolant test, so maybe my guess is way off base.

Again, I don't know if you even have an EGR on that engine, but I have had this problem on one of my diesel pickups.

 

What about loosing coolant at 7th injector? Leaking coolant into EGR cooler and get burned?

 

Here's my 2 cents worth..if you are losing coolant, the system is being pressurised. If it does it more when it's hot and under load it sounds like it is being pressurised from the combustion chamber. When you said the coolant had been tested for carbon and showed zero, maybe you should get it tested again. Get a sample of coolant and have it tested. If there are no contaminants then it can't be from the combustion chamber, but everything seems to point that way.

 

its easy to test the egr cooler.

 

to make sure you need to take of and pressure test. But first do this. in the morning before starting the truck disconnect outlet side of egr cooler and look for coolant. you may see red water like spots. Do you have red coolant ? Again to make sure 100 % you need to take it off and pressure test .

 

egr cooler has inlet and outlet ports for coolant. take off the cooler. close one of the ports. apply 50-80 psi pressure another port and insert cooler in the water. if you see bubbles are coming out then cooler is bad.