NAPA sells a kit to check for the presence of exhaust in the coolant. Do a compression test. Get a new mechanic when you find what I think you will find.
This wreaks of a blown head gasket.
If you continually have a problem with one cylinder and the compression test and the test for exhaust in the coolant comes up clean then it is time to check the injectors if you find no other problems though I really think you have a combustion leak into the coolant system. Coolant is at 14-18psi but it may not be enough to push coolant into the cyclinder. The explosion when the cyclinder fires is at several hundred PSI and can blow combustion gasses into the coolant if the HG fails.
Most places that work on Diesels can get injectors cleaned and flow tested. Usually around $20-25 and injector and in most cases they can save the injector. If you have to remove the head I highly recommend doing this.
I have not personally used this place but their site seem on the up and up and the prices in line with what I have paid.
Had this done on my 4Runner when I rebuilt it. 2 injectors had restricted flow and all had atomization issues with 200k on the motor. That motor runs smooth as glass now between the injector cleaning and having the motor balanced. it will idle smooth at 400 rpm if I crank down on it and it has a cam in it that is not stock.
Here is how I learned about this:
I had an issue where I had ONE injector go lean and burn the valves on my 89 Supra. Never threw a code but would boil the coolant around that cylinder and make it run warm. The ECM was able to dump enough extra fuel in the other cylinders to hide the problem from the O2sensor.
Regularly inspect plugs on any high mile motor. If they all do not look close to the same you have something going on. On my supra the plug in the cylinder then went lean was always showing different. I was too dumb to figure it out until I burned the valves and lost compression in that cylinder.
Running wet in one cylinder will cause lubrication issues AND the ECM will lean the other cylinders out trying to get a good reading at the O2. Wet cylinder can cause lubrication issues, crack rings and score cylinders.
Let us know what you find and complete the thread with the final resolution.