Wow. That's one tough vehicle to keep going when you ignored the Check Engine light for that long. How are you expecting to know when a new minor problem occurs that when also ignored, turns into a serious one?
There's two problems with ignoring any warning light. First of all, the obvious one is when the Engine Computer detects a problem, it sets a diagnostic fault code, and if that problem could adversely affect emissions, it turns the Check Engine light on. Sure, a small leak in the evaporative emissions systems wont affect how the engine runs, (it just affects the air you're breathing), but when a totally different problem develops, the Check Engine light is already on, so you won't know another problem exists.
The bigger problem is the computers constantly run self tests on sensors and output circuits. They compare sensor readings and operating conditions to each other to figure out when something is wrong. One of the conditions that must be met to set a fault code is that certain other codes can't already be set. For example, it knows when the engine has been off for at least six hours, the coolant temperature sensor and the intake air temperature sensor had better be reading the same temperature. It also knows the engine had better not be running at 4,000 rpm when the throttle position sensor says it's closed and at idle. If a code is already set for the intake air temperature sensor, the computer has nothing it can trust to compare to the coolant temperature sensor's readings, so some of the tests on it are suspended. Some failures will not be detected, and will not set a code.
When you finally get the first problem repaired, any suspended tests that use that as a reference will resume. THAT'S when some of those other problems will first be detected, set a code, and turn on the Check Engine light again. You incorrectly assume the mechanic is incompetent, but in reality, he had no way of knowing there was another defect when there was no fault code. With a Check Engine light that has been on for two years, you can be chasing problems for a while, and there's no telling if anything was damaged by ignoring those problems. Also, some of those tests will resume right away, but some only run under very specific conditions and could take from hours to days before the next problem is detected. The problem isn't that the mechanics don't know what to do. They're getting confused by what appears to be recurring problems over and over, but they are likely faced with multiple problems that have to diagnosed each time as they show up.
The loss of coolant you described sounds like a cylinder head gasket is leaking. Combustion gases can sneak into the cooling system and push coolant into the reservoir, or it can be drawn into the cylinder where it's burned and goes out the exhaust. When that gas goes into the cooling system, it can pool under the thermostat causing it to not open, and the engine overheats. Thermostats have to be hit with hot liquid to open. Hot air wont do it.
There's two tests for this. The first one involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, that liquid will turn bright yellow.
If that test is negative, you can add a small bottle of dark purple dye to the coolant, then search a few days later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If a head gasket is leaking, you'll see the dye inside the tail pipe.
Wednesday, February 26th, 2014 AT 1:07 PM