The reduced mpg is probably due to the misfire, not the evap problem. You have a general misfire, not specific to one cylinder. If specific to one cylinder, you can swap stuff around, like the coil packs, to see if the problem follows the swap. But a general misfire is harder to diagnose.
What’s happening is the computer is firing the spark plug and sometimes noticing that the crank shaft isn’t speeding up a little due to the firing. Which it should, right? An explosion in the cylinder should force the piston down and speed up the crankshaft.
So you occasionally have no explosion. What you need for an explosion is
- a good spark
- at the right time
- with compressed fuel and air in the cylinder
- and the correct mixture of fuel an air
The problem is almost certainly among the above list. A good mechanic should have the equipment to measure for good spark. A diy’er might use one of those in-line spark testers. Spark problems can come from crank and cam sensor too. And from the igniter module. Timing is something that is usually pretty easy to check. And compression should be next on the list if the spark is good and at the right time. You may have developed leaky valves or rings. Mixture is harder to determine, but shops would start with making sure there’s a new engine air filter, then a fuel pressure test probably, then verify all the injectors are pulsing like they should.
If the TC lock-up clutch was stuck on all the time, it seems to me the engine would die at stoplights. Since it doesn’t, maybe that’s some kind of sensor problem, and the clutch is actually ok. Do you notice anything weird going on, like a brief or lasting rumbling sensation, at speeds between 35-50 mph?
I don’t see any reason why the evap problem can’t be addressed independent of the TC. Beats me why they said that. Federal regulations require that cars self-test themselves, to insure gas fumes don’t escape from the fuel tank into the air. It’s a complicated system that prevents this from happening, and differs from manufacturer to manufacturer how they do it. But the car either pressurizes or forms a vacuum in the airspace above the tank, then measures to verify that condition holds. Otherwise it will flag a leak. Another common evap problem is caused b/c it can’t vent that space like it want. It’s uses a solenoid valve called a “purge valve”, and purge valve replacements are a common thing here to address evap problems.