The dash coolant temp gauge at the 1/2 mark is at the correct spot after the engine warms up. That’s not likely a problem. For a proper “firing” there has to be several things in place and working inside the cylinder
- Correct amount of air and fuel flowing into the cylinder
- Correct compression of the mixture above
- Healthy spark
- Occurring at the right time
- Resulting in a “bang”, which forces the crankshaft to accelerate
- Which is detected by the engine computer via the crank position sensor
- Quick and thorough venting of the exhaust gas products
So something’s amiss with all that. The big clue in your favor of a quick diagnosis is that it only affects one cylinder. That makes it unlikely to be associated with airflow into the intake manifold for example.
hmmm … given what your shop has already determined, and it sounds like they know what they’re doing, I think the first thing I’d do is make sure that cylinder’s spark looks good by visual inspection. Next, I’d test to see if the injector is getting electrically pulsed like it should. After that either I’d follow the idea above of swapping that injector with another one, or I’d do an injector balance test, to see if that injector wasn’t putting out the expected amount of gas per pulse, or too much. I wouldn’t purchase and install a new injector quite yet. I’d probably do a fuel trim test before installing a new injector. Injectors, especially Toyotas which are known to have a pretty much bullet proof fuel injection system, rarely fail. Especially true if the fuel filter has been properly serviced at the recommended intervals.
This didn’t happen soon after a fuel filter replacement did it?
What does your shop say about your diy’er “fix” on the vacuum line?