With all due respect NIPPER> I have been building and repairing all types, styles, sizes and brands of vehicles for 25 years. I do not have comebacks. I do not replace anything unnecessarily. I have a very large customer base.
You would be surprised at how many vehicles I get in my shop that have been elsewhere and were misdiagnosed by a mechanic. A parts exchanger that didn't pay attention to math and science.
Electrical theory. Study. When is the last time you changed a fuse that was rated V? Its amps. I didn't say anything about an alternator.
The hesitation is there and is amplified by the operation of the AC. Therefore, the AC is out of the equation. It is narrowed to Air, Fuel, Fire and the controls of each one in relation to the timing of what the operator wants and what the engine can provide.
The fuel map of any modern fuel injected engine is mathematically calculated using information gathered from the temperature sensor, AF sensor or short band O2, camshaft position sensor, MAF or MAP depending on the vehicle setup for calculating air input or intake pressure, the TPS, gear position and speed sensor. It calculates the engine load using the same inputs.
Fuel pressure and flow are two different things.
The temperature sensor can effect operation at high ambient temperature but not low in a case where the output is fixed all the time. Case in point, a 99 outback with 2.5 dohc won't start unless the throttle is held slightly open. Had a dealership tech call me after he replaced the expensive idle control motor and he had the same problem. No start. I told him to look at the temp output. He called me back and said it started right up after replacing the ECT. The old one was feeding 365F to the computer so the computer was keeping the valve closed to prevent it from starting and damaging the engine.
If you don't know how a computer system and all the relative inputs function for each system, guessing is your worst option.
That's why I said check.
I have posted a good technician reference to fuel trim on this forum. You may want to read it. Modern engines do NOT run lean unless there is a problem. All ECU/ECMs are programmed for the engine to run at the prime stoichiometric equation for the fuel it runs. If it can't, there is a problem.
Cheap gas, high ethanol content, water, trash, dirt, clogged filter, poor pump effect this equation. As well as the sensors I've already mentioned.