Suzuki: ECM, mass air flow sensor, mass air flow

Question
If the ECM is faulty and is causing acceleration hesitancy and hard starting- would the engine light be on?
2000 Vitara 2.0L 4 Cyl
has new fuel pump, plugs, coils, and mass air flow sensor.

Answer
Not necessarily - a defective ecm might fail to display codes or illuminate the malfuncr tion indicator light.
Have you measured the resistance curve of the engine temp sensor?  If it is out of spec, it could cause a lean condition at cold start, and during warmup, at least until the oxy sensor goes in-loop.  Both the temp sensor and the Throttle Position Sender could be faulty, or either one singly, and be involved in causing a symptom like that.