Hello. Welcome to Just Answer. Please allow me to assist you. First, high test gas is detrimental in this engine. The engine is not designed for this octane and will cause drive-ability issues. Use 89 octane only.
Second, teh correct spark plugs is also critical. Use only factory recommended NIPPONDENSO spark plugs. Other plugs will cause poor acceleration and rough running
Third, uses only genuine Toyota electrical components (air/fuel sensors, oxygen sensors, coil packs, etc). On Toyota cars, inexpensive replacement parts are well known for contributing to poor engine performance.
If the car is indeed running bad, the reason should show up on a scan. If there are problems, the check engine light should be on and the problem reasons stored in the computer. If a scanner does not show problems, perhaps the scanner is not good enough to extract History and Pending codes, which are critical to proper diagnosis.
One thing to remember: The timing can be one tooth off. This will cause poor performance, and, on a 1998 engine, will NOT have any codes in it.