With a NO START, we start out with a can of CARB cleaner as a fuel (it's safer), spray it in the intake and if it starts, we have a fuel problem. If it doesn't start, we may have an IGNITION problem. We then check for fire with a test plug. No fire = NOT START. Next comes a compression test on at least 1 cyl. If good, then the only thing left is exhaust restriction. What it takes for a four stroke engine to run is FUEL, FIRE, COMPRESSION, and EXHAUST.
Think in terms of these 4 words and it will all fall into place and the engine will run. If this helps maybe we can help with the system that is failing.
Good luck
Paul

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84 500 SEL (307,xxx miles)