It takes AIR, FUEL,COMPRESSION,IGNITION (at the correct time),and EXHAUST. One or more is missing. Too little air or too much because of big vacuum leaks. Not enough fuel because of stuck air sensor plate or pluged cold start injector or possibly no power to the cold start injector. Compression low. Ignition problems because of gas fouled plugs which need to be replaced. And finally, the exhaust pluged up by varments (I have seen it happen).
Most cars I work on that won't start are sent to me because of something the owner or other shop doesn't understand. They say it must be a computer problem so send it to Paul. Most could have been fixed by just checking the 5 things above and fixing the ones that are bad.

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84 500 SEL (307,xxx miles)