Car cranks but won't start until I removed gas cap and fuel pump relay?

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Lost power while driving and car shut off; it has strong crank but will not start. Changed fuel and air filter, spark plugs are new..getting spark from block. Tested fuel injectors..working. Fuel pump relay tested ok. Hear fuel pump working as well as getting plenty of gas. Timing belt and alternator were replaced a few months ago. Battery new. I tried removing gas cap and fuel pump relay and car did start for several seconds before dying...what should I look for next? Thank you.