97 Thunderbird won't start. What could it be?

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I have been searching and searching and can't figure out why this car won't crank. The IFSS is working as it should. The fuel pressure is at 39PSI when the key is in the on position. There is fuel coming from the Schrader valve. I replaced the fuel filter. Checked all the relays and they all work. I used multiple batteries and they are all pumping out enough juice throughout. Checked the spark plugs, they are not fouled out. They are all in proper sequence. The MAF was cleaned. We even tried using starter fluid into the intake, and it will crank, but not stay running. The fuel pump IS getting 12V. The intake manifold had been replaced here recently, and since the replacement, it has not ran. Another thing is the fuel looks almost a greenish color when we changed out the filter, so we drained more to check it. It is a yellow/green color. I am thinking the gas is old, and has lost its octane, but I am trying to not have to drain the tank. I am, unfortunately, trying to take the shortcut for now. What is next?