gen 4 '95 nissan maxima running too lean on fuel

My Maxima is a '95 model, Automatic it has 211,000 miles, It has been slow to start when cold for some time, cranks fast just don't like to start.
Once it starts it runs perfect till it warms up, after it's warm it starts to lean out on fuel till it's near impossible to drive. Overall in good shape, good compression, does'nt use oil. No ECM codes acts like intermittent injector missfire, gets worse the more I drive. I'm thinking it may be a weak fuel pump? whats the fuel pressure supposed to be? Someone suggested the thermal coupler or thermal valve, Whats that? Thanks very much for any help.
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