Engineers figure this stuff out...

 

They sure do

 

I still think it's a PSI thing...disconnect your fuel line and drop it in a bucket and turn your key on

 

There is nothing that monitors the fuel pressure, the pump is just on a timed circuit from the PCM when the key is initially turned on, and when the engine is started up the pump kicks back on continues to run.

In the case of fuel trim, thats coming from the O2 sensors telling the PCM that the engine is burning too much or too little and it adjust to compensate. When it can't adjust itself say from too little fuel for a prolonged amount of time, a code will pop up.

Even today very little cars have fuel pressure monitoring systems. The new F150's do have that system and a couple other cars.

 

Thanks that is what I was looking for. I read it on another web page and have directed others to a bad PMC before.