Unmount your isolator, turn it guage side up, bleed the fuel side pressure into a paper towel. take off the connection to the gauge and stick the eraser end of a pencil in carefully and push the diaphram back down. Retighten the fuel side bleeder. Refil the gauge side with coolant, ensure the tubing to the gauge is full of coolant and reconnect.

Turn on the ignition and see what you get for fuel pressure.

 

I would put a gauge on the rail and test the pressure there. If you have never changed your pusher pump (lift pump) that could be the problem. If it's the fuel gauge isolator, most gauge companies, if the isolator was originaly designed for gas, won't warrantee them or guarantee that they will work with diesel. Most people I know that run gauges use the electrical fuel gauge, no isolator, or they run a programmer that monitors the fuel pressure. I recommend to anyone that is running a 98.5 to 2002, they change their pusher pump if they haven't already, it will save a bunch of money in the long run.

 

i had the same problem on my 01 2500 .....i was idling at about 12-14 lbs and all of a sudden my fuel press dropped down to maybe 1-2 lbs and then 0lbs, i changed my my fuel filter and nothing and then i replaced my lift pump and was idling at about 15lbs i would lean toward the lift pump they are known to go bad in those models at least people i talk to,,,,i had the x monitor digital gauges and would use nothing else