My 1999 5.4l runs great then after driving about 20 minutes, surges & stalls if you try to accelerate. Then the idle surges from 600 to 1500 (back and forth). Connected a fuel pressure gauge, and it shows 30 then drops to near 0 then bounces back to 30 then near 0, then back to 30 etc. No codes are reporting. Any ideas what to check? Let it cool down, process repeats.

Has new fuel pump & filter.

 

Fuel pressure should never drop to zero and 30 psi is still too low.
The electric pump runs independent of the engine.
The only effect the engine has on pressure is on the regulator vacuum applied but cannot cut fuel off, only vary it over a 5 to 8 psi range if the regulator is good.
Better review the fuel system operation.
Good luck.

 

Any idea what component could cause the fluctuation in the fuel pressure? This only starts after driving for about 20 minutes. Still no codes.

 

12 volt supply through the fuse, fuel pump relay contacts, crash sensor contacts, wiring and fuel pump.

 

Relays & electrical tested fine. Could this be catalytic converters or a fuel line getting hot? Still stumped. Any help appreciated.