Engine mopar mpi on 258 not holding residual fuel psi;

[mojac11]

just bo't a '73 CJ5 with a 4.2 engine & MOPAR mpi conversion; fuel pump not holding residual psi; - if it sits awhile it loses psi & have to remove the fuel line & "suck" fuel up to the fuel rail, then it works if started regularly - where is the pressure residual valve? What parts do i need to replace or get?

 

[Patrick]

Sounds like a very dirty fuel filter and clogged strainer. The fuel rail normally pressurizes immediately after ignition is turned on (not cranked) regardless of how long the engine was turned off. Listen for the fuel pump coming on for a few seconds as soon as ignition is turned on but not cranking the engine. You could also have a bad pump. Note: Residual pressure is not used or necessary to start your engine. Good luck.

 

[mojac11]

thanks - i posted that above based on what the P.O. told me, having no idea how the MPI worked; afterwards i found some other threads explaining the system & had concluded "maybe" the rail was air locked & needs to be purged cuz the P.O said he always got it started by removing the pressure line, which obviously would introduce air into the rail; since then i can report the fuel pump whirs as it is supposed to on initial key-on, - however no fuel is delivered out the open line - next i'll have a look at the filter as you suggest & check for fuel at the pump inlet ... today, also found the install instructions in a binder stuffed in the back with a pile of other jeep repair books - at this point i know mpi works basically the same as my 3.2 Motronic Porsche so i am in somewhat familiar territory

 

[TSB8C]

A whirring pump with no fuel coming out an open line is either post-pump filter or fuel inlet to the pump for sure. You're on the right track. Is the pump mounted in the tank or is it external relying on the stock sock filter and pickup tube in the tank? If it's external, mounting location such as distance from the tank and height could be factors. I initially had issues with my externally mounted pump strapped to a frame rail. Now using an in-tank pump and no issues whatsoever.