91 Sidekick No start

[randyz71]

91 Sidekick 8v wont start unless fuel return line disconnected from FPR then starts and runs fine. once reconnected no start

[fixkick]

91 Sidekick 8v wont start unless fuel return line disconnected from FPR (not vacuum but return) then starts and runs fine. once reconnected no start

the return line is clogged and or FPR is bad.

when you remove any return line , nothing happens odd and is NORMAL, for sure.
now the worse case.
cars old have this bad habit of 2 things wrong at once,  for sure 30 years old
the cause can only be  proved with a fuel pressure gauge.

case in point,  a 1/2 clogged injector (or ECU running it way lean, inputs to ecu dead wrong etc. ) failure 1
then the FPR return line is clogged, causing crazy high fuel pressure the injector loves now that  it is clogged.
but yours is opposite,  it likes FPR line removed. doing that should do nothing but does, so the lower pressure cause engine fuel mix to be more correct.
3 things wrong? is possible.

Id use a fuel pressure gauge, to see if the pressure is in spec, hose on and off. (return hose)
that is what I'd do , so we can work from baseline of facts on that.
34 to 41 psi is spec, key on  3 seconds. or idle. or WOT or any time, it is stuck solid in this range and changes only with ALTITUDe.
good luck... finding all things wrong easy.

[randyz71]

After removing gas cap it now is running and no fuel in return line. Will replace gas cap and then do pressure tests.

[fixkick]

the FPR is bad, it is jammed shut after 30 years they do that sooooooo easy.
the FFR must flow fuel all the time engine running, or at the 3 second key on prime time.
make sure the gas cap internal vacuum breaker is working, or tank goes to vacuum over night cools and the gas tank collapses.... nasty this is.
like this.

https://fixkick.com/INJECTORS/PUMP/dead-...l#collapse

fpr bad or pump super weak.

the shunt test on the pump is 50 PSI or 55,
if that fails the pump is no good. for sure done with good FPR. or FPR removed.
when you key on the and it runs, the pump pushes fuel to the FPR the FPR opens and regulates at 34 to 41 PSI by its spring inside and flows to the exit port back to the tank, in a loop.
the pump must always be more powerful than the FPR spring, and the piston in the FPR not jammed closed 100% if that happens (either) the return line is dead.

1: weak pump
2: FPR dead, and stuck closed.
3: return line clogged, but with FPR hose return removed it must still flow fuel at the exit port of the FPR.