M20 missing and backfire

[chri]

Hi guys need some help

Car starts but missing and back fire
Changed external hpfp and fpr but still same problem

When closing fuel return pipe with a grip tool, engine works perfect

Any help

[AlexSutherland]

Hopefully you found the issue with the miss fire. If you've changed the fuel pressure regulator and the fuel pump, I would start conducting tests on the fuel lines. There's a few tests you can do. One is using a fuel pressure gauge and connect it to the fuel pump. See how much psi you're getting (4.3 psi or more).

The other test is the "fuel delivery rate" disconnect the return hose from the FPR and connect a hose to it. You need to measure the amount of volume that comes out of the FPR after 30 seconds (has to be at least 875 m l (30 oz.)).