Depending on the output pressure of the lift pump. You might want to install the old pressure relief valve for testing. I do not know what pressure the replacement ones are calibrated at. Yet obviously higher than the originals.

If you see output from that valve it is okay to leave it in for the testing. What I was thinking was if there is air involved it cannot partially clear through a constantly closed relief or overflow valve. All of the air instead would have to get processed through the injector lines. Just a thought though.

Another indication was did you observe any difference at all with the new relief valve? If none the fuel pressure supplied could have been too low to open either of them. Or perhaps is not enough to open the enhanced one. It s pretty obvious you probably have some form of fuel starvation with the times posted. You might need a fuel pressure gauge to get a clear picture of what is going on or to reduce possibilities.

Also keep in mind that too much blow by pressure can partially or totally shut down the injection pump. I have no ideal if you can run these engines at speed with the oil filler cap off as a test. I would make sure at least my crank vent hose was not badly gunked up internally.

There was another test with some possible risks for too much blow by. Basically it was to close off the vent hose and see how long the engine ran until it quit. You would have to look it up but ten seconds was about the normal amount on an old engine I think I remember. It was also suggested that this test might damage some seals but I have no opinion on that. Many used it when going out to examine an example of a car with this engine in it.

I have used it in the past myself. Too much base pressure starts to control the injection pump shutoff. No ideal if this was intentional in design or not.