LQ4 into a 3rd Gen/1972 Nova by frojoe

4 minute(s) of a 891 minute read

Been driving the car around a bunch during the dry days this past October..

The main issue I'm dealing with right now is fuel pressure dropoff for no logical reason. When I'm starting at 2500-3000rpm and slowly climb with constant throttle, the fuel pressure, the fuel pressure drops thru the floor.

I've tested:
- tank full of gas, so fuel starvation into the pumps can't be it
- both in-tank pumps *sound* like they're on (from my tuned hearing, sounds louder than just a single pump running).. however I still need to climb under the car and test-hotwire each individual pump to ensure they're both running
- test pull with FPR connected to manifold pressure
- test pull without FPR connected to manifold pressure

My thoughts so far:
- drop in fuel pressure isn't due to some strange action of the FPR influenced by a weird vacuum/boost signal
- even if only a single pump is running, a 6L engine @ 3000rpm not in WOT condition should easily be fed by a single 300lph pump without the pump being a limiter
- I have a Summit brand fuel filter that hasn't been inspected or replaced since installed in the original single turbo setup circa ~2011.... maybe filter is causing restriction and hampering fuel flow
- since I changed from a semi-dead-head fuel rail setup (with FPR before the rails), maybe with full volume of flow running into FPR it is somehow overrunning the FPR spring... doubtful?
- I want to test fuel pressure before and after rails with existings FPR-after-rails setup to see what the pressure differential is due to the rails... not sure what this would tell me other than a point of curiousity
- I want to temporarily re-plumb the rails to be like the old setup, with FPR before the rails and fuel flow thru the rails "dead-headed" and not "thru-flow"... this would simply be a comparison to the old "working" setup

Here are two datalog screengrabs:

This one shows the fuel pressure (faint yellow line) climbing in stages as the MAP goes up, with the FPR connected to the intake manifold. The fuel pressure starts dropping JUST before the calculated fuel flow starts ramping up to compensate...

This datalog shows the fuel pressure slowly falling as the engine RPM's increase at a steady rate, to the point that the fuel flow can't sustain and the fuel pressure drops off...