First-time poster/long-time lurker. Am working on a 1990 Grand Wagoneer (AMC 360) project that already had EFI (GM TBI) installed on it when we got it. Have been working thru running/driving issues ever since, and have it the best it's ever run, with the exception of a couple issues: 1) Rough idle/hesitation at cold start for a minute after startup 2) Backfire at high RPM at highway speeds, when running at 70 mph, let off throttle, decel a bit & get back in it; can hear & feel backfire thru the exhaust - sounds like popcorn popping. Am running Affordable's GM TBI fuel injection system, along with 1320electronics.com bluetooth OBD1 scanner & have tuned initial timing (10 deg), base idle(750), along with IAC & TPS. Have found/fixed numerous - having replaced all vacuum lines in the engine bay, and added a Schrader valve to the fuel supply line, about 12 inches back from the throttle body. Am thinking i may have a fuel delivery issue. When cold(sitting overnight), fuel pressure at prime shows 12 lbs.; once primed, the fuel pump cuts off, and pressure instantly drops to zero. The pressure drop seems key to the issue, as it seems like it should hold pressure. To figure out which direction to look (forward or backward), i've run a few tests: 1) inspected the fuel injectors for leaking - so far, ive seen a nice pulsing spray pattern, with no leaks/dribbling. 2) Changed fuel filter - saw no real change. 2) Crimped the supply line between the fuel pressure gauge & the throttle body, and primed the system: to see if the fuel pressure drops indicating an issue with fuel pump, or its check valve. Pressure rose to 60 lbs & held - indicating the issue is on throttle body side of the equation. 3) crimped the return line near the throttle body & primed the system; & saw the pressure rise to 60 lbs & hold, which which makes me think fuel pressure regulator. Have installed a FPR rebuild kit a few weeks back, and saw no change. Am stumped. It seems like the fuel line should hold pressure at prime & not instantly drop off. Everything i read says, that seeing a pressure spike when crimping the return indicates an issue with the fuel pressure regulator, and crimping the supply ahead of the throttle body proves the fuel pump check valve is good. Where to go from here?