High fuel pressure

[Maverick H1L]

Back story:

3100 hybrid engine has been yanked. Harness, exhaust to cat and all. 2 bad pistons in 1 and 4.

Replacement 3.1 put in. 2.8 F-body harness and 1227302 ECM.

So far I have replaced:
ICM (bad wire in harness EST circuit is causing the code 42 but I got a good module anyways)
Core freeze-out plugs (2 were popped, 2 more were rusted through)
Knock sensor has been removed and replaced with a standard drain plug
Thermostat and gasket
Guts of alternator
Battery
RH exhaust manifold was replaced before engine yanked from donor. All manifold bolts replaced. AIR tube plugged with plug to prevent having to retighten the stupid thing every so often.
Power steering pressure line replaced due to split in hose
Had to repair O2S bung on crossover pipe due to being torn and also a crack next to it welded.
Fuel injectors, FPR, and rail replaced with NOS. Yes, BRAND NEW. Found sitting on a shelf at the dealer I work at. Injectors are 15.4# 523-5136.

Still running on external Bosch fuel pump. Still using Magnaflow 93441 cat with Summit adapters to fit on crossover pipe. Had custom fuel hoses from chassis lines to engine made and am using AN pipe fittings.

Problem: Getting 50 PSI fuel pressure. Should be not more than 47. Nominal I have seen is 43 engine not running. Engine running like puke. The MAFis throwing a code 34 but that is beside the point. Going to deal with that shortly. Anyways, this is a stock FPR with clear lines. Need to get the fuel pressure down to get the rest of the setup completed with proper idle. Right now it's hunting pretty bad at first and then just running like puke when hot.

[neophile_17]

Is that Bosch pump meant to supply 700hp? You may be overloading the regulator if it's oversized.

[Maverick H1L]

Doubtful as the fuel pressure is within specs once engine warms up and it runs fine (about 42-44 PSI). It's cold starting and warmup that has fuel pressure issues. Almost like a lack of engine vacuum when warming up as pressure regulator is responsive to vacuum pump with pump running continuously during vacuum pressure testing. I know I said before it ran like crap when hot but not really now that it has good injectors in it and it has been timed relatively close to where it should be.

[Maverick H1L]

Update:

Fuel pressure STILL too high. OVER 50 when pump running and engine not now and still 43 PSI running.

So far I have replaced the fuel pump with a Walbro with 43 GPH flow rate (just about stock though still slightly high from what I can find online).
Blown out fuel return lines and hoses right from the port in the FPR. Sat on the pressure for a good 15 seconds. Twice.

Still having the cold start issue with hunting idle. Swapped IAC for known good one AND cleaned seat in TB. Engine vacuum about 15" Hg when running (if the gauge on my cheapo vac pump is relatively accurate).
Code 34 is gone. Pulled terminals from MAF connector and used a screwdriver to tighten terminals. Haven't had code 34 since.
Code 42 is gone. New wire for EST control run but not hooked to anything because all 4 wires in that module connector tested good when wire was run (not sure why?).
No codes.
Spark plugs replaced. NGK platinums gone for R43TSK plugs (can you believe they're DISCONTINUED now?). Ignition coil swapped for another. No change.
Random miss apparent. Bouncing tach and also tach seems to be off (93 Camaro tach). ICM already replaced. I know there is a quick connect T-tap in the coil tach wire, probably for an aftermarket tach. Going to be fixing that wire soon.
Haven't done compression test or cylinder deactivation test yet. Coming soon. Transmission needs work first (leaking ATF).

Fresh out of ideas here. Almost like too much spring in FPR. Not liking the 6 PSI over where I have found the fuel pressure should be.

[Guest]

I assume this is a 3rd gen F-body since I believe that is what you have. I see no mention of what the "stock regulator" is from...

For GM regulators pre 2000 in most applications and all port 60 degree Vy applications that I'm aware of pre 2000, the 43.5 PSIG is pump on engine off, or no vacuum applied to the regulator, running it should be down about 39 or so at idle.

If the pressure stays high you have a blockage in the return, no question.

To test take the return line off at the regulator, and add a hose to run it into a pail, turn on the pump and see what the pressure is. If it's still high, which is not common then the regulator is to blame.

[Maverick H1L]

1988 IROC-Z28, gone back to stock 3.1L LHO run by 1987 1227302 HNL

2 fuel rails have this problem, as stated in first post. One rail is NOS. All 12 injectors are NOS.

Pump on engine off is ~52 PSI. Until pump shuts off where pressure drops because external pump doesn't have a check valve. Running is 43. Tested on old 3.1 in my 87 was 41 PSI engine off and 37 running. That was with factory fuel sender and pump and it ran pretty good. Installed pump is only, from what I have seen, about 4 GPH higher than factory and a couple higher than the TPI pump. Walbro GCL-603 pump fitted with AN fittings instead of hose barbs.

This fuel system, with the exception of the regulator and rail, is perfectly capable of running down to as low as 22 PSI as tested previously with the hybrid.

Again, return side of system has been blown out. Once, waited a couple hours, hit it again. At least 15 seconds each. About 100 PSI. From the return port in the FPR valley in the rail all the way to the tank. And both between FPR port and end of engine pipe and end of return hose connecting to said pipe all the way to tank just to be sure of nothing in the engine pipe. If I had my old fuel rail from the 3.1 I'd pull that FPR diaphragm to see if it does the same. Something doesn't make sense here.