287Klex

Hello all and thanks in advance for any advice that might lead me to solving my hard start condition on my 1991 LS400. Here is the issue:
Upon starting my LS it takes a few seconds of cranking for the engine to start. About 2 seconds when cold. Up to 5 seconds once the car is warm. Aside from this there are NO other running issues whatsoever.
I have traced the issue to a fuel pressure drop. If I prime the fuel system by jumping the B+ and Fp terminal in the DTC under the hood the car will start within a half a crank cold or hot, as it should.
With a fuel pressure gauge connected to the drivers side rear fuel rail, a fuel pressure test revealed that at idle I have 32psi regardless if the engine is cold or warmed up. If I unplug the vacuum line to the FPR it jumps to 38psi (both numbers to spec per repair manual).
I pinched off the return line, shut off the engine, and the pressure dropped to 0psi immediately. I pinched off the feed line, shut of the engine and the pressure held steady at 32psi for more than 20 minutes.
Connecting the fuel pressure gauge to fuel line under the car, BEFORE the filter and AFTER the fuel pump yielded the same results.
Connecting the fuel pressure gauge to the fuel pump directly showed that the pump held 50psi for more than half hour.
What is between my fuel pump and fuel filter than can be leaking? Also, if this is a fuel siphoning back to the tank or pressure leak issue, should I install a one way check valve just before the fuel filter?
This a problem because it causes delayed starts which become worse in warm weather. Unlike many other cars, the LS400 fuel pump doesn't "prime" with ignition on. It starts when the starter is engaged.
Facts: FPR is new. Fuel pump and fuel filter under the car are new (old one failed at 290,000 mi). Injectors are rebuilt and tested leak free (maintenance item at 325,000 mi). Ignition system is new (plugs, wires, caps and rotors) maintenance items.
How do I fix this problem?

LScowboyLS

sounds like a faulty Fuel Pump ECU or resistor, since it works fine with B+ jumpered to Fp (which makes the fuel pump run at full capacity all the time - no safety off, Fuel Pump ECU bypassed) - although occasionally the culprit has been some part of the EVAP system, or else a faulty gas cap, restricting flow. - a 91 is old enough that it may just have a fuel pump resistor (p/n 23285-50010 -- $50 at Sewell Lexus) in lieu of a complete fuel pump ECU (I seem to recall 92 was first year for fuel pump ECU), but the principle is the same. - Yamae can comment more on this.

You can actually drive the car with B+ and Fp jumpered like this in a pinch, but I would highly advise not getting into an accident that involves the fuel system

287Klex

With regards to the EVAP system, I was listening around the engine at idle and could hear a very faint hissing noise. It turns out I left the vacuum line from the charcoal canister disconnected when I did the park plugs. However I know in the recent past this was properly connected so I so not think this was my problem. The cap seals tight and the car did pass the EVAP test the last time I did my smog check here in CA.
I did install a one way check valve right before the fuel filter as well as a new filter. Results: With the car stone cold after sitting for 6 days it started right up. With the fuel pressure gauge connected I ran about my usual driving patterns. After the first stop and a 15 minute period with the car shut off I returned to find fuel pressure was still at 36 psi. Ran about for another 10 minutes to the next stop, shut car off for 20 minutes, came back, and pressure held steady at 36 psi. started up within a half second of turning the key. I did this 4 times. Each time the car fired up within a half second. It felt "alive". No pressure drop. By this time the car was warmed up and running on closed loop mode.
As I write this the car has been sitting for an hour and the fuel pressure reads 32psi. I think the problem has been solved. i had been after this issue for two years. Although it doesn't take 2 years to trouble shoot a problem like this what kept me from doing it "right", that is, with a fuel pressure gauge, was the fact that our cars do ot have a convenient test port. I added a permanent one and plan to share part numbers and prices on here for other to benefit from my experience.
If there are any changes to my results I will repost here. Also, to make the repair more permanent, I will connect the check valve with the proper double flare style fitting as opposed to the hose and clamp set up I have there now.

LScowboyLS

I am not sure how adding a check valve really gets to the source of the problem - a check valve should not be necessary.

I know you just want the car working, but it would be instructive to figure out why a check valve solves this fuel pressure issue. I always like to get to the bottom of a problem, that is what makes you a world-class tech!