Fuel pressure advise

1999 4.0 aw4 np231 rustys air tube, flowmaster catback

Symptoms: hard to start, rough idle when cold for like 20 seconds, i noticed raw fuel smell in my air filter at an oil change once.

I had a check engine light come on today and the code was po301 cylinder misfire #1. I cleared the code and will see if it comes up again. Cap, rotor, spark plugs all were changed about 8k miles ago. Also sporting some old accel 8mm wires which are still in pretty good shape.
After checking the code I went home and did some searching and found 2 threads on the code that were not much help besides doing a compression test. After checking wires and such i figured to hit the valve on the fuel rail just to see what would happen. Nothing, no pressure whatsoever. Started it and had extended crank like usual.

How long do leaky injectors take to leak down all that pressure?
Should i look into replacing parts on the pump first?
Could the code be part of the problem?

Also MAP sensor was changed last week, and i have been trying to run ethenol free gas for the last month.

Thanks in advance.
Sorry for the long read.

I would actually ohm out that plug wire for cylinder #1, rather than assuming that it is good. And I would pull the #1 plug out and have a look and pull another one or two for comparison if necessary.

Having a bad check valve is extremely common for a 1999 fuel pump assembly. It allows the pressure to drop pretty quickly when you turn the engine off. Mine went south at around 85k. Because the pressure is leaking down doesn't mean you have an actual fuel pump problem. The fuel pressure leakdown you are experiencing is most likely not related to the cylinder #1 misfire. More on the checkvalve problem below. It is so common that I have a "cut and paste" answer at the ready! Good luck...
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The check valve is located on top of the gas tank and is part of the fuel pump assembly on 95.5-2001 vintage 4.0 XJs. When the check valve starts to fail, it reduces fuel pressure to a point where an extended crank is needed to start the engine. It is a very common failure. I experienced it on my 99 at around 75K.

A good (and simple) way to troubleshoot this problem is called “the poor mans prime”

1. Turn key to ON position (do not crank the engine!)
2. The fuel pump will energize and run for a couple of seconds
3. Turn key to OFF position
4. Repeat above a two more times
5. NOW crank the engine over

If the engine starts quickly and cleanly after performing this procedure, you may have the check valve issue. If it starts and runs poorly for a few seconds, it could also be a leaky fuel injector resulting in the bleed down and the stumble upon startup is the engine clearing the excess fuel that has leaked into the cylinder because of the faulty injector.

Also be aware that a bad battery can give you this symptom. Sounds strange but it definitely does happen. Have battery load tested if you have any questions; your XJ does NOT like low available battery voltage. Be sure battery connections and posts are CLEAN.

Some troubleshooting tips for extended crank times:

Hook up fuel pressure gauge on the schrader valve on the fuel rail

Start engine and bring to normal operating temperature.

Observe test gauge. Normal operating pressure should be 49.2 psi (plus or minus 5 psi)

Shut engine off.

Pressure should not fall below 30 psi for five minutes.

If pressure falls below 30 psi, it must be determined if a fuel injector, the check valve within the
fuel pump module, or a fuel tube/line is leaking. An adaptor tool/hose included with the fuel pressure gauge can help you with this. Consult gauge manual for more information on this but here is basically how it works:

*Turn the engine off and immediately clamp the fuel line at the adaptor hose. Watch the pressure gauge and see how long it takes to lose pressure.

*If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - possibly the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky fuel injector.

You can limp a check valve problem along indefinitely if you wish as it doesn't mean the fuel pump itself is going bad. Normally, the check valve is replaced as an entire assembly as you have to drop the gas tank to access the assembly. Fuel pump assembly consists of fuel pump, regulator, check valve, filter. If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket assemblies don’t hold up and fail prematurely.