Starting Issue

2000 4.0, AW4, 85k miles
Cone filter, 2.5 inch exhaust
Short in O2 sensor circuit, I would assume it runs in open loop mode

The issue is that when starting, the engine will crank over many times than usual, maybe for 10 seconds on some occasions before actually starting. But after it starts, it will run just fine. It doesnt hesitate or feel like it is lacking in power. It only has a problem with starting. On top of that it only happens about 60% of the time, independent of how long it has been off.

Don't really know where to begin diagnosing this problem. Obviously you need a few things to start an engine:
It cranks over, starter seems to be working fine.
I haven't checked plugs yet, but considering the engine works just fine after it has turned on, that seems to be an unlikely culprit.

That leaves the fuel system. I don't smell leaking gasoline, so hopefully all of my fuel lines are with out leaks. Nor does it misfire after startup, so I would hesitantly rule out injectors. Then the last part being the fuel pump, I am not exactly sure how to test that for proper functionality, but once again, it works fine AFTER startup.

I don't really know where to look next.

Also, heat soak ran through my mind. I have had that particular problem before and solved it for the most part by wrapping the fuel rail and a good portion of the injectors in insulating foil.


Suggestions?

First of all, FIX THE SHORT IN THE 02 circuit! Trying to troubleshoot problems when you already have known issue introduces variables that can make you chase your tail!

You should really rule out the checkvalve before looking elsewhere. It is a very common problem on the 97 and newer XJs. Here is more.
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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.

Listen to BirchlakeXJ, he has spoken...wisely.

Agreed.

Replace that O2 sensor before you do anything else, they're ~$40 on RockAuto. Listings are EXTREMELY confusing though, some of the companies state it's for the front bank but goes on cylinder 4 and nonsense like that. Use a code reader to figure out which O2 sensor needs replacing. Probably will improve your gas mileage, my MJ went from ~13 to ~16-17mpg when I replaced the upstream sensor and my 98 XJ actually started running properly instead of choking and stalling and stuttering.

Try cleaning the throttle body. My son's 97 wouldn't start easily unless the accelerator was depressed once before starting. After cleaning the T-body with a new IAC, it starts normally. It had 165k miles, and compared to my old Renix, the T-body was relatively clean.